FAST TWIST BARREL QUESTION
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Ned Ludd
Silver $$ Contributor
As far as .224" barrels, there have been a number of F-TR shooters using .223 Rem that have experienced jacket failures with heavy .224" bullets such as Berger's 90 VLDs and Hornady's 88 ELDMs. Studies have been carried out by Berger and perhaps others that suggest the bullet jacket can become white hot where it is engraved by the rifling lands. So hot, in fact, that the surface of the lead core underneath the land engravement grooves in the jacket actually melts. The molten lead will attempt find a crack or pinhole in the jacket, thus sometimes spraying out, leaving what have been referred to as "comet tails", or curved blackish spray patterns around the bullet holes on the target face, if the distance is not too great (i.e. < 100 yd or so). Eventually, the stresses on the bullets often cause the jacket to fail, after which the jacket/core remnants can start to tumble. There are a number of factors that can contribute to jacket failure, including barrel length, twist rate, and bore/groove configuration:
Barrel length - barrels 30+" in length seem to be much more prone to jacket failures than barrels of 26" or shorter. This is likely due to the increased friction from the extra barrel length.
Twist rate - twist rates of faster than 7.0 have been associated with jacket failures.
Jacket thickness/toughness - it is clear that the bullet jackets used by certain manufacturers of heavy .224" bullets are thicker/tougher than others, and those bullets rarely, if ever, fail under conditions that would almost certainly cause jacket failures with a different manufacturer's heavy bullet.
Bore/groove configuration - It has been suggested that certain land/groove configurations such as 5R/5C/ratchet rifling are slightly less likely to promote jacket failure as the "corners" at the top of the lands are not perfectly square and thus a little less sharp (i.e. greater than 90 degrees).
Perhaps even more important is the bore/groove diameter. For years the .224 barrels I used were all 5R 0.218"/0.224". However, in discussing the potential causes of these jacket failures and how to mitigate them with other F-TR .223 Rem shooters over a period of several years, the notion has emerged that going with a 0.219"/0.224" bore/groove configuration is the simplest fix. The 0.219" bore diameter may not seem like much of a difference, only .0005" less tight per side. However, those using 0.219" bore barrels have noted that many of the other features associated with jacket failures such as barrel lengths of 30+" and twist rates faster than 7.0 no longer seem to be an issue with the 0.219" bore. In other words, some of the contributing factors to jacket failure were cumulative effects that collectively would increase the odds of jacket failure, but that were no longer sufficient to cause the problem by themselves when a slightly less tight bore was used. In fact, some that went the 0.219" bore barrels were able to spin heavy .224" bullets with twist rates as fast as 6.5 or even 6.25; twist rates that almost certainly would have caused jacket failures with a 0.218" bore barrel. So simply using a slightly larger bore made the other contributing factors effectively "go away". This is not exactly true in that they don't actually "go away", but are themselves insufficient to cause jacket failure in the absence of the tighter bore configuration. This is good evidence that excessive friction is the primary culprit in terms of jacket failures.
I have no idea how much play in terms of bore diameter is possible across a wide range different cartridges/calibers. Certainly various bore diameters are available in .30 cal barrels. If a bore diameter .001" larger is actually available in the caliber of choice for those experiencing bullet jacket failures, going with the larger bore diameter may be a relatively simple way to minimize/prevent those failures.
Barrel length - barrels 30+" in length seem to be much more prone to jacket failures than barrels of 26" or shorter. This is likely due to the increased friction from the extra barrel length.
Twist rate - twist rates of faster than 7.0 have been associated with jacket failures.
Jacket thickness/toughness - it is clear that the bullet jackets used by certain manufacturers of heavy .224" bullets are thicker/tougher than others, and those bullets rarely, if ever, fail under conditions that would almost certainly cause jacket failures with a different manufacturer's heavy bullet.
Bore/groove configuration - It has been suggested that certain land/groove configurations such as 5R/5C/ratchet rifling are slightly less likely to promote jacket failure as the "corners" at the top of the lands are not perfectly square and thus a little less sharp (i.e. greater than 90 degrees).
