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Discussion Starter · #1 ·
Hello,
I don't know if there's an easy answer here or a resource someone could point me towards. I have a Bison Armory 18" SS barrel with 1:11 twist. Does anyone have a suggested round (weight) and how to calculate? Just target shooting +/- 100yards.
Thanks for your help.
 

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You can shoot any grain factory loads offered or hand load from 77-130 grain.

Loading and shooting heavier bullets Bison has 1:7 twist barrels that will also shoot the lower grain and heavier bullets well, and there's load information for loading 200 grain subs with the 1:7.

Sent from a final firing position, the crosshairs are on you!
 

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Discussion Starter · #5 ·
Thank you!

Thanks for the quick responses. I'm dying to try the new barrel. I previously had a 16" barrel that loved Federal 90g but hated the Hornaday 119g. It was odd. I hope the new barrel cures this. Glad to hear the range in weight will work since I've bought so many different ones.
 

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My bison 16" shoots the S&B 110 fmj pretty well. It also does well so far with 95 TTSX handloads. FWIW.

Ignore the .223 holes as I was working up a Mini-14 on the same target.

The S&B were from a completely clean barrel, the groups tightened up as I went through the 10 fouling rounds.

That small group is 29.4 gr of H322 in S&B brass, 95 gr TTSX, COAL 2.24. BR-4 primers. Going to reshoot that one again.

Shot at 100 yds, the small dots are 1" diameter.

Organism
 

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Twist rate stability calculator

I plugged a few numbers into the calculator, and most of the loads for 6.8 come back with a marginal factor with the twist rate my barrel has.

It seems that the calculator does not take into account the profile of the projectile.

I have always been of the belief that for a given bullet length, a boat tail was a more stabile projectile.

Also, it seems that when inputting the bullet length, the presence of a plastic tip should not be considered or at least not the entire length of the tip.

What say yee?
 

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Charts and formulas are just a guide. An 11-12 twist will shoot any 80-130gr bullet in a 6.8 except a 130 TTSX. The 13 twist barrels I made in 08 would shoot almost everything I tried at that time except the Barnes 120gr Solid(brass). It would shoot 130gr Sierra BT or Speer 130s. I did not try the 130TTSX, 130 or 140 Berger because they did not exist then.
 

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Charts and formulas are just a guide. An 11-12 twist will shoot any 80-130gr bullet in a 6.8 except a 130 TTSX. The 13 twist barrels I made in 08 would shoot almost everything I tried at that time except the Barnes 120gr Solid(brass). It would shoot 130gr Sierra BT or Speer 130s. I did not try the 130TTSX, 130 or 140 Berger because they did not exist then.
I don't doubt that my barrel will stabilize the bullets that have been/will be shot through it. No oblong holes so far even with the most mild loads.

My question was more to the validity of a one size fits all formula that does not factor in projectile profiles.

I have run the numbers on several bullets in 5.56, .243, and .264 mag. More than a few return a result of marginal, even though the bullets shoot well.
 

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What is the longest range you have fired the 6.8 with heavy bullets and measured group size? Is it possible that as the bullet rotation slows with longer flight times that stability degrades?

I believe this is a problem in the 5.56 that shows up at 1000 yards and beyond using the popular 1/8 twist. I think I read that the USAMU will only use 1/7 or faster barrels for this reason.
 

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What is the longest range you have fired the 6.8 with heavy bullets and measured group size? Is it possible that as the bullet rotation slows with longer flight times that stability degrades?

I believe this is a problem in the 5.56 that shows up at 1000 yards and beyond using the popular 1/8 twist. I think I read that the USAMU will only use 1/7 or faster barrels for this reason.
I would think that shooting at 1000 yds would be very heavy for caliber bullets and would need a fast twist barrel to stabilize them. It is my understanding that bullet rpm's slow less than fps.

The farthest I have shot is 200 yds with the 120SST. No indication of bullet yaw at that range. Group size was about 2.75 inches.

Perhaps the 120's would become unstable at very long ranges, but for me the rifles purpose is hunting.

Doubtful I would be taking shots at much over 200 yds.
 

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What is the longest range you have fired the 6.8 with heavy bullets and measured group size? Is it possible that as the bullet rotation slows with longer flight times that stability degrades?

I believe this is a problem in the 5.56 that shows up at 1000 yards and beyond using the popular 1/8 twist. I think I read that the USAMU will only use 1/7 or faster barrels for this reason.
Some of the very long high bc bullets used for that distance actually require a 1:7 twist. The idea is to stay supersonic rather than transition to subsonic and subject the bullets to being caught and passed by sound waves.
 

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Discussion Starter · #14 ·
What is the longest range you have fired the 6.8 with heavy bullets and measured group size? Is it possible that as the bullet rotation slows with longer flight times that stability degrades?

I believe this is a problem in the 5.56 that shows up at 1000 yards and beyond using the popular 1/8 twist. I think I read that the USAMU will only use 1/7 or faster barrels for this reason.
I've only been out 100 yards with the new barrel I can't ever imagin going for more than 500yds.
 

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Some of the very long high bc bullets used for that distance actually require a 1:7 twist. The idea is to stay supersonic rather than transition to subsonic and subject the bullets to being caught and passed by sound waves.
I would think that any sound waves catching up to any bullet would have dissipated enough to have no if any effect at all. I could be wrong.
 

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Hello,
I don't know if there's an easy answer here or a resource someone could point me towards. I have a Bison Armory 18" SS barrel with 1:11 twist. Does anyone have a suggested round (weight) and how to calculate? Just target shooting +/- 100yards.
Thanks for your help.
K: As the others mentioned, your 1in11 twist is adequate to stabilize a wide range of .277 bullets, see the picture posted at the top of the reloading subforum. I presume you are handloading your own ammo. BTW, its the length of the bullet, not its weight, that determines what twist is best. See any of the online charts based on Greenhill's formula.

In any event, there is another factor to consider. Any particular 6.8 barrel with a 1in11 twist will show a preference for certain bullets over others. The only way to test which one your rifle likes best, insofar as accuracy, is to buy a box of each likely suspect and try them with moderate loads. I suggest you start with Barnes 95 grainers, Speer 100s, Sierra 110s, Hornady 110 HPBTs, Sierra or Nosler 115s and Hornady 120 SSTs, as your budget permits. Hodgdon H332 or Accurate 2200 powder and CCI 450 or Rem 7-1/2 primers. The object is to develop a pet load then practice until you can put 10 rounds into less than an inch CTC at 100 yards/meters. good luck - CW
 

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K: As the others mentioned, your 1in11 twist is adequate to stabilize a wide range of .277 bullets, see the picture posted at the top of the reloading subforum. I presume you are handloading your own ammo. BTW, its the length of the bullet, not its weight, that determines what twist is best. See any of the online charts based on Greenhill's formula.

In any event, there is another factor to consider. Any particular 6.8 barrel with a 1in11 twist will show a preference for certain bullets over others. The only way to test which one your rifle likes best, insofar as accuracy, is to buy a box of each likely suspect and try them with moderate loads. I suggest you start with Barnes 95 grainers, Speer 100s, Sierra 110s, Hornady 110 HPBTs, Sierra or Nosler 115s and Hornady 120 SSTs, as your budget permits. Hodgdon H332 or Accurate 2200 powder and CCI 450 or Rem 7-1/2 primers. The object is to develop a pet load then practice until you can put 10 rounds into less than an inch CTC at 100 yards/meters. good luck - CW
Thanks , will be doing just that
 
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