Help me understand Rate of Twist!

hunter-4-life

hunter-4-life

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Ok here it goes. I know every rifle has a different ROT. I also know that the ROT affects how lighter or heavier bullets fly. Ex some rifles don't have a fast enough ROT to stabilize heavier bullets and others are too fast for lighter bullets.

Ultimately what im asking is what bullet weights would be "ideal" in my 308 win with a 24" 1-12" twist barrel? It shoots sub moa with 150's but id really like to load some 208gr A-Max's for some longer range fun, but if my ROT isnt fast enough to stabilize the bullets it wont be very accurate at 400+ yards.

Hope you guys understand what im tryin to spit out here. Merry Christmas all, and thanks for the help!
 
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hunter-4-life said:
Ok here it goes. I know every rifle has a different ROT. I also know that the ROT affects how lighter or heavier bullets fly. Ex some rifles don't have a fast enough ROT to stabilize heavier bullets and others are too fast for lighter bullets.

Ultimately what im asking is what bullet weights would be "ideal" in my 308 win with a 24" 1-12" twist barrel? It shoots sub moa with 150's but id really like to load some 208gr A-Max's for some longer range fun, but if my ROT isnt fast enough to stabilize the bullets it wont be very accurate at 400+ yards.

Hope you guys understand what im tryin to spit out here. Merry Christmas all, and thanks for the help!
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Most rifles that are made for shooting 200 grain bullets and heavier are 1 in 10 twist. I'm guessing the best bullets for the 1 in 12 will be 150 to 180 grain. However you never know until you try it. Just read where a guy couldn't get a rifle to shoot and the bullets that should have never worked shot the best. Its not just the twist rate that effects stabilization, it is also velocity... Some guns with a 1-14 twist with lower speeds will tumble bullets, if you speed them up they are just fine.
 
That 308 shooting 208 Amax would definitely have lower speed. Either the rifle would need a exceedingly long throat....and rifles with long throats that are designed for long projectiles would not have a 1-12 twist....or the projectile would have to be seated very deep in the case for the COAL to work in the magazine and throat....thus loosing powder capacity. Probably talking sub 303 British velocity in that case. It would seem that the 208 Amax would be a better match for a 300 WM or Rum.
 
nowarningshot said:
That 308 shooting 208 Amax would definitely have lower speed. Either the rifle would need a exceedingly long throat....and rifles with long throats that are designed for long projectiles would not have a 1-12 twist....or the projectile would have to be seated very deep in the case for the COAL to work in the magazine and throat....thus loosing powder capacity. Probably talking sub 303 British velocity in that case. It would seem that the 208 Amax would be a better match for a 300 WM or Rum.
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Ok thanks. I know it is a large bullet for a 308, even 180 is pushin the limits but I thought id ask. Im going to build a long range rig here in the near future as well and its goin to be a 300 of some sorts, probably a RUM.
 
hunter-4-life said:
Ok here it goes. I know every rifle has a different ROT. I also know that the ROT affects how lighter or heavier bullets fly. Ex some rifles don't have a fast enough ROT to stabilize heavier bullets and others are too fast for lighter bullets.

Ultimately what im asking is what bullet weights would be "ideal" in my 308 win with a 24" 1-12" twist barrel? It shoots sub moa with 150's but id really like to load some 208gr A-Max's for some longer range fun, but if my ROT isnt fast enough to stabilize the bullets it wont be very accurate at 400+ yards.

Hope you guys understