Magic RPM Number = 720

[runfiverun]

case capacity and shape do influence part of the equasion.
felix outlined excellent parameters in a thread recently.
in either the shooters or cast bullet subs.
the 30-30 case size/shape is pretty ideal, the neck don't have to be quite that long but it isn't hurting.
filling the case with powder don't hurt either.

[Nrut]

Naw......the 16" twist of the 32WSPL couldn't be making a difference..... now could it?

Larry Gibson

That was easy..

[Nrut]

case capacity and shape do influence part of the equasion.
felix outlined excellent parameters in a thread recently.
in either the shooters or cast bullet subs.
the 30-30 case size/shape is pretty ideal, the neck don't have to be quite that long but it isn't hurting.
filling the case with powder don't hurt either.

The 30-30 case may be the ideal size/shape for the .30 cal. boolit..
What case would be the ideal size/shape for other cal. like the 35, 40, and 45 keeping in mind (or not) as the boolit dia. increases the boolit strength increases as a function of its dia.
runfiverun, I missed felix's thread which you speak of.. Thanks for the heads up.. I will search..

[runfiverun]

i remember thinking once how much powder[ 4895] would be the ideal for a 223 bullet
and finally come up with a number.
then years later i read p.o. ackleys books [some years after i had met him and talked to him several times, as he lived and worked very close to where i grew up.]
anyways he come up with some numbers that seemed ideal for efficient calibers.
the case size is going to be influenced by what you need them to do.you want 2k or 3k
for instance the 358 winchester it holds @ 52 grs of rl-19 but with a 250 cast boolit it seems much more accurate at 48.5 [at about 100 fps difference]
does this mean i should push the shoulders back more to help slow the powders burn.
or just to reduce the case size down to my 48.5
would i even be able to tell?
or would just going to a case full of rl-25 be a better option.
dunno. the knowledge part for me still lies in figuring out my options.
which is the better option for me has got to be in the powder, as i don't have a way to test case shapes.
so i have see what things do the old way, i gotta try them.
if i were to design a case for something, it sure would look a lot different than it would have, say, 2 years ago though.

[Zeek]

Well, what I was getting at is that theoretically a fairly soft CBoo with reasonable length can deform as it leaves the muzzle if the muzzle pressure is higher than it's upset pressure. This type of effect can blurr or complicate the magic number. . . . the point is there other factors involved and my question would be how do those factors change or influence the 'standard baseline'? The shape of the pressure curve frigzample? The volume of muzzle blast gas? And so on.

In order for any load to work, its Max PSI (after ~1" to 3" of PPBoo travel) must exceed that alloy yield strength. Otherwise, there is no obturation and sealing. There is quite a bit of expansion going on well-prior-to the PPCBoo's reaching the muzzle (but that stops WELL before it reaches the muzzle), AND the powder should have nearly completed its burn by then. So, in order to have a muzzle pressure that exceeds the alloy's yield strength, you would have to have a WAAAAAAY TOO HOT load going, so that you still had that much pressure left by the time the PPCBoo got to the muzzle.

I don't see that happening. Rather, if you really want additional problems to consider, how about dealing with the HOW/WHY of the bullet's proceeding forward, still backed by considerable pressure, after the pressure drops below the alloy's yield point (no more obturation, so the sealing should be really crappy, inviting blow-by and patch/CBoo damage). Now THAT is a for real head scratcher.
Zeek

[shooter93]

For 35 caliber I've been dabbling with a 35 krag. Slightly more case capicity than a 358 Win and slightly less than a 35 Win. Long necked case. It seems to be a decent size case for the 35 caliber. As for pressure and obturation once the bullet reaches barrel size it isn't obturating anymore till it exists the barrel and depending on the load and barrel length the bullet can expand beyond barrel size.

[felix]

Keep in mind that both boolit and barrel expand together, as well as the bolt. Hopefully, only the elastic limit of the projectile was exceeded! ... felix

[Zeek]

Keep in mind that both boolit and barrel expand together, as well as the bolt. Hopefully, only the elastic limit of the projectile was exceeded! ... felix

Yes. A related issue, according to Mic McPherson, is that the barrel heating that we see is primarily a result of that localized barrel expansion as the bullet moves down the length of the barrel, rather than from the hot gasses, which have FAR too short a duration of contact with the barrel interior to convey that much heat. That expansion/contraction cycle works the barrel metal and the near-instant heat that we see is the result.
Whoodah Thunkit strikes again!
Zeek

[303Guy]

That expansion/contraction cycle works the barrel metal and the near-instant heat that we see is the result.