Perhaps even more important is the bore/groove diameter. For years the .224 barrels I used were all 5R 0.218"/0.224". However, in discussing the potential causes of these jacket failures and how to mitigate them with other F-TR .223 Rem shooters over a period of several years, the notion has emerged that going with a 0.219"/0.224" bore/groove configuration is the simplest fix. The 0.219" bore diameter may not seem like much of a difference, only .0005" less tight per side. However, those using 0.219" bore barrels have noted that many of the other features associated with jacket failures such as barrel lengths of 30+" and twist rates faster than 7.0 no longer seem to be an issue with the 0.219" bore. In other words, some of the contributing factors to jacket failure were cumulative effects that collectively would increase the odds of jacket failure, but that were no longer sufficient to cause the problem by themselves when a slightly less tight bore was used. In fact, some that went the 0.219" bore barrels were able to spin heavy .224" bullets with twist rates as fast as 6.5 or even 6.25; twist rates that almost certainly would have caused jacket failures with a 0.218" bore barrel. So simply using a slightly larger bore made the other contributing factors effectively "go away". This is not exactly true in that they don't actually "go away", but are themselves insufficient to cause jacket failure in the absence of the tighter bore configuration. This is good evidence that excessive friction is the primary culprit in terms of jacket failures.
I have no idea how much play in terms of bore diameter is possible across a wide range different cartridges/calibers. Certainly various bore diameters are available in .30 cal barrels. If a bore diameter .001" larger is actually available in the caliber of choice for those experiencing bullet jacket failures, going with the larger bore diameter may be a relatively simple way to minimize/prevent those failures.
GREAT INFO!! THANKS FOR SHARING YOUR KNOWLEDGE AND EXPERIENCE!!
jelenko
Gold $$ Contributor
Ned - Wasn't the finding that it wasn't the increased 'toughness' of the jacket as it was the thicker jackets reduced the heat transfer to the lead core?
Ned Ludd
Silver $$ Contributor
I've never heard that, but it could certainly be the case. I'd imagine jacket thickness, as well as the particular alloy used could help prevent jacket failures. However, I'd don't know any of the specifics why certain manufacturer's jackets seem "tougher" than others. Increased thickness and lesser heat transfer seems plausible. I only know that in my hands, some manufacturer's bullets have experienced jacket failures, where as other manufacturer's bullets did not when loaded at the same pressure/velocity.
My understanding that caliber is not the issue as much as sectional density and ballistic coefficients when it comes to aerodynamic stability.
I.E. a 107 grain 24 caliber and a 250 grain 30 caliber may require the same rate of rotation to stabilize.
I haven't studied the mathematical formula however studying a projectile that requires a 1 in 7 twist, in 243, if it is say 4 calibers long, (.972" long), then a 308 that is 4 calibers long 1.52" long would require a 1 in 7 twist.
From memory, I think 240,000-275,000rpm is the correct window to be in. A lot of really good shooters used to preach (not sure if this is still true) using the slowest twist rate possible to stabilize the bullet for maximum accuracy. Someone will surely add to what I’ve said though and provide more detailed info.
Dave
This is inline with my findings too. But my girlfriend is shooting a new 31” Bartlien 7.5 twist 6 dasher barrel that is just ridiculously accurate right now.
Check your math in the last sentence. I think .308 at 4 calibers length should read 1.232”, and not 1.52”
JEFFPPC
Gold $$ Contributor
Take her out to dinner, maybe she will let you use it.
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Haha. Not likely. I’m the chief armorer for her, I get to touch the rifle only when tuning, reloading, or tuning. lol.
I didn't use real math it was hypothetical, a 107 grain 6MM I believe is really 5 calibers and a 275 grain 308 would maybe be 1 5/8" or maybe 1 3/4". The point being equal sectional densities most likely will need equal to similar twist rates.
I have an idea what is meant by "4 bullet calibers" but would like to hear from those that really know.
No, stability is unrelated to either RPMs, or to bullet blowups.
