Is long bearing length the key?

[Marlin Junky]

I've shot a total of about 50 boolits cast from an LBT 30-180LFN mold through an old 336A with a long throat and a 4-groove, 1:10" barrel and I'm amazed how well they shot even at over 2300 fps. Today I shot 4, 5-shot groups at 50 yards with iron sights and all 4 groups were within 1". Boolits tested 16.0-16.5 on the Lee hardness scale and were propelled by 34.2 grains of DP-74 (like AA2520).

I think the LBT advantage is the long bearing surface, especially the .2" (or more) bearing surface ahead of the first lube groove. Not only was the accuracy better than good, the barrel remained clean even though this particular design only holds at most .4 grain of lube. Lube used was BAC purchased from White Label Lube.

MJ

[MtGun44]

Sounds good. My one LBT rifle mold is a 130 gr 7mm which has shot 1" 5 shot groups at
50yds with several loads in Ruger 77 in 7x57. No leading.

[Bret4207]

Bearing length means the boolit is supported better in the barrel. The tipping point is when the barrels twist isn't right for the length. I don't think LBTs design have any magic qualities. I think you found the happy confluence of a boolit that fits (is supported) in a twist that handles the length.

Shorter bearing length can work but IME boolit runout is critical and case fit in the chamber is too (tight necks).

[runfiverun]

in some rifles the longer bearing area is a must.
in #1 wifes 0-6 after all the measuring and fussing i thought that the rcbs silh boolit would be the ideal partner.
nunuh nope not even close.
however the rcbs-150-fngc with the much longer bearing surface was the boolit the rifle wanted.
the dimensions are identical except for the length of the nose,and bearing area.

[StarMetal]

Bearing length means the boolit is supported better in the barrel. The tipping point is when the barrels twist isn't right for the length. I don't think LBTs design have any magic qualities. I think you found the happy confluence of a boolit that fits (is supported) in a twist that handles the length.

Shorter bearing length can work but IME boolit runout is critical and case fit in the chamber is too (tight necks).

I'll have to say I'm with Bret on this one. I have shot some bullets that practically have no bearing length and were very accurate...to boot at high velocity. I've also found bullets with really long bearing length to be very accurate also.

You just hit the right combination for your barrel.

[Marlin Junky]

What I should do is shoot the same boolit through my M70 4-groove .30-'06 (1:10 twist) that has shallower rifling to see if I can surpass its accuracy limit of about 2000 fps. My feeling is the long bearing surface between the ogive and first groove is more able to withstand rotational forces better than a bore rider with a short leading band. I'll see if I can dig up some old '06 records.

MJ

[Larry Gibson]

MJ

I add another viewpoint; The longer the bearing surface (ncluding a tight fitting bore riding nose in the actual bore) the more support the bullet has during accelleration. The more support the less obturation, tipping, uneven setback, or whatever you want to call it that the bullet will sustain. The longer the bearing surface the better the alignment of the bullet especially if the fit is correct to the throat and the bearing surface fills the throat to the leade. The better the sustained alignment to the bore and the less damage to the bullet during accelleration the more accurate it will be.

If the bullet fits the throat to the leade and is of sufficient tensile strength to not deform (ergo; if the deformaty is uneven the bullet becomes unbalanced). This is why we use strong (harder) alloys as we increase velocity (increasing velocity is increasing the initial inertial forces that cause the bullet to deform during accelleration. Using a ductile alloy is also correct. To hard and alloy with too high an antimony content can cause chipping if the driving bands by the lands. These of course can fly off with the lube when the bullet is in flight. This obviously unbalances the bullet and inaccuracy is the result. I Bass's thread on bullets stripping he touches on another searious area of bullet deformation at higher velocity/accelleration. The longer the bearing surface the more resistance to stripping a given alloy will have.

There are a few other reasons a longer bearing surface is better, especially at higher velocity/accelleration but I've touched on the important ones.

The higher the RPM the more inaccurate an unbalanced bullet will be. A long bearing surface is the first step to mitigating that problem. Your example of the 180 gr bullet in your 10" twist Marlin at 2300 fps is a very good example. If one pays attention to what I said is necessary to push the RPM threshold a long bearing surface on the bullet is one of the first requirements. You have pushed the RPM threshold with that bullet in your rifle. Good on you:-)

Larry Gibson

[Marlin Junky]

Larry,

I agree with you on internal ballistics (e.g., over stressing the boolit as it traverses the barrel); however, once the boolit leaves the barrel I don't believe you can over-stabilize it with too much RPM. In other words, if the boolit leaves the barrel in beautiful shape, it doesn't matter if it's doing 250,000 or 1,000,000 RPM. I could be wrong, but I haven't seen evidence to the contrary. The only thing that'll change my mind at this point is an in depth study incorporating high speed photography.

Regards,
MJ

[lead Foot]

The only thing that'll change my mind at this point is an in depth study incorporating high speed photography.