The barrel heat reaches the exterior pretty quickly. But there's a problem. My hornet. With the load I was using the barrel heating was every bit as much as a 303 Brit - more actually. But, by increasing the powder charge by a small amount, the barrel remained cool. Also, the suppressor walls heated to the same degree or remained cool to the same degree (almost). Even better, the barrel and suppressor heating was even from throat to muzzle/suppressor. Isn't elastic expansion is friction free? No friction = no heating. And why does a moderate low pressure PPCBoo load heat the barrel so much? Remember that a PPCBoo is supposed to develop a higher velocity than a j-word with the same powder charge. That means less friction (over-all). It just keeps getting simpler! Hee hee.

[45 2.1]

Naw......the 16" twist of the 32WSPL couldn't be making a difference..... now could it?

Larry Gibson

All depends on how much time and effort someone puts into finding out what does what. There are a lot of things that are not considered by the majority of people. After all, we have 1:38 twists in 44 caliber rifles....don't we.

[Larry Gibson]

All depends on how much time and effort someone puts into finding out what does what. There are a lot of things that are not considered by the majority of people. After all, we have 1:38 twists in 44 caliber rifles....don't we.

That we do, and with the correct weight cast bullets they shoot extremely well to their maximum velocity. But then do any of those 38" twist .44s, even the .444 Marlin which has the largest case capacity, push cast bullets anywhere close to the RPM we're talking about? The 444 Marlin at a max velocity 2400given a 240 gr bullet is only running around 45,000 RPM. That is a far cry from the 140 - 150,000+ RPM being discussed.

Larry Gibson

[leftiye]

Your point???

[Nrut]

Now that leftiye is here I wonder if "Tiger"/Ralf is going to show up to complete the RPM discussion circle.. I miss Tiger! I hope he is OK..
Oh forgot Joe, bass and a few others as well..
Some background for you Zeek in case you missed it..
Note the locked RPM threads in these links from 2008...
1-- http://castboolits.gunloads.com/showthread.php?t=38808
2-- http://castboolits.gunloads.com/showthread.php?t=31517
---- not locked
3-- http://castboolits.gunloads.com/showthread.php?t=33859

Zeek & Larry
All I take away from this RPM discussion is that generally speaking it is easier to get cast boolits to shoot accurately (1" or less at 100yds.) at HV (over 2300fps) in slower twist barrels, but there are ways to get around that if you have the knowledge and I don't, YET..
That's what makes this hobby so much fun..
It distracts our minds from the reality of our demise.. Some people have to use alcohol or drugs or religion to do that.. Probably cheaper than casting boolits though..

I hope the moderators let this thread run! (not that we'll learn anything about shooting cast at HV)

[Larry Gibson]

Nrut

All I take away from this RPM discussion is that generally speaking it is easier to get cast boolits to shoot accurately (1" or less at 100yds.) at HV (over 2300fps) in slower twist barrels, but there are ways to get around that if you have the knowledge and I don't, YET..
That's what makes this hobby so much fun..


That is basically correct except that you don't "get around" it. I used to think we could cross over it or "get around" it also but that is incorrect. Extensive testing with 6.5 and .30 cals has led me to believe that with some advanced casting and loading techniques we simply push the RPM threshold higher. I.e., in the 10" twist '06 loading Bass and I did with 311291 we both were able to push it to 21-2200 fps and maintain decent accuracy, still not what we could get down under 1950 fps though. However, the RPM threshold still ended up biting us and accuracy went south. It is the same with any cartridge or caliber, you can push the RPM threshold higher with advanced loading techniques. Still, with regular cast bullets and normal cast bullet loading techn iques (like those listed in the manuals which most loaders use) the RPM threshold is in the 140,000 RPm range. That is with the faster and medium burning powders and the normal alloys most casters use.

