Lija 1:7 30cal

[AZShooter]

A friend and I had a bad experience with 1 in 7 twist 3 groove 7mms. In short order Dan's 7 STW damaged the 180 berger hunting bullets causing them to land way off the mark at 100 yds at just over 100 rounds through the barrel. Same for my 7 rem mag with round count a right around 200. At the advise of Bryan Litz I switched to 180 target bullets which lasted another 100 rounds or so then the accuracy died again. Moved to monos or bonded which held together.

In contrast had a 1 in 7.5 5R 6mm-284 that never had an issue for the life of the barrel with either 107 SMK or 105 Berger hybrids.

IMO high RPMs combined with a 3 groove is a recipe for disaster.


RPM formula: (MV x 720) divided by twist

7 Rem mag 303,428
7 STW 318, 857
6mm-284 with 105s 336,000 107 SMK 331,200
The 5R had higher RPMs but never had an issue.

I agree with NEMThunter get a 5R and stay away from fast twist 3 groove barrels.

 

[MagMan]

A friend and I had a bad experience with 1 in 7 twist 3 groove 7mms. In short order Dan's 7 STW damaged the 180 berger hunting bullets causing them to land way off the mark at 100 yds at just over 100 rounds through the barrel. Same for my 7 rem mag with round count a right around 200. At the advise of Bryan Litz I switched to 180 target bullets which lasted another 100 rounds or so then the accuracy died again. Moved to monos or bonded which held together.

IMO high RPMs combined with a 3 groove is a recipe for disaster.

I was in the same boat with the same barrel in a 7RUM. Blew apart before 100 yards.

 

[HARPERC]

Each bullet type come with unique characteristics to overcome. The extremes reveal these.

 

[RH300UM]

We have been dealing with this in @HARPERC 's 6.5 Ultracat. The long bullet that we designed for the 6" twist shot great until we reached 3300 fps. Lost stability. We first thought we were slipping in the rifling so we caught one to examine the bullet engraving. No slipping, nice clean engraving. The dynamic stability looks to be the culprit. The faster the bullet is going the more force is exerted on it causing it to tumble. The longer a bullet gets for caliber the more prevalent the problem gets.

Only downside to faster than needed twist that we have seen is that it causes more torque on the rifle, making it more difficult to shoot.

Kirby Allen ran into this some years ago. If you have a case capable of higher velocities and a conventional twist to match bullet weight then stability goes down.
Example. We have found that a 195 berger 7mm bullet degrades stability once it hits 3200fps in a 8.4 twist.
Moving to a 9 twist and pushing velocity over 3300 fps smoothed things out

 

[RockyMtnMT]

Kirby Allen ran into this some years ago. If you have a case capable of higher velocities and a conventional twist to match bullet weight then stability goes down.
Example. We have found that a 195 berger 7mm bullet degrades stability once it hits 3200fps in a 8.4 twist.
Moving to a 9 twist and pushing velocity over 3300 fps smoothed things out

Miller stability formula shows that we have over 1.5 sg. McCoy's formula calls for much higher twist rates for the same bullets. This formula uses material density in the calculations which makes sense to our testing.

Dynamic stability really makes your head swim!

 

[HARPERC]

......Too bad I'm not a rocket scientist.......

I think Berger has a couple, and their 6.5 is still on the board.

 

[RockyMtnMT]

Took me a sec. Drawing board. I'm not super fast.

 

[HARPERC]

Just some perspective on how difficult a task you've undertaken!

 

[RH300UM]

Just some perspective on how difficult a task you've undertaken!

RockyMthMt
You like it or like/hate it?

 

[RockyMtnMT]

RockyMthMt
You like it or like/hate it?

I can think of a lot worse things to spend time and energy on trying to figure out! Just wish we had more time to spend just on it.

 

[Gord0]

Throwing out a possible theory, or something to test if someone has high speed video capabilities. We've all seen a bullet leave the barrel in slow motion. It has that wobble to it. I wonder if the combination of high velocity, and really long bullets causes too much of this wobble, and the fast twist destabilizes this mess. Also causing the bullet to not only spin around it's axis, but also rotate, or "wobble" around it's center of gravity causing air pressure to want to turn the bullet backwards even more.