Very interesting points from Larry and Marlin Junky. These new high speed video cameras is an amazing invention and would concur and deniy a lot of theory. When traveling my boolits 2000fps + I a long bearing works best for me.
Lead foot;

[StarMetal]

Larry,

I agree with you on internal ballistics (e.g., over stressing the boolit as it traverses the barrel); however, once the boolit leaves the barrel I don't believe you can over-stabilize it with too much RPM. In other words, if the boolit leaves the barrel in beautiful shape, it doesn't matter if it's doing 250,000 or 1,000,000 RPM. I could be wrong, but I haven't seen evidence to the contrary. The only thing that'll change my mind at this point is an in depth study incorporating high speed photography.

Regards,
MJ

Bullets will fly two ways, nose down, nose up. Over stabilization picks the worse of those two. As much as everyone know's I'm the RPM bandit there is a point of two much just like there is a point of not enough.

I' have to ask, I've reached nearly 2900 fps with the air cooled 6.5 Kurtz in my Swede without stripping, without leading, and staying on the targer paper, not the larger backer board. In other words it stayed on my printing paper target. That was with 50/50 air cooled alloy. I ask what is the dang limit of that alloy. Appparently 28-2900 fps isn't. I've also pushed the Saecon 140 grain 6.5 bullet which has little bearing band surface to speak of to high velocity in shallow button groove rifling with no stripping, leading, and in this bullet's case...with accuracy. Now I will have to say the bore on the rifle is dead on .256 and the bullet nose is .256 and it doesn't obturate the nose, nor slump it any into the grooves because I've recovered more then one fairly undamaged bullet to see that it doesn't. Now those bullets do shorten some from compression buy the powder pressure but only to 1/32 past the front bearing ban which is the base or beginning of the bore rider nose. So I'm saying what Larry just explained about a tight fitting bore rider doen't hold true to that rifle. Unless he's calling my dimensions tight for that bullet, but there's no rifling marks on the nose to speak of except for what I've noted.

[303Guy]

StarMetal, I would say that you have confirmed what Larry said. Your boolit compressed evenly. If the nose was loose or too short, it might have slumped to one side. (That's what I call 'slumping' - when it slumps to one side). On your question,
I ask what is the dang limit of that alloy. That's something I would like to understand. That's because I insist on using 1 in 10 twist rifles - all I've got. What Marlin Junky posted is therefore very relevent to me. I would like to drive a lighter boolit faster in one of my Lee Enfields (I need a specific use for it[smilie=1: ). I cannot shorten the boolit but I can make the forward section very hollow. Theoretically, that will move the centre of mass toward the outer perifery, thus increasing its stability. But, will it deform in flight due to lack of a core to hold it together? Only one way to find out what the limits are! But then, with such a hollow, I could comfortably use a stronger alloy - it'll be a varmint boolit anyway.

[Marlin Junky]

But, will it deform in flight due to lack of a core to hold it together? Only one way to find out what the limits are! But then, with such a hollow, I could comfortably use a stronger alloy - it'll be a varmint boolit anyway.

The only way it'll deform in flight is if the pressure differential between the cavity and outside the boolit exceed the shear strength of the alloy. But I guess everyone already knows that.

MJ

[AZ-Stew]

Just to throw a little fuel on the fire, I have a .30 Loverin design (311466) that's almost ALL bearing surface. There's no "bore rider" nose, but the multiple driving bands make the rest of the boolit essentially bearing surface. Being very short, the nose that does exist is too short to "slump". Not enough mass.

In my Rem 788 .30-30, it does not shoot as well as the RCBS .30-150FN.

How does that fit into your discussion?



in post #7

If the bullet fits the throat to the leade and is of sufficient tensile strength to not deform (ergo; if the deofaty is uneven the bullet becomes unbalanced). Larry Gibson

Larry,

Not ragging on you, but I want to understand your point. I suspect "deofaty" is a misspelling or mistyping of a word, but for the life of me, I can't figure out what it is. Please fill me in. Maybe spell checker (ABC checkmark) at the top right of the response window will help.

Regards,

Stew

[Larry Gibson]

AX-stew

That bullet has a very long bearing surface for sure. But if most of the bullet is in the case with the base of the GC below the neck/shoulder junction then it's going to have uneven obturation befor it gets into the throat much less into the bore proper. Ideally you case necks should have minimal clearence in the chamber neck for expansion. The chamber throat should be be long enough so the bullet is also a slip fit with .0005 - .001 clearence and the front dirving band should just kiss the leade. Preferably the bullet should be seated to max mag length also if possible to get as much bullet into the throat which should match this length. Then the alloy should be just strong enough to with stand the velocity/accelleration achieved with no or very minimal obturation. If you are driving both bullets cast of the same alloy at the same accelleration rate then apparently the RCBS bullet better fits this criteria in your M788 than does the Lovern bullet.

BTW; "deofaty" is certainly a misspelling that slipped right on by me. It is supposed to be "deformed". Thanks for catching that, I have edited my post and corrected it.

