Bullets Spinning Apart
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lancetkenyon
Well-Known Member
A LOT! I doubt you can run them fast enough with enough twist unless you are shooting a custom twist 1:7.5" from something like a 7AM (or similar) at 3500+fps.
I had some 6mm 85gr V-Max vaporize from a 1:7.5" 6SST @ 3560fps. If I remember correctly, that was about 341,760RPM for a thin jacketed varmint bullet.
I had some 6mm 85gr V-Max vaporize from a 1:7.5" 6SST @ 3560fps. If I remember correctly, that was about 341,760RPM for a thin jacketed varmint bullet.
At while ago, I had a 1 in 7 3 groove in 7 Rem mag. A friend built a 7 STW with same barrel configuration. We both shot 180 VLDs. The 7 STW began to damage bullet jackets at round count 150 or so. His bullets would diverge from POI by inches at 100 yds. Mine did the same thing at just over 200 rounds.
I emailed Bryan Litz and he suggested I switch to 180 target VLDs with their heavier jackets. That helped for a while then the bullets would show signs of damage and shoot several inches off POI at 100 yds. I tried moly coating the bullets and it worked for a while.
Other members told me before I built the rifle not to use a 3 groove with such a fast twist and they were right. The 3 groove is the issue as it damaged the jackets in the throat area once it got rough. Accubonds and TTSX bullets shot fine in that barrel after I quit using the VLDs.
RPMs:
7 mag 180 VLD @ 2950 fps 303,428
7 STW " @ 3100 fps 318,857
I emailed Bryan Litz and he suggested I switch to 180 target VLDs with their heavier jackets. That helped for a while then the bullets would show signs of damage and shoot several inches off POI at 100 yds. I tried moly coating the bullets and it worked for a while.
Other members told me before I built the rifle not to use a 3 groove with such a fast twist and they were right. The 3 groove is the issue as it damaged the jackets in the throat area once it got rough. Accubonds and TTSX bullets shot fine in that barrel after I quit using the VLDs.
RPMs:
7 mag 180 VLD @ 2950 fps 303,428
7 STW " @ 3100 fps 318,857
At while ago, I had a 1 in 7 3 groove in 7 Rem mag. A friend built a 7 STW with same barrel configuration. We both shot 180 VLDs. The 7 STW began to damage bullet jackets at round count 150 or so. His bullets would diverge from POI by inches at 100 yds. Mine did the same thing at just over 200 rounds.
I emailed Bryan Litz and he suggested I switch to 180 target VLDs with their heavier jackets. That helped for a while then the bullets would show signs of damage and shoot several inches off POI at 100 yds. I tried moly coating the bullets and it worked for a while.
Other members told me before I built the rifle not to use a 3 groove with such a fast twist and they were right. The 3 groove is the issue as it damaged the jackets in the throat area once it got rough. Accubonds and TTSX bullets shot fine in that barrel after I quit using the VLDs.
RPMs:
7 mag 180 VLD @ 2950 fps 303,428
7 STW " @ 3100 fps 318,857
I emailed Bryan Litz and he suggested I switch to 180 target VLDs with their heavier jackets. That helped for a while then the bullets would show signs of damage and shoot several inches off POI at 100 yds. I tried moly coating the bullets and it worked for a while.
Other members told me before I built the rifle not to use a 3 groove with such a fast twist and they were right. The 3 groove is the issue as it damaged the jackets in the throat area once it got rough. Accubonds and TTSX bullets shot fine in that barrel after I quit using the VLDs.
RPMs:
7 mag 180 VLD @ 2950 fps 303,428
7 STW " @ 3100 fps 318,857
I have a Lilja 3G 1:8" in my .270 AI propelling the 175 Matrix VLDs at near 3100 FPS (2993 FPS current accuracy); no problem thus far with nearly 200 rounds through. I have another 3G 1:9" in ..308 cal but have not explored it further yet.
LumbermanP
Member
It typically takes 310,000-315,000 for jacket core seperation. FPSx720/twist rate
J E Custom
Well-Known Member
Different bullet designs and makers will take different RPMs and you just have to find the bullet that will handle the velocity/RPM.