Twist rate does matter.
The cal matters about blowups because a turn of 30cal is more of everything than a turn of 22cal.
The cartridge can matter because it's the source of X amount of energy. A 6.5 grendel wouldn't usually apply as much heat as a 264WM, even with both loaded to same pressure.
Outside of bullets blowing up, let's say you don't have that problem,, then fully stable can be more accurate than marginally stable. I've seen that directly here at sea level.
Pretty sure Bryan Litz has demonstrated that an adequate twist and even considerably tighter than adequate is desirable over an under twist condition.
Keep in mind that Twist requirements are unrelated to RPM.
Twist is displacement per turn, while RPM represents turns per time.
No matter the RPM, if your displacement per turn is too high for a bullet, it will be unstable.
And no matter the RPM, if your displacement per turn is correct for a bullet, it will be stable.
Pretty sure Bryan Litz has demonstrated that an adequate twist and even considerably tighter than adequate is desirable over an under twist condition.
Keep in mind that Twist requirements are unrelated to RPM.
Twist is displacement per turn, while RPM represents turns per time.
No matter the RPM, if your displacement per turn is too high for a bullet, it will be unstable.
And no matter the RPM, if your displacement per turn is correct for a bullet, it will be stable.
My only disappointment with rifle twist are ones that are too slow to stabilize the bullet...going sideways in the paper at short range.
I prefer fast twist barrels. 6.5 twist 224 Valkyrie for 95 MK will also accurately shoot 40 gr BT. 7.5 twist on dasher, &ARC, shoot light bullets accurately. My last two 308 bulids are 9 twist and 8 twist.
The only bullet that would consistently blow up in a standard factory 9 twist in 17 Remington was the 30 gr Berger, it never made it to 100 yards...ever. But devastating on close range squirrels. My dasher 7.5 twist accurately shoots 58 gr v max at 4000 fps or 70 gr Nosler at 3700 fps ...todays bullets have held together really well in fast twist barrels and high velocity. Most of these are premium aftermarket barrels with 5 R or ratchet rifling...supposed to easy on the bullet,...and maybe that's true, as I haven't had bullet blow up in years. According to Hornadys radar the same longe range bullet fired from a fast twist barrel (8 twist ) has a significantly higher BC throughout it's range of flight than the slower (10 twist)... something to consider if shooting long range. When experimenting with 5.56 in AR 20" barrel 95 Sierra MK the 7 twist (mag feed) wasn't fast enough, at 2450 fps, needed a 6 twist. The game you play and the bullets generally used for that particular game will dictate the twist rates needed...or a fast general purpose twist rate, for a more variety of bullets, and shooting endeavors for particular single rifle.
I prefer fast twist barrels. 6.5 twist 224 Valkyrie for 95 MK will also accurately shoot 40 gr BT. 7.5 twist on dasher, &ARC, shoot light bullets accurately. My last two 308 bulids are 9 twist and 8 twist.
The only bullet that would consistently blow up in a standard factory 9 twist in 17 Remington was the 30 gr Berger, it never made it to 100 yards...ever. But devastating on close range squirrels. My dasher 7.5 twist accurately shoots 58 gr v max at 4000 fps or 70 gr Nosler at 3700 fps ...todays bullets have held together really well in fast twist barrels and high velocity. Most of these are premium aftermarket barrels with 5 R or ratchet rifling...supposed to easy on the bullet,...and maybe that's true, as I haven't had bullet blow up in years. According to Hornadys radar the same longe range bullet fired from a fast twist barrel (8 twist ) has a significantly higher BC throughout it's range of flight than the slower (10 twist)... something to consider if shooting long range. When experimenting with 5.56 in AR 20" barrel 95 Sierra MK the 7 twist (mag feed) wasn't fast enough, at 2450 fps, needed a 6 twist. The game you play and the bullets generally used for that particular game will dictate the twist rates needed...or a fast general purpose twist rate, for a more variety of bullets, and shooting endeavors for particular single rifle.
what is your velocity??? Roger
I did not chrono these loads yet.
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