Larry Gibson

[Marlin Junky]

Just to throw a little fuel on the fire, I have a .30 Loverin design (311466) that's almost ALL bearing surface. There's no "bore rider" nose, but the multiple driving bands make the rest of the boolit essentially bearing surface. Being very short, the nose that does exist is too short to "slump". Not enough mass.

In my Rem 788 .30-30, it does not shoot as well as the RCBS .30-150FN.

How does that fit into your discussion?

I'm not sure, but I think we're missing something. I wanted to stress (you'll have to re-read my opening post) that the long groove diameter section of an LBT LFN that is before the lube grooves (I don't have one handy, or I'd post a pic) is a heck of a lot stronger than any leading band on typical boolit designs. I think the 30-180LFN has a groove diameter section before the lube grooves > .2" long. Most Loverin driving bands are < 1/16 long and I would suspect are more prone to deformation, or stripping, or whatever we want to call it. Anyway, there was more to the context of my original post than just bearing surface. I should have called it something like "Groove Diameter Initial Engraving Interface Surface", or something... sometimes this stuff makes me crazy. [smilie=f:

MJ

[Larry Gibson]

Larry,

I agree with you on internal ballistics (e.g., over stressing the boolit as it traverses the barrel); however, once the boolit leaves the barrel I don't believe you can over-stabilize it with too much RPM. In other words, if the boolit leaves the barrel in beautiful shape, it doesn't matter if it's doing 250,000 or 1,000,000 RPM. I could be wrong, but I haven't seen evidence to the contrary. The only thing that'll change my mind at this point is an in depth study incorporating high speed photography.

Regards,
MJ

MJ

You don't have to take my word for it but it is very possible to over stabilize any bullet. Just read bonafide information written on ballistics by ballisticions not internet opinions. Even J bullets can be over stabilized. The better the quality of the bullet, i.e. concentric and balanced the less the high RPM or overstabilization will affect it accuracy wise. This is why we see some very good accuracy with high quality bullets out of fast twists. Both Sierra and Hornady have very good sections in their manuals or on the web sites explaing it. The higher the RPM per a given velocity the less accurate a bullet will be. Now some here will probably want to tell of 1/2" groups shot with their favorite 8" twist rifle. That is very good no doubt about it. However, ponder me this; how many bench rest shooters are shooting fast twist barreled guns? The answer is simple; none. They want the bullet perfectly stabilized not over or under stabilized.

Now the very same thing applies to cast bullets. The more perfectly balanced and concentric we can keep the cast bullet during accelleration the more accurate it will be at any velocity but particularly at high velocity and high RPM.

I do not have a high speed camera and even if I did a still shot would show little about stability. It would have to be a video taken of a distance. I can however measure BCs. A measured BC is a measurement of how efficiently the bullet flies through the air. The higher the BC the more stable a given bullet is. I have run this test several times. Using the same .308W loaded ammo in 3 different twist rifles and measuring the BCs the 12" twist gave the highest at .525. The 14" twist gave .489 and the 10" twist gave .484. Guess which one shot best at 300+ yards. The same test with 190 gr MKs had the 10" twist giveing the highest BC and also shooting the best at 300+ yards. Then the same test with 155 gr Palma MKs the 14" twist gives the highest BC and the best accuracy at 300+ yards.

Not enough then let's switch to the .223. The same test with 55 gr BT match bullets give the highest BC to 12" twists. Using FB'd match bullet and the highest goes with the 14" twist barrels. Switch to 68 - 69 gr match bullets and the 9" barrel gives the highest BC. Go to the *) gr match bullets and the 8" barrel gets the nod. Use regular Hornady, Speer, Nosler or Sierra bullets and it is always the same; a particular twist will give the highest BC, ergo the best stabilized bullet, with a given load when tested in 7, 8, 9, 12 and 14" twist rifles (yes I have all those twists .223s).

It is indeed very possible to over or under stabilize a bullet. It will have some effect on accuracy. How much depends on the quality of the barrel, the load and the quality of the bullet. That fast steppin' 40 gr bullet in a 7" twist .223 that I'm sure we'll hear about would probably shoot much more accurately with a 72 -84 gr bullet. How do we know? Well, just how many 600 yard matches are shot with fast steppin 40 gr bullets? How many 300 yard matches? Okay, how many 200 yard matches? Your answer is why. that fast steppin 40 gr bullet will not hold accuracy as the over stabilization leads to non linear dispersion and increased wind drift. In other words it just ain't accurate beyond 100 yards as a bullet that is adequately stabilized by that 7" twist.

Starmetal wants to say I am wrong about RPM because he can hold an apparent 8.5x11" group at 2800 fps. Well ok, if that is "accurate" to him that's fine. To me that sucks, I don't care what the velocity is. I'll also be the 200 yard accuracy is non linear also. If he backs off to 2300 fps his accuracy will be much better. Better still if he backs off to 1800 2200 fps his accuracy will be very good and the linear dispersion of the 200 yard group will be in line with the 100 yard group. In other words he will have adequately stabilized bullets to maintain accuracy at longer ranges.

I know none of this will change your mind, hasn't before and I don't expect it to now. I'll just ask that you conduct a simple test. Take that Marlin with the 10" twist and that 2300 fps load. Shoot a 10 shot group at 50 yards. The shoot a 10 shot group at 100 yards. It should be twice the size of the 50 yard group in linear correct. I'll bet it won't be. Then shoot a 10 shot group at 200 yards. It should be twice the size of the 100 yard group if linear, right? I'll be it will be considerably larger. Why? Simply because you've exceeded the RPM threshold. There are some bullets that you can shoot well at 200 yards aat 2300 fps but I don't think that RCBS bullet is one of them. Go ahead and try it and learn for yourself.

Larry Gibson

[Larry Gibson]

MJ

Additionally, if you look at a picture chart of Lyman moulds you will see numerous very streamlined point mould designs that should have high BC and shoot very well at long range. Seveal come to mind 287377, 311329, 311365, 311413 and 323366. The problem is; they don't shoot well at all at anything approaching regular velocity. All of them shoot ok in the 1500 - 1800 fps range. The reason is; a lack of bearing surface as mentioned in other posts.

Larry Gibson

[StarMetal]

Larry,

Why you say all that about me. In the post way ahead of yours I said that over stabilization, or simply put, put too much spin isn't the best.

Now to put things in context I used the 28-2900 fps thing to show that I hadn't reached the stress limit on the 50/50 aircooled 6.5 Kurtz. If I had it wouldn't have stayed on the target paper. You try to tell everyone I use that as an accuracy statement. Okay, Okay, so what about the very very small groups with the ALL my 6.5's and my 7mm-08 all at high velocity? You don't tell them about that. You only try to make me look bad by taking something I state in another context and say "Look what Joe is calling accuracy". There's no dealing with you is there?

Long ago you came up with your RPM Threshold theory. You said that is where one would get the accuracy. I've proved that wrong, that you could get accuracy at HV, with so many rifles now and loads I've lost count.

Please quit taking selected statements of mine out of context.

[303Guy]

The only way it'll deform in flight is if the pressure differential between the cavity and outside the boolit exceed the shear strength of the alloy.I was thinking of centrifugal distortion, not air pressure differential. But that's a good point! The two forces might combine to cause distortion and loss of accuracy. There is also air friction heating of the boolit which would weaken it.

[StarMetal]

I'm still running what Larry said around in my head. We were talking about bearing length having to do with accuracy. I think that when you exceed the stress limit of your alloy and it's bearing length, which more bearing length should raise the stress limit of the alloy, then accuracy will disappear. Now with that said that apparently I hadn't reached the stress limit of the Kurtz with AC 50/50 because they were still staying on the 8x10 printer paper. What I didn't say is what size they printing. They were printing 6-7 inches. Remember now these were going 28-2900 fps. To be more specific they were averaging 2879 fps. Now below is an excerpt from Larry Gibson's last 6.5 Swede test:

I’m going to save you all the pain of the gruesome details. I fired the first 6 loads of 41.9 gr through the “dimes thickness” load of 43.4 gr of 3100 and then quit the test. This load was totally erratic and extremely inaccurate. To give you an idea I’ll list the data of the first test group and the last one.

41.9 gr
2536 fps/35/88/50,000 psi

43.4 gr
2531 fps/37/83/51,100 psi

That’s a 2.5 gr in crease in powder with no increase in velocity but a definite increase in pressure. The additional loads in between were just as erratic.

Accuracy;

What accuracy? I was using a target frame 24” wide and 36” tall. Of the 30 test shots fired at it only 1 shot hit the target frame. From the bullet impacts all around the target frame it appeared the accuracy was around 4 - 6 foot groups.


How come my 2879 fps printed 6-7 inches?? [smilie=s:[smilie=1:

[Marlin Junky]

MJ

You don't have to take my word for it but it is very possible to over stabilize any bullet. Just read bonafide information written on ballistics by ballisticions not internet opinions. Even J bullets can be over stabilized. The better the quality of the bullet, i.e. concentric and balanced the less the high RPM or overstabilization will affect it accuracy wise. This is why we see some very good accuracy with high quality bullets out of fast twists. Both Sierra and Hornady have very good sections in their manuals or on the web sites explaing it. The higher the RPM per a given velocity the less accurate a bullet will be. Now some here will probably want to tell of 1/2" groups shot with their favorite 8" twist rifle. That is very good no doubt about it. However, ponder me this; how many bench rest shooters are shooting fast twist barreled guns? The answer is simple; none. They want the bullet perfectly stabilized not over or under stabilized.

Now the very same thing applies to cast bullets. The more perfectly balanced and concentric we can keep the cast bullet during accelleration the more accurate it will be at any velocity but particularly at high velocity and high RPM.

I do not have a high speed camera and even if I did a still shot would show little about stability.

I'm looking for boolit surface deformation right after it leaves the muzzle to see how twist correlates to deformation.