I have had two rifles that would exceed some bullets ability to stay together. One was a 31" barreled 7 STW that could move a 140 grain bullet beyond 3800 ft/sec velocity. it could push through the 3700 ft/sec barrier but on approach to the 3800 mark the bullets started shedding there jackets as the bullet left the muzzle.I tried other brands of bullets, but they suffered the same problems. I then decided to try mono metal bullets and found the accuracy lacking even though they remained in once piece.
The other rifle was/is a 223 WSM in a long barrel that can exceed 4600 ft/sec with the right bullet. all others Give it up around 44 to 4500 ft/sec
Twist rates and velocity + bullet construction end up being the deciding factor.
J E CUSTOM
I have had two rifles that would exceed some bullets ability to stay together. One was a 31" barreled 7 STW that could move a 140 grain bullet beyond 3800 ft/sec velocity. it could push through the 3700 ft/sec barrier but on approach to the 3800 mark the bullets started shedding there jackets as the bullet left the muzzle.I tried other brands of bullets, but they suffered the same problems. I then decided to try mono metal bullets and found the accuracy lacking even though they remained in once piece.
The other rifle was/is a 223 WSM in a long barrel that can exceed 4600 ft/sec with the right bullet. all others Give it up around 44 to 4500 ft/sec
Twist rates and velocity + bullet construction end up being the deciding factor.
J E CUSTOM
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cmarchand
Member
Try the David Tubb TMS (Throat Maintenance System) if in fact you have throat problems.
Buzzsaw
Well-Known Member
some seeing this with the .22 creedmoor shooting too fast through a fast twist. forget the 55 gr they blow up on exit
Randy Tidwell
Well-Known Member
I shoot allot of fast twist barrels. At one point I was experiencing some 105 VLD bullets coming apart in flight. I was running them at about 3250 fps from a version of 6mmAI
I called Berger about it and was told that they should not be driven over 3150 from an 8 twist barrel because they spin apart.
So, I decided to calculate just what the RPM of a bullet would be from an 8 twist barrel at 3150 fps. I wrote a quick little spread sheet to make this calculation.
283,500 RPM, holy crap, this can't be right? Think about the centrifugal force on the thin copper jacket at that RPM, no wonder they were coming apart.
I called my son that has a Masters in Math to confirm my calculation. I only gave him the twist rate and velocity. He confirmed I was correct.
The formula to calculate RPM is:
Velocity * 60 *12 / Twist Rate
3150 fps * 60 = 189,000 feet Per Minute
189,00 fpm * 12 = 2,268,000 Inches per minute
2,268,000 inches/min / 8" Twist Rate = 283,500
I tried to attach a spread sheet that you can make RPM calculations for different barrel twist rates and velocity, but the extension is not acceptable.
If anyone wants a copy of the spread sheet, send me your email address, I will be happy to send it to you.
I called Berger about it and was told that they should not be driven over 3150 from an 8 twist barrel because they spin apart.
So, I decided to calculate just what the RPM of a bullet would be from an 8 twist barrel at 3150 fps. I wrote a quick little spread sheet to make this calculation.
283,500 RPM, holy crap, this can't be right? Think about the centrifugal force on the thin copper jacket at that RPM, no wonder they were coming apart.
I called my son that has a Masters in Math to confirm my calculation. I only gave him the twist rate and velocity. He confirmed I was correct.
The formula to calculate RPM is:
Velocity * 60 *12 / Twist Rate
3150 fps * 60 = 189,000 feet Per Minute
189,00 fpm * 12 = 2,268,000 Inches per minute
2,268,000 inches/min / 8" Twist Rate = 283,500
I tried to attach a spread sheet that you can make RPM calculations for different barrel twist rates and velocity, but the extension is not acceptable.
If anyone wants a copy of the spread sheet, send me your email address, I will be happy to send it to you.
Orange Dust
Well-Known Member
1+ Rough throat will cause this in a hurry. Don't believe it, get an 8 twist 22-6mm and shoot a cup and core bullet through it after the throat gets a little heat checked.
