RPM Threshold barrel twist/velocity chart

[dtknowles]

I bought a used bench rest rifle in 30 BR long with a 1x16 twist barrel but I only have one mould in the right weight range and it does not group. I need to figure out what to buy for a mould to get this rifle to shoot.

Tim

[Larry Gibson]

Thanks Larry, the time and effort that you put into this isappreciated. I'm curious about if you ever found the RPM threshold to movedownward as bullet diameter increases? ...........

One would think so? However I've not got the answer to that yet. Thereare several things at play; if we have an equal defect weight wise it ismost often a much larger % of the mass of the smaller diameter bullet if thebullets are the same length. If we lengthen the smaller bullet to equalizethe weight then where in relation to the center of balance is the defect? If we make the defect equal in mass % then can we detect it with visual or evenby weight sorting? Also is the defect a surface defect or an internalone? If internal then what is it's shape and at what distance is it fromthe COG, COF and center of spin? Answers to those questions I'm notsure of yet.

What I have found is in establishing the RPM threshold with regular castbullets of ternary alloy using the slower fast burning powders to the slowmedium burning powders is the RPM Threshold most often falls between 120,000and 140,000 RPM. This was arrived at by analyzing load and test accuracyresults of .22 cal rifles to rifles of .375 caliber with twists of 7 to12" most often. With the variables involved it is hard to nail downan answer to your question......yet. As we continue with sound testinglimiting the variable to one specific question, as best we can anyway, we getcloser to a lot of answers.

For example; I have controlled the RPM keeping it under 140,000 with the14" twist Palma rifle. When psi's are kept under 40,000 accuracy isexcellent. However, when psi increases to 42,000 psi I get flyers. Not alot out of the group but still flyers. The RPM is still below the RPM threshold at 136 – 137,000 RPM yet theflyers begin. Based on engineercalculations this probably where, at that pressure, some of the bullets beginto plasticize under that rate of acceleration. Thus another variable is thrown in. My solution is to increase case capacity with the 30x57 (30 XCB) to keepthe psi under/at 40,000 psi while increasing velocity. We shall see.

Larry Gibson

[Larry Gibson]

I bought a used bench rest rifle in 30 BR long with a 1x16 twist barrel but I only have one mould in the right weight range and it does not group. I need to figure out what to buy for a mould to get this rifle to shoot.

Tim

What length barrel?

What bullet?

Larry Gibson

[dtknowles]

What length barrel?

What bullet?

Larry Gibson

I will go out an look for a mould number it casts at 130 grains it has one grease groove and a crimp groove with a long ogive and a bevel base. Clearly this is not a great choice of bullet for this rifle but it works OK in my SKS. The barrel is 22 inches long, I think the barrel might be shot out as it will only shoot jacketed into 0.5 MOA at 100 yards with 130 grain Bergers hollow points. I don't really know as the rifling is not far ahead of the chamber and I don't have a BoreScope.

Tim

[dtknowles]

What length barrel?

What bullet?

Larry Gibson

Mould is Ideal 311 410.

Tim

[btroj]

One would think so? However I've not got the answer to that yet. Thereare several things at play; if we have an equal defect weight wise it ismost often a much larger % of the mass of the smaller diameter bullet if thebullets are the same length. If we lengthen the smaller bullet to equalizethe weight then where in relation to the center of balance is the defect? If we make the defect equal in mass % then can we detect it with visual or evenby weight sorting? Also is the defect a surface defect or an internalone? If internal then what is it's shape and at what distance is it fromthe COG, COF and center of spin? Answers to those questions I'm notsure of yet.

What I have found is in establishing the RPM threshold with regular castbullets of ternary alloy using the slower fast burning powders to the slowmedium burning powders is the RPM Threshold most often falls between 120,000and 140,000 RPM. This was arrived at by analyzing load and test accuracyresults of .22 cal rifles to rifles of .375 caliber with twists of 7 to12" most often. With the variables involved it is hard to nail downan answer to your question......yet. As we continue with sound testinglimiting the variable to one specific question, as best we can anyway, we getcloser to a lot of answers.

For example; I have controlled the RPM keeping it under 140,000 with the14" twist Palma rifle. When psi's are kept under 40,000 accuracy isexcellent. However, when psi increases to 42,000 psi I get flyers. Not alot out of the group but still flyers. The RPM is still below the RPM threshold at 136 – 137,000 RPM yet theflyers begin. Based on engineercalculations this probably where, at that pressure, some of the bullets beginto plasticize under that rate of acceleration. Thus another variable is thrown in. My solution is to increase case capacity with the 30x57 (30 XCB) to keepthe psi under/at 40,000 psi while increasing velocity. We shall see.