I can however measure BCs. A measured BC is a measurement of how efficiently the bullet flies through the air.

That's true, but it doesn't prove your next statement:


The higher the BC the more stable a given bullet is.

No, BC is a drag coefficient, it is not directly related to stability; i.e., shoot a VLD jacketed bullet though a very slow twist at a very slow velocity and it will tumble soon after leaving the muzzle.


I have run this test several times. Using the same .308W loaded ammo in 3 different twist rifles and measuring the BCs the 12" twist gave the highest at .525. The 14" twist gave .489 and the 10" twist gave .484.

Do you actually believe that BC is dependent on twist when a 14% change in twist is responsible for only a 1% change in BC? How do you know the change in BC wasn't due to a slight change in average velocity resulting from a change in atmospheric conditions during your test? What were the respective muzzle velocities from the 12, 14 and 10 inch twists? Perhaps the muzzle velocities varied based on other internal barrel conditions (e.g., smoothness, internal dimensions, etc.) Perhaps the faster twist consumes a larger portion of the bullet's kenetic energy? Perhaps the faster twist increases pressure and also velocity? If BC is directly related to twist, why did the bullet shot from the 12" have a higher BC than the bullets shot from both the 14" and 10" twists? Larry, you just can't pick and choose your "facts" to suit your theory. Please, we don't need any more Al Gores in the world. :lol:


Guess which one shot best at 300+ yards. The same test with 190 gr MKs had the 10" twist giveing the highest BC and also shooting the best at 300+ yards. Then the same test with 155 gr Palma MKs the 14" twist gives the highest BC and the best accuracy at 300+ yards.

Not enough then let's switch to the .223. The same test with 55 gr BT match bullets give the highest BC to 12" twists. Using FB'd match bullet and the highest goes with the 14" twist barrels. Switch to 68 - 69 gr match bullets and the 9" barrel gives the highest BC. Go to the *) gr match bullets and the 8" barrel gets the nod. Use regular Hornady, Speer, Nosler or Sierra bullets and it is always the same; a particular twist will give the highest BC, ergo the best stabilized bullet, with a given load when tested in 7, 8, 9, 12 and 14" twist rifles (yes I have all those twists .223s).

It is indeed very possible to over or under stabilize a bullet. It will have some effect on accuracy. How much depends on the quality of the barrel, the load and the quality of the bullet. That fast steppin' 40 gr bullet in a 7" twist .223 that I'm sure we'll hear about would probably shoot much more accurately with a 72 -84 gr bullet. How do we know? Well, just how many 600 yard matches are shot with fast steppin 40 gr bullets? How many 300 yard matches? Okay, how many 200 yard matches? Your answer is why. that fast steppin 40 gr bullet will not hold accuracy as the over stabilization leads to non linear dispersion and increased wind drift. In other words it just ain't accurate beyond 100 yards as a bullet that is adequately stabilized by that 7" twist.

Starmetal wants to say I am wrong about RPM because he can hold an apparent 8.5x11" group at 2800 fps. Well ok, if that is "accurate" to him that's fine. To me that sucks, I don't care what the velocity is. I'll also be the 200 yard accuracy is non linear also. If he backs off to 2300 fps his accuracy will be much better. Better still if he backs off to 1800 2200 fps his accuracy will be very good and the linear dispersion of the 200 yard group will be in line with the 100 yard group. In other words he will have adequately stabilized bullets to maintain accuracy at longer ranges.

I know none of this will change your mind, hasn't before and I don't expect it to now. I'll just ask that you conduct a simple test. Take that Marlin with the 10" twist and that 2300 fps load. Shoot a 10 shot group at 50 yards. The shoot a 10 shot group at 100 yards. It should be twice the size of the 50 yard group in linear correct. I'll bet it won't be. Then shoot a 10 shot group at 200 yards. It should be twice the size of the 100 yard group if linear, right? I'll be it will be considerably larger. Why? Simply because you've exceeded the RPM threshold.

I'd like to continue with this but it has already consumed too much of my time and no one has even touched on my idea that the LBT LFN design is good for HV work. Perhaps I need to call Veral and talk to him. Also, your next statement indicates you're not even paying attention to the subject boolit that I was trying to develop a discussion around.


There are some bullets that you can shoot well at 200 yards aat 2300 fps but I don't think that RCBS bullet is one of them. Go ahead and try it and learn for yourself.

Larry Gibson

[Larry Gibson]

Larry,

Why you say all that about me. In the post way ahead of yours I said that over stabilization, or simply put, put too much spin isn't the best.

Now to put things in context I used the 28-2900 fps thing to show that I hadn't reached the stress limit on the 50/50 aircooled 6.5 Kurtz. If I had it wouldn't have stayed on the target paper. You try to tell everyone I use that as an accuracy statement. Okay, Okay, so what about the very very small groups with the ALL my 6.5's and my 7mm-08 all at high velocity? You don't tell them about that. You only try to make me look bad by taking something I state in another context and say "Look what Joe is calling accuracy". There's no dealing with you is there?

