Bore rider boolits 101 needed

[runfiverun]

glad you come back with the second post.
that one [and a couple ahead of it] explains why the rpm threshhold can be overcome.
and why a bore riding nose won't go much past it.

you have to think in terms of up and down from the bottom of the chamber if you use a fatter boolit how does it really help?
if you use thicker brass?
if you change the shape of the boolit from paralell to a sloping shape in the throat area and size to scuff the area ahead of the chamber.

look at tom's picture.
now look at all the white surrounding the boolit and in front of the case.
now make the area around the neck bigger,and add some more white in front of the chamber.
that's more representative of what you are really dealing with.
see how the shape of the boolit would change?
it would end up looking more like youngda9's boolit.

now the whole round is stuck hanging off the nose of the boolit with all that white showing around everything.
worse yet it's holding the case on an angle too, all the way back to the bolt face,so that when the round is fired it's shooting it's gas charge [and the base of the boolit] at the roof of the throat.
need i go on?

oh.. this thread is gonna get linked to my 'something that may help' sticky too.
this will make the 4th or 5th thread dealing with throats and alignment tacked on the back of it.

[Larry Gibson]

Of course some of the RPM effects can be overcome by bullet design/bearing length, hardness, and where and when peak pressure is applied and how long sustained. Obturation and deformation are not the same thing, anybody can have a bullet obturate the bore/seal it, but if that same obturated bullet deforms far enough and pushes nose to one side(which does happen no matter) then the major contributing factor to inaccuracy is rpm'S. There's a fellow here who has been all over this like white on rice for many many years. Facts are facts no matter how nyou try to twist(pun intended) it.

Very well put.

Centrifugal force could indeed cause nose "slump" to one side as DrCaveman suggests. However it is generally conceded that the "slump is caused more by increased acceleration where there can be weak spots in the bullet (softer alloy in some locations than others or hard antimony spots from too much antimony and too little tin) or the driving bands collapse on one side. This would cause an unsupported bore riding noet to tip to one side. The former and latter can easily be seen in HV/high RPM and higher pressure loadings. The latter is easily seen with too soft alloys.

Many who push the velocity and RPM threshold up use a softer (more malleable actually) alloy that is hardened (WQ'd or HT'd) so the bullet will set back evenly during acceleration. However, as acceleration increases it overcomes the bullets elasticity and the bullet obdurates and setbacks (slumps) unevenly. This unbalances the bullet. Of course the unbalanced bullets accuracy is adversely affected and a new, higher RPM threshold has been found for that bullet/load combination. At some point, using a cast bullet of ternary alloy, if the practical velocity is not reached with the cast bullet then an RPM threshold will be found when accuracy will get poor very quickly.

It is indeed the centrifugal force acting on the imbalances of the bullet that cause the inaccuracy in flight......at the RPM threshold.

Larry Gibson

[MBTcustom]

All I know, is the best most accurate brass is the one that is still in the chamber of the rifle, that has just been fired. Every thing we do to the case (eject, tumble, size, trim, debur, reload, chamber, and fire) has the ability to detract from that perfection. The boolit comes into play also, in very similar order.
At the end of the day, I realize that the brass itself is a weakness in accuracy design, and the breech seating black powder guys actually have the best "case" scenario (pun intended).
However, I think we can get pretty close, but things need to be considered that are usually taken for granted.
Not that I am some great target shooter, but I know where the problems are.
The big question is how do you accelerate a soft boolit into a controlled condition without damaging it in the process?
Sometimes this feels like using a sledge hammer to drive a finish nail into a piece of Sheetrock and examining it with a microscope to observe any damage! LOL!

[geargnasher]

Very well put.

Centrifugal force could indeed cause nose "slump" to one side as DrCaveman suggests. Centrifugal force causes "slump"? I don't understand how that can be. However it is generally conceded that the "slump is caused more by increased acceleration where there can be weak spots in the bullet (softer alloy in some locations than others or hard antimony spots from too much antimony and too little tin) or the driving bands collapse on one side. This would cause an unsupported bore riding noet to tip to one side. The former and latter can easily be seen in HV/high RPM and higher pressure loadings. ...and a few other anomalies.....The latter is easily seen with too soft alloys. Other things, like improper powder burn rate selection and some of the things Runfiverun covered are more usual culprits unless there is a major issue with large voids from poor casting technique.

Many who push the velocity and RPM threshold up use a softer (more malleable actually) alloy that is hardened (WQ'd or HT'd) so the bullet will set back evenly during acceleration. NO, that isn't why that alloy is chosen. However, as acceleration increases it overcomes the bullets elasticity and the bullet obdurates and setbacks (slumps) unevenly. First, if you launch it properly, there is no setback. Second, what would guarantee that it will slump UNEVENLY, if you force it to do so? Talk to the black powder guys about how uneven their nose slump is based on 1,000-yard targets. Perhaps something else is at play to damage the boolit's noses? This unbalances the bullet. Lots of things can "unbalance" the boolit, not a good thing. Of course the unbalanced bullets accuracy is adversely affected and a new, higher RPM threshold has been found for that bullet/load combination. Higher threshold with more unbalanced boolits? At some point, using a cast bullet of ternary alloy, if the practical velocity is not reached with the cast bullet then an RPM threshold will be found when accuracy will get poor very quickly.