Larry Gibson

at at some point shooting lots of groups from a bench with a 375 or larger bore rifle just stops being fun. A 300 gr plus bullet at 2400 isn't much fun to shoot lots of groups with, is it?

I sure don't want to be the guy testing a 458 Win mag over long strings from the bench.

[runfiverun]

i'll loan you my browning 45/70 with crescent steel butt plate,, that should soften the blow a tick.

[btroj]

Bullets are easier to ship than a rifle, how about I send you 200 bullets in the 425 gr range. Let me know how they do in multiple 10 shot groups.

Crescent steel butt plate, 4 words guaranteed to bring tears to the eyes.......

[MBTcustom]

Bullets are easier to ship than a rifle, how about I send you 200 bullets in the 425 gr range. Let me know how they do in multiple 10 shot groups.

Crescent steel butt plate, 4 words guaranteed to bring tears to the eyes.......

Proof that the old timers weren't pushing the RPMTH very much at all.
I've got a buddy with a 458Lott with a steel butplate. It's a real pretty rifle till you see your fillings laying on the bench. LOL!

[runfiverun]

no need to send boolits we run the same load in our 45/70's already...
I dunno the twist rate, so I can't do the math on the rpm's either [but neither can the rifle, so it's fine]

[w30wcf]

Larry,
Thank you for sharing all the information that you have experienced in your "Tale Of Three Twists". Very educational and interesting!

w30wcf

[dtknowles]

Is there another (lower) threshold for plain base bullets?

Tim

[GhostHawk]

Thanks to all who have contributed to this incredibly interesting thread.

This right here, is the meat and potatoes of this site, and the reason so many of us keep coming back.

[runfiverun]

ghost hawk:
the meat and potatoes of this site used to be figuring out way's around the rpm windows.

[Eutectic]

ghost hawk:
the meat and potatoes of this site used to be figuring out way's around the rpm windows.

You can say that again, Lamar !!!

Eutectic

[Larry Gibson]

ghost hawk:
the meat and potatoes of this site used to be figuring out way's around the rpm windows.

If one bothers to continue researching what was done and discussed after this thread was original published you'd find this site is still about that. Even before this thread was originally published here several of us were actually pushing the RPM threshold up. Except for the unfortunate fact a few still think and insinuate the RPM Threshold is a "limit" of some sort much has been done to push the RPM threshold up. Those of us who are serious in this endeavor conduct appropriate tests and report the results here in this forum. Others make claims but fail to document those claims through appropriate testing.

Progress is being made. When we understand the actual problem then many times we can solve it. We now know how to push a ternary naked cast bullet to higher RPM before losing accuracy. However, we are only pushing the RPM threshold up as we still are not achieving equal velocity with the same accuracy that the rifles are capable of with jacketed bullets. We also know that if we control the RPM via a slower twist barrel even higher velocity with excellent accuracy can be had. We are even approaching jacketed bullet velocities and accuracy with ternary alloyed naked cast bullets. Numerous members are also expanding our abilities with PP'd cast bullets, with harder alloyed and HT's bullets and now PC'd bullets are showing some real promise. Yes, there is a lot of progress being made on this forum.

Some of us just chose to get on with that progress. Others have chosen to complain and leave because they feel their "old timer" status is threatened. I was on this forum for several years under the old site and have been on this site for a long time. I don't care what a members age is, or what country he is from, or if the spelling isn't great or how many posts he's made. If he has a legitimate question I can help with I will offer advise and try to help. I do a lot of testing to not only provide accurate cast bullet loads for my rifles and handguns but to answer many of the questions as to "why" things happen when we shoot cast bullets. There are the laws of physics and ballistics which we are not really going to get around. Conversely there are many techniques and different ways to get things done. Those are the variables we deal with most often and there are lots of ways to do the same thing. However, when we bump up against a law of physics or ballistics we are not going to change that. What we can do is understand what is happening and then change the variables. That is how we are pushing the RPM threshold up. That is the progress that is being made; we understand the problem and know we can not change the problem so we change the variable to simple move the problem up (in this case to a higher RPM and/or velocity).