Long ago you came up with your RPM Threshold theory. You said that is where one would get the accuracy. I've proved that wrong, that you could get accuracy at HV, with so many rifles now and loads I've lost count.

Please quit taking selected statements of mine out of context.

Joe

You are being way overly sensitve here. There was no criticism of you. I used your example to make a point the same way you used Bass's. Nothing is quoted out of context. What I said about the accuracy of your loads holds true; slow ithe velocity and RPM down and the accuracy will improve. That's a simple enough fact. Quit taking everything I say about this as a personal criticism, it is not.

BTW; I'm getting close, I think. I'll post my latest test results on the 6.5 thread later today.

Larry Gibson

[Larry Gibson]

MJ

You might want to go to Sierra's web site and read their take on BCs and stability. My apologies for wasting your time.

Larry Gibson

[Greg in Malad]

http://i818.photobucket.com/albums/zz110/greginmalad/100_0518.jpg

Marlin Junky,
Here is a picture of the bullets I sent you. I think the reason they shoot so well is a simple case of alignment. Bullets that fits snug in the throat are in alignment with the bore, if they travel down the bore in perfect alignment they exit the muzzle straight.

[StarMetal]

Joe

You are being way overly sensitve here. There was no criticism of you. I used your example to make a point the same way you used Bass's. Nothing is quoted out of context. What I said about the accuracy of your loads holds true; slow ithe velocity and RPM down and the accuracy will improve. That's a simple enough fact. Quit taking everything I say about this as a personal criticism, it is not.

BTW; I'm getting close, I think. I'll post my latest test results on the 6.5 thread later today.

Larry Gibson

Larry you are wrong about your reply. The load that was doing 2879 fps was used as an example that I hadn't reached the stress limit of the 50/50 air cooled alloy yet in my Swede. I was not an example of accuracy which you stated that way. Simple as that. I believe I can answer my own question as to why that load at least stayed on the paper, 6-7 inch group none the less and yours went off the whole target frame at much less velocity is testimonial to 45 2.1's excellent casting ability. The bullets he cast me must be pretty dang near perfect, perfect being nonexistent.

Not over sensitive. You noticed there was no harshness and no name calling what so ever.

As I said you can spin a bullet TOO fast. It is easier to obtain good accuracy with slower twist barrels, but it also possible to obtain good accuracy with fast twist barrels at HV.

I think on your last test where you did get a 1 inch group I believe, that I would consider that getting in the ballpark of good accuracy.

[Bass Ackward]

What makes this complex is generally ....us. WE focus on details when we really need to look more at the big picture. And the big picture doesn't mean applying jacketed theory to cast.

Cast shooters tend look at stabilization in two factors. Length vs twist rate and velocity. Since lead is a limited strength material, over stabilization is almost NEVER achieved with lead because launch quality is affected and muzzle stripping begins long before that happens.

The biggie for cast is center of balance. Take equal weight bullets. The slower you want a bullet to perform, the more weight you want on the back to make it easier to stabilize. The penalty you pay is that it takes away bearing length needed to maintain bore center at anything but low pressure and velocity levels. At the very least the bullet will become MUCH more lube, fouling, and load sensitive as velocity increases. This results in failed launch characteristics that RPMs won't be high enough to correct.

If you want to go faster, the more forward you push the weight. LBT does this with those heavy forward drive bands and short, unsupported nose lengths, thus establishing C of B forward. It also strengthens the slug to deal with bore fouling and maintain bore center. Of coarse the limitation there is you will need a faster twist to stabilize this at slower velocity levels and it probably will be much more load and lube sensitive down low.

Coarse, regardless of design, when stripping begins to occur on the back of the slug as it exits the muzzle, that is your max velocity level with that slug hardness, powder combination. PB goes first. When it does, RPMs fail to increase with velocity which is different than jacketed theory.

[Greg in Malad]

Bass Ackward,
The LBT bullets shoot very well at low speed(1400 fps) for me. The biggest drawback to Veral's design is bullet weight is limited by throat length.

[felix]

Well done, John! ... felix

[303Guy]

Theoretically, we need more twist rate for slower boolits, specially at transonic velocities. Driving a boolit faster does not increase its stability because at higher velocity more spin is needed. That changes when one starts going to hyper-velocity - above 3000 plus fps. A lead boolit with its short length for weight is more stable than a jacketed and the fatter and shorter the nose the more stable still. The intensity of the muzzle blast is a factor to consider. It could be that at lower powder charges and hence velocity, the muzzle blast is more intense due to late peak pressure developement. Increase the powder charge and hence velocity and the muzzle blast intensity decreases while the spin increases. With an earlier peak pressure development, the boolit is obturated in the bore earlier too, giving it better bore alignment. (By obturated I don't nesessarily mean 'upset' or plastic deformation).

Another thing about cast boolits are the lube grooves at the rear of the boolit - this moves the centre of balance forward.