Best indicator is when testing loads the accuracy tightens up and then gets worse. You know you are pushing the RPM threshold as the accuracy gets worse. You'll know if you've actually exceeded the RPM threshold when a 200 yard group does not exhibit linear expansion to a 100 yard group. We can push the RPM threshold and still maintain reasonable accuracy and linear expansion. I do this all the time with my hunting cast bullet loads. However, if we exceed the RPM threshold then as range increases the accuracy gets worse and worse.

Testing HV loads at 50 yards does not give us any really valid accuracy information. It is too easy to think a flyer is your fault, especially with open or milsurp sights. We should remember that all flyers are telling us something unless we can absolutely say that shot went to call. In several tests of really imbalanced bullets the 50 yard groups were under 2" and were still what many call "good". However, those imbalanced bullets were demonstrating how the centrafugal force of too much RPM adversely affects accurcy on a linear basis and accuracy at 100 and 200 yards was far greater than linear. The 200 yard groups were on the order of 16"+. The tested ammo normally produced 3" groups at 200 yards. Besides 50 yards not being the best distance to test HV loads 5 shot groups are really not enough either. Spear says 5 shots are not "statistically valid" and I have found that to be the case. They suggest 7 shots as a minimum. I generally use 10 shots these days. It really seperates the wheat from the chaff and eliminates that 'cloverleaf" group that is simply the result of "random selection".

Cast bullets are a lot of fun. Loading and shooting cast bullets at HV can also be fun but at the same time can be very frustration, probably more frustrating than fun

Larry Gibson

[leftiye]

But now we're in the paper patch forum..........................(don't forget!) It does seem that the "RPM barrier" exists in the part of the boolit that contacts the rifling. A "paper jacket" seems to pretty much obviate the problem. N'est ce pas?

[45 2.1]

Hahahahahahaha.............. It seems that you need to learn some more Larry. Don't let what you do know get in the way of learning something else, even if it is contrary to what you think.

Most of the people i've taught could not shoot HV with their own cast boolits, but could with mine. They learned how to cast good boolits finally, then progressed to excellent boolits thru the correct techniques. Even Harry Pope stated late in his life that not one in a thousand could cast a boolit worth shooting. Heed his thoughts and learn to cast boolits worth shooting, not "common cast boolits". If you can't do that or get results like others, contact those people to learn how......................

[45 2.1]

But now we're in the paper patch forum..........................(don't forget!) It does seem that the "RPM barrier" exists in the part of the boolit that contacts the rifling. A "paper jacket" seems to pretty much obviate the problem. N'est ce pas?

Good observation.......... But it is more indicative of launching it crooked instead of any RPM excuse.

[Larry Gibson]

But now we're in the paper patch forum..........................(don't forget!) It does seem that the "RPM barrier" exists in the part of the boolit that contacts the rifling. A "paper jacket" seems to pretty much obviate the problem. N'est ce pas?

That is correct. The PP offers much more support to the bullet and with some designs there is little room for any uneven obturation to unbalance the bullet. What some don't realise, understand or haven't learned yet is that a bullet entering the barrel "crooked" becomes unbalanced in flight because the center of gravity and the center of form do not coincide with the center of spin. That is the why and the where that centrafugal force of RPM has it's adverse affect. The higher the RPM the greater the adverse affect and thus the worse accuracy is.

The RPM threshold of PP'd alloy bullets is higher than that of regular cast bullets. The PP keeps the heat away and lessens the obturation. Yet even with a PP'd bullet the hfaster the velcity and the higher the RPM the harder we must make the bullet. Given the same design and cartridge a pure lead PP bullet cast of lead can not be driven as fast as a WW cast bullet and neither of them as fast as a Linotype alloyed bullet. We all know that.

Larry Gibson

[Larry Gibson]

Hahahahahahaha.............. It seems that you need to learn some more Larry. Don't let what you do know get in the way of learning something else, even if it is contrary to what you think.

Most of the people i've taught could not shoot HV with their own cast boolits, but could with mine. They learned how to cast good boolits finally, then progressed to excellent boolits thru the correct techniques. Even Harry Pope stated late in his life that not one in a thousand could cast a boolit worth shooting. Heed his thoughts and learn to cast boolits worth shooting, not "common cast boolits". If you can't do that or get results like others, contact those people to learn how......................

Let's keep this civil this time, ok?