It is indeed the centrifugal force acting on the imbalances of the bullet that cause the inaccuracy in flight......at the RPM threshold.

Larry Gibson

Wouldn't it be better not to get the imbalances in the first place than allow them to happen and try to mitigate the effect with slow twist?

Gear

[geargnasher]

The big question is how do you accelerate a soft boolit into a controlled condition without damaging it in the process?
Sometimes this feels like using a sledge hammer to drive a finish nail into a piece of Sheetrock and examining it with a microscope to observe any damage! LOL!

An egg in a potato cannon would be an appropriate analogy.

Gear

[Larry Gibson]

Wouldn't it be better not to get the imbalances in the first place than allow them to happen and try to mitigate the effect with slow twist?

Gear

That certainly is the trick as we have discussed many times before. However, even you find a velocity level where accuracy diminishes regardless of what you do to mitigate it. Guess what? That is the RPM threshold for that load. The higher a velocity (with a given twist) that we can mitigate any imbalances to the bullet the higher the RPM threshold.

However, at a certain point with any cartridge/cast bullet combination capable of 140,000+ RPM with a "standard" twist barrel it is much easier to use a barrel with a slower twist if practical. Such as using a 12 or 14" twist in a .223 or a 12, 13, 14 or even a 16" twist in a .308W or '06. What is "practical" simply depends on how much you really want to spend shooting cast bullets to HV with a semlence of consistent accuracy easily. Or we can instead strive to mitigate imbalances using a "standard" twist and maybe get results with a lot of frustration and other equipment expense using the "standard" twist such as a 7 or 9" for the .223 or a 10" twist for the .308W or '06 these days. Just depends on how you want to get there.........easy way or the hard way..........the easy way is with the slow twist...........

Larry Gibson

[swheeler]

Very well put.

Centrifugal force could indeed cause nose "slump" to one side as DrCaveman suggests. However it is generally conceded that the "slump is caused more by increased acceleration where there can be weak spots in the bullet (softer alloy in some locations than others or hard antimony spots from too much antimony and too little tin) or the driving bands collapse on one side. This would cause an unsupported bore riding noet to tip to one side. The former and latter can easily be seen in HV/high RPM and higher pressure loadings. The latter is easily seen with too soft alloys.

Many who push the velocity and RPM threshold up use a softer (more malleable actually) alloy that is hardened (WQ'd or HT'd) so the bullet will set back evenly during acceleration. However, as acceleration increases it overcomes the bullets elasticity and the bullet obdurates and setbacks (slumps) unevenly. This unbalances the bullet. Of course the unbalanced bullets accuracy is adversely affected and a new, higher RPM threshold has been found for that bullet/load combination. At some point, using a cast bullet of ternary alloy, if the practical velocity is not reached with the cast bullet then an RPM threshold will be found when accuracy will get poor very quickly.

It is indeed the centrifugal force acting on the imbalances of the bullet that cause the inaccuracy in flight......at the RPM threshold.

Larry Gibson

And put the least little void, imperfection, or soft spot in the bullet, your group just went TU. That's why most of these HV cast groups shown are SINGLE groups, lets see 5x5 or better yet 10x10 groups.

[DrCaveman]

Wow, good discussion guys. I have re-read the entire thread about 5 times and I catch new things each read.

The RPM threshold is interesting, and ~maybe~ I have found it with my marlin. Of course there have been many other variables explained here to which I may not be paying proper attention, and I should refine my practices a bit to help narrow things down.

At least the rpm issue involves simple math, since I know the barrel twist and have a chronograph. It seems that I may need to get a little more consistent in my alloy in order to take results from one shooting which may have shown me an 'rpm threshold' but all that will change if my next batch is a little heavier or lighter with tin or antimony %.

Hmm, lots to ponder. I think I will try the 'more appropriate powder' direction first. Rx7 worked great using GC boolits in the 30-30, I will see what's what with down loaded plain base boolits.

After I shoot up all those I will revisit lapping or beagling the mold and possibly water dropping. I suppose I could just go cast some and water drop now...hmmm...casting or loading...maybe I make a pot of coffee and do both tonite

[DrCaveman]

Ok, to heck with that thought about rx7. I want to get this thing working without gas checks, and my load notebook shows that 18.5 gr was terrible.