Also, a short, fat round nose with a hollow nose should be more stable still as there is less mass at the nose for small irregularities to 'wobble' the nose in flight. That's my theory, anyway.

[felix]

303Guy, not theory, but factual. But, keep in mind the more pointed a projectile is (or lube groove proximity), the less twist is required. Conversely, the more pointed the base (or lube groove proximity), the more twist is required. ... felix

[Larry Gibson]

Larry you are wrong about your reply. The load that was doing 2879 fps was used as an example that I hadn't reached the stress limit of the 50/50 air cooled alloy yet in my Swede. I was not an example of accuracy which you stated that way. Simple as that. Yes Joe, it is as simple as that. You used it for your reasons and I used your level of accuracy example for mine. Two different reasons. Get the picuture? If so then quit arguing with yourself. It is as simple as that.I believe I can answer my own question as to why that load at least stayed on the paper, 6-7 inch group none the less and yours went off the whole target frame at much less velocity is testimonial to 45 2.1's excellent casting ability. The bullets he cast me must be pretty dang near perfect, perfect being nonexistent.

Not over sensitive. You noticed there was no harshness and no name calling what so ever.

As I said you can spin a bullet TOO fast. It is easier to obtain good accuracy with slower twist barrels, You think so?but it also possible to obtain good accuracy with fast twist barrels at HV.

I think on your last test where you did get a 1 inch group I believe, that I would consider that getting in the ballpark of good accuracy. Go to my 6.5 thread and you'll see where I'm at. I'm done on this thread Joe. There is no argument with you here from me anyways. I will not waste MJ's time anymore. Catch me on the 6.5 thread.

Larry Gibson

[Marlin Junky]

http://i818.photobucket.com/albums/zz110/greginmalad/100_0518.jpg

Marlin Junky,
Here is a picture of the bullets I sent you. I think the reason they shoot so well is a simple case of alignment. Bullets that fits snug in the throat are in alignment with the bore, if they travel down the bore in perfect alignment they exit the muzzle straight.

That's true Greg; however, I have other boolit designs that group very well, just not at 2300-2400 fps from relatively fast twist rifles. At this point I really don't have enough data to draw any conclusions, I just know these boolits surprised me in terms of how well they shot (at over 2300 fps) with so little lube and I think the long (strong) groove diameter front section has something to do with it. I need to put a few of these through my '06... more on this later. In the mean time, I should probably order this mold from Veral. Greg, do you own the 170 grain LBT LFN as well?

Thanks again Greg,
MJ

P.S. I want to emphasize that this boolit holds only .4 grain of lube... those are very shallow grooves.

[Marlin Junky]

The biggest drawback to Veral's design is bullet weight is limited by throat length.

That's the same darn problem those new fangled copper patched boolits have! You should sell that silly mold to me after all Greg. :smile:

MJ

[Marlin Junky]

MJ

You might want to go to Sierra's web site and read their take on BCs and stability. My apologies for wasting your time.

Larry Gibson

I'm sorry Larry but I really don't care about BC right now. I knew that BC was an aerodynamic quantifier when I was 15 and if we really want to make progress on this RPM thing, everything else needs to remain constant. Besides, for now I'm only interested in my boolits leaving the muzzle in good condition.

Thanks for trying just the same,
MJ

[Bass Ackward]

Bass Ackward,
The LBT bullets shoot very well at low speed(1400 fps) for me. The biggest drawback to Veral's design is bullet weight is limited by throat length.

Greg,

You sorta miss the fact that 1400 fps is in the 30 caliber zone, especially with PB. And out to 100 it's really the launch. So 1400 isn't outta whack.

But shoot that 1400fps out to 300 or 400 yards where velocity drops and it will eventually destabilize before a bore ride would. That's the key to focus on.

The same goes with high velocity where base stripping begins. You might do well at 100 and then begin to destabilize before you reach your next range goal. People often see this as over RPM when it is really under RPM for the bullet balance point. As the base strips, the B of C moves forward requiring either velocity to remain constant which it won't or a higher twist rate much as Felix pointed out.

[Marlin Junky]

Greg,

You sorta miss the fact that 1400 fps is in the 30 caliber zone, especially with PB. And out to 100 it's really the launch. So 1400 isn't outta whack.

But shoot that 1400fps out to 300 or 400 yards where velocity drops and it will eventually destabilize before a bore ride would. That's the key to focus on.

So spin it up a little more to compensate for a more forward CG, right?


The same goes with high velocity where base stripping begins.

Wouldn't we need to be as, or more, concerned about leading edge stripping at HV, Bass?


You might do well at 100 and then begin to destabilize before you reach your next range goal. People often see this as over RPM when it is really under RPM for the bullet balance point. As the base strips, the B of C...
B of C is the same as CG (center of gravity), right?
...moves forward requiring either velocity to remain constant which it won't or a higher twist rate much as Felix pointed out.