So instead of those whining and crying that many do realize there is an RPM Threshold and it is real, how about getting on the band wagon and lets change the variables and move it up. I've already moved it way up by changing the twist variable and am now shooting a ternary alloyed 311466 at 2600 fps with excellent accuracy to a tested and verified 300 Yards. I've posted numerous 10 shot groups to demonstrate that. Another forum member has recently shot that rifle/load at 300 yards and can attest to it.


So how about us continuing to progress instead of regressing?

Larry Gibson

[garym1a2]

Does this mean that I would have better chance of success with a 30-06 to get high velocity than the 308 as I can increase the velocity and not have as much pressure as the 308 would?

One would think so? However I've not got the answer to that yet. Thereare several things at play; if we have an equal defect weight wise it ismost often a much larger % of the mass of the smaller diameter bullet if thebullets are the same length. If we lengthen the smaller bullet to equalizethe weight then where in relation to the center of balance is the defect? If we make the defect equal in mass % then can we detect it with visual or evenby weight sorting? Also is the defect a surface defect or an internalone? If internal then what is it's shape and at what distance is it fromthe COG, COF and center of spin? Answers to those questions I'm notsure of yet.

What I have found is in establishing the RPM threshold with regular castbullets of ternary alloy using the slower fast burning powders to the slowmedium burning powders is the RPM Threshold most often falls between 120,000and 140,000 RPM. This was arrived at by analyzing load and test accuracyresults of .22 cal rifles to rifles of .375 caliber with twists of 7 to12" most often. With the variables involved it is hard to nail downan answer to your question......yet. As we continue with sound testinglimiting the variable to one specific question, as best we can anyway, we getcloser to a lot of answers.

For example; I have controlled the RPM keeping it under 140,000 with the14" twist Palma rifle. When psi's are kept under 40,000 accuracy isexcellent. However, when psi increases to 42,000 psi I get flyers. Not alot out of the group but still flyers. The RPM is still below the RPM threshold at 136 – 137,000 RPM yet theflyers begin. Based on engineercalculations this probably where, at that pressure, some of the bullets beginto plasticize under that rate of acceleration. Thus another variable is thrown in. My solution is to increase case capacity with the 30x57 (30 XCB) to keepthe psi under/at 40,000 psi while increasing velocity. We shall see.

Larry Gibson

[btroj]

So instead of those whining and crying that many do realize there is an RPM Threshold and it is real, how about getting on the band wagon and lets change the variables and move it up. I've already moved it way up by changing the twist variable and am now shooting a ternary alloyed 311466 at 2600 fps with excellent accuracy to a tested and verified 300 Yards. I've posted numerous 10 shot groups to demonstrate that. Another forum member has recently shot that rifle/load at 300 yards and can attest to it

How is changing to a slower twist rate pushing the rpm threshold? You got more velocity for the same rpm but that doesn't "push" anything regarding the rpm threshold.

The "twist variable" isn't really a variable to the majority of members here. Many here won't rebarrel a rifle but will change alloy, mould, or powder along with modifying load technique.

Once a barrel is made the twist is no longer a variable, it is set in steel.

[Larry Gibson]

Does this mean that I would have better chance of success with a 30-06 to get high velocity than the 308 as I can increase the velocity and not have as much pressure as the 308 would?

That it does if both the '06 and the .308W have the same length barrels with the same 10" twist. Proved that numerous years ago when Bass Ackwards and I were testing the LBT bullet and the 311291 and I also did additional testing with the 311466. Both rifles had the same actions and the barrels were from the same manufacturer with the same rifling at 24" with 10" twists. Both rifles shot essentially the same accuracy level just under the RPM Threshold. When we pushed the RPM threshold I could get 100 - 200 fps more out of the '06 than the .308W with the same accuracy with all the bullets because the '06 could use a bit more of the slower burning powders than the .308W at the same psi level. The bit more powder at the same psi meant the higher velocity as the time/pressure curve was still a bit slower in the '06.

Not so easy if the '06 has a 10" twist and the .308W has a 12" twist though. The slower twist/RPM of the 12" twist balances out the case capacity with lower psi of the 10" twist '06.

Larry Gibson

[popper]

I'm trying to decide between 10 or 12 in 300BO, 150 gr like the 041. Powder choice is a problem in that little case. I know the boolit will take the pressure, it's a matter of a good burn. I did a 3d plot of fps, weight, ft-# @ 100 for acceptable load ranges but I can't add twist and it's effects, not accurately anyway.