Guess I will try to max out a unique load. 8.5 seemed very wimpy, but maybe that's what I am stuck with. Maybe I push it a little further, though the speed of unique compared to 'fast rifle' powders seems a bit off-track. Still think 2400 may be my ticket, I shall also tinker with charges there.

Rx7 shot like a CHAMP when gas checks were applied. Maybe this says something about my process. 25.5 gr, near max, prob about 2000 fps (didn't have chrono yet), led me to remark: "best load. Use for hunting."

Good thing I took notes, sometimes they make more sense 6 months later than they did when I took them.

[btroj]

It isn't just velocity. Pressure AND pressure curve matters. I like 2400 in a 30-30 for up to about 1750 fps. If I want more I go to a slower powder. For max velocities I like to go slow as possible. For hunting I use RE15. This eases the push on the bullet.

[runfiverun]

nobody said it was easy.
if you care to take the time to make things work, you will have no more and no fewer flyers than you will with storeboughts.

i believe i have shown multiple groups every time i have shown any groups.
i think the last target i showed was 100 shots on the same target from the same day [from my 8 mauser] 5x20 shot groups using J-lube a 180 gr boolit and 32 grs of 4895 with a dacron filler/ no it was dryer lint filler.
it was showing the individual groups pretty clearly and how they shifted slightly throughout the day [due to mirage in the scope] but the entire group[s] was within 1-1/2"s.
pretty much what i expect from this rifle on a consistent basis even with jaxketed.

i don't expect a rifle to magically start shooting 1/2" groups just because i am using cast [sometimes it happens]

[DrCaveman]

Forgot I had imr 4227. A minimum load with this should put me around 1600 fps, with a relatively slow pressure curve, if I understand.

Loaded up 10@ 16.4 gr, no gas check. Lithi bee lube, boolit nose fits snug into muzzle but requires nothing more than a push...no whack or hard hand pressure needed. No wobble.

Driving bands are averaging about .310, some show 311 and even 314 at widest. Did not size these ones at all. Wish they averaged out bigger but apparently my mold cavities aren't quite equal. A good note is that they are larger than before minor lapping.

I should probably take better note of which boolit fell from which cavity, but that requires some serious discipline after the mold is up to temp and everything is moving so nice

[shredder]

"The big question is how do you accelerate a soft boolit into a controlled condition without damaging it in the process?
Sometimes this feels like using a sledge hammer to drive a finish nail into a piece of Sheetrock and examining it with a microscope to observe any damage! LOL!"

Nice! Man this discussion was getting pretty technical until this came up. Now I can relate to that.

Just kidding, I have my first real bore rider design, a lyman 311332, and am here soaking up all the great info.

[Larry Gibson]

"The big question is how do you accelerate a soft boolit into a controlled condition without damaging it in the process?

That is the question alright. Easiest answer, with a given barrel, is to use a lighter weight cast bullet with as slow a burning powder at close to 100% load density that ignites and burns consistently at the psi involved. If one can change the barrel to a slower tiwst and longer length (26 - 30") then that will also recuse the psi and accelleration rate for a specific velocity along with reducing the RPM.

If one has the "given" barrel such as on a milsurp, custom or factory rifle of a "standard" or faster twist then you are adapting the load to that barrel. If you are able to rebarrel with a slower twist and longer barrel then you also are adapting the barrel to the load. With one you've one way to go but with the other you can work from both ways which affect the "accelleration" which was the question.

Larry Gibson

[runfiverun]

if i were using a non gas check boolit and was looking for accuracy i'd target a velocity of @ 1200 fps then slowly move forward.
in some instances 1400 is doable,in others 1600.
you just get to a point where the base allows too much deformation [gas escape]
and it's ability to hold the rifling.
marlins round bumps are not helping the situation in any way.
have reasonable expectations for your combination.
i went through this in my 375 marlin with a couple of plain base boolits,i got to where i had decent accuracy at around 1400+ but the groups were lower than my sights would compensate for at 50 yds.

[DrCaveman]

Runfiverun

Marlin's round bumps? Are you talking about the rifling grooves?

The more studying I do is leaning me more toward your stated 1200 fps goal for starters. Dunno why I thought I could pump em at 1600-1700 without following boolit design (adding gas check), guess arrogance...like you say, have reasonable expectations.

And, like you, my super light loads (8.5 unique) shoot so low that it is hard to compensate with the sights. There's a good chance that I already found a load that grouped well, but since I couldn't find paper after 5 shots nor hit a clay pigeon at 50 yds, so I gave up on that one.

Now trying to bring POI closer to factory rounds by increasing velocity. But trying to cheap out and use a small-ish powder charge and no GC.

Well, the cake is being baked, but I don't know if I deserve to eat it yet.

Better load up some more to meet this new realistic goal. Good thing there is 100% chance of rain today, and its 40 outside. Good time for casting and loading more. Shoot Sunday, that's my weekly spiritual revitalizer. All due respect.