[Bass Ackward]

1. So spin it up a little more to compensate for a more forward CG, right?

2. Wouldn't we need to be as, or more, concerned about leading edge stripping at HV, Bass?


3. B of C is the same as CG (center of gravity), right?

1. Yea, spinning it up so that the bullet remains above the destabilizing point all the way to the target what ever that distance is. But sometimes it can be more accurate to start a slug off slower and have the design carry it stable down range. Depends on the gun and the purpose of the load.

2. If you were working "up" a load as defined as starting low and coming up, you would see stripping of the base long before leading edge problems assuming your lube was good and your hardness was at or above what was required to prevent fouling. On The other hand, if someone simply wants to pick a load and try that, then I suppose that could happen first. Remember, the most prevalent form of gas leakage occurs because of fouling. If one rifling's contribution is compromised, the remaining ones in contact must perform the task.

The more fouling, the higher the odds for failure, so it is easy to understand that faster velocity will develop more fouling problems. So will the requirement for more shots. (heat) Someone requiring 10 shots to validate a load, for what ever reason, is clearly at a disadvantage also.


3. Sorry, should have read C of B. Center of gravity can be difficult for people to see or imagine. So when it comes to shooting, the easier we can make it,the more people can benefit, so it is however it is easier for you to grasp the point.

C of B allows the human eye to find the length center of the bullet. And once that is done the eye can reasonably determine which half is heavier fairly easily if it is apparent. (semiwadcutter) Or the bullet can be placed on a sharp edge if a person's eye can't perform the task. The EXACT location is only of use to a designer or to a person that wants to work the velocity extremes which is defined as out of "the zone". Otherwise he must rely on historical reputation, luck, or observe failure and thus declare it a "poor" design. (for his purposes)

[Marlin Junky]

If you were working "up" a load as defined as starting low and coming up, you would see stripping of the base long before leading edge problems...

Does that base stripping happen at the muzzle?

MJ

[AZ-Stew]

Having stepped in it, and preferring clean shoes, I shall now step out of it.

Regards,

Stew

[GabbyM]

http://i818.photobucket.com/albums/zz110/greginmalad/100_0518.jpg

Marlin Junky,
Here is a picture of the bullets I sent you. I think the reason they shoot so well is a simple case of alignment. Bullets that fits snug in the throat are in alignment with the bore, if they travel down the bore in perfect alignment they exit the muzzle straight.

That bullet looks like a Loverin missing a few lube groves.

[Greg in Malad]

GabbyM,
It works like a Loverin design with a much stronger front driving band.

MJ,
I don't have the 170-LFN mold. Veral's 170-LFN lever action mold has a longer slimmer nose and a crimp groove. My other LBT mold is a .360-190-LFN that I had built for my M77 .350 Rem.
I have been avoiding the RPM debate, but. I gave some Lyman 225415's to a friend to try in his 22-250. He brought back a target that had 6 holes 7/8" apart, fired from 100 yds. Muzzle velocity was around 3400 fps. would anybody like to tell me what the RPM was[smilie=1:
Dont get your feathers ruffled guys, I'm not trying a stir the pot.

[StarMetal]

174,857 rpm if the twist is 14 and 204,000 if the twist is 12.

[Bass Ackward]

Does that base stripping happen at the muzzle?

MJ


Yes. When it starts, it is directly under the check. Then as velocity increases it moves further and further up the bullet. Picture a bullet exiting from the muzzle in your mind and see how less and less of it must continue to rotate the slug and hold it forward as it exits. Picture a PB in the same situation. Would you imagine that a PB would have a lower limit than a checked slug? And you could put plastic wads that would protect the base from pressure and still it doesn't raise velocity limits appreciably. Muzzle stripping.

This is the driving idea to fast powders and cast so that acceleration is mostly completed before bullet exit. Then there is little force, but you have to run slower powders to get the velocity without the pressure, so for higher velocity the slug is still accelerating upon exit unless your barrel is sufficiently long with "long" defined by caliber and bore diameter of coarse. With the slower powder comes more heat too.

Don't let exceptions confuse you. There are no rules, so you WILL see accurate PB above 1600 fps periodically or a bore ride at 2800. But everything has to be perfect.

[303Guy]

Bass Ackward, I would never have thought muzzle stripping was possible! Thanks for the enlightenment. Might I ask how you came to learn of this phenomenon?

P.S. I enjoy reading your posts - you have amazing insight. StarMetal and Larry Gibson are mine fields of information too! (Howcome no-one picks me out for saying 'mine fields' instead of 'mines'?:mrgreen:) I haven't forgotten Felix - he's a very clever man. So's Molly. :drinks:

[NHlever]

Well, a lot of very scientific, and high tech info here! The Marlin barrels that I have worked with tend to vary end to end in bore dimensions. I've had better luck with long bearing surface boolits, and bullets too, but I just suspected that they "absorbed" those bore variations better than short boolits. I had one that was so bad that short bullets would tumble at 25 yards while the Hornady 170's would shoot under 1 1/2" at 100 yards. My cast 311041's would stay well under 2". Simple me.