As to the second part of your comment regarding good cast bullets you are absolutely correct. I do shoot lots of cast bullets, regular ones at that, at HV velocity (2300 -2600+ fps) with very good accuracy. Casting a good bullet is the obvious place to start. If it cast with imbalances then it will have those imbalances and even more from the effect of accelleration when it is launched. The centrafugal force of the RPM will then have a greater adverse effect on a poorly cast bullet. See, even you can learn something. I have said over and over again the way to push the RPM threshold is to lessen the imbalances in the bullets. So why would I, or any one else who is attempting HV with cast bullets start with less than the best they can cast? Doesn't make sense to me and I don't use less than the best in my HV loads. BTW; Pope was also correct but I don't think that casting knowledge was as available or wide spread as it is today.

It's nice to know that you can cast perfect bullets. I tried every which way from Sunday to get some of starmetal's 6.5 Kurtz bullets to test. They must also be perfect based on his claims of accuracy and your confirmation of his accuracy. Sooooooo....let me ask you to send me 100 of your perfect 6.5 Kurtz bullets so I can compare them to my own 3rd GB 6.5 bullets in quality of casting and in shooting accuracy at HV. I would gladly pay for the bullets and the shipping. How about it?

Larry Gibson

P.S.; this is the PP forum so if 45 2.1 does send me some of his bullets to test I'll post the results on the appropriate forum. I think we should keep this discussion pertinant to how RPM also affects PP'd bullets and not get into discussing regular cast bullets here. The principle is the same but the RPM threshold is higher with PP'd bullets and since i am back into PP'd bullets I find it an interesting topic also. I've learned quite a bit from the several who have pushed PP'd bullets into HV.

[45 2.1]

Let's keep this civil this time, ok?
Civility differs from place to place and according to age group also. We were civil in our own age group. The younger age group grew up in a somewhat different world than we did and has a different idea about that.

As to the second part of your comment regarding good cast bullets you are absolutely correct. I do shoot lots of cast bullets, regular ones at that, at HV velocity (2300 -2600+ fps) with very good accuracy. Casting a good bullet is the obvious place to start. If it cast with imbalances then it will have those imbalances and even more from the effect of accelleration when it is launched. The centrafugal force of the RPM will then have a greater adverse effect on a poorly cast bullet. Someday, if you live long enough, you'll see that getting them started into the barrel, straight in line, every time has much more to do with HV accuracy. Once you get to a certain point, your booting them out of alignment the way your going at it. See, even you can learn something. I have said over and over again the way to push the RPM threshold is to lessen the imbalances in the bullets. Hahaha, thats B19. So why would I, or any one else who is attempting HV with cast bullets start with less than the best they can cast? Thats would be the best you can cast, not what is required to shoot accurately. There is a difference there. Everybody I taught changed from what they were casting to the way I did it and got much better accuracy. Doesn't make sense to me and I don't use less than the best in my HV loads. BTW; Pope was also correct but I don't think that casting knowledge was as available or wide spread as it is today. Pope was an advocate of breech seated boolits............ conditions were different than now.

It's nice to know that you can cast perfect bullets. I didn't say that, I just know what makes a difference in the alloy and methodology used to get accuracy when casting them. I tried every which way from Sunday to get some of starmetal's 6.5 Kurtz bullets to test. Those were ones I sent him. They must also be perfect based on his claims of accuracy and your confirmation of his accuracy. Sooooooo....let me ask you to send me 100 of your perfect 6.5 Kurtz bullets so I can compare them to my own 3rd GB 6.5 bullets in quality of casting and in shooting accuracy at HV. I would gladly pay for the bullets and the shipping. How about it? I sent all my aged 6.5mm boolits to Joe so he would have enough to learn how to get them to shoot. I don't have anymore cast. Besides, they would have to be aged then.

Larry Gibson

P.S.; this is the PP forum so if 45 2.1 does send me some of his bullets to test I'll post the results on the appropriate forum. I think we should keep this discussion pertinant to how RPM also affects PP'd bullets and not get into discussing regular cast bullets here. The principle is the same but the RPM threshold is higher with PP'd bullets and since i am back into PP'd bullets I find it an interesting topic also. I've learned quite a bit from the several who have pushed PP'd bullets into HV.

The idea a PP boolit is different than a "normal" cast boolit is ludicrous. The people here shooting full jacketed velocity cast boolits wrapped in paper and getting excellent accuracy know a few things. Learn from them and don't blame failure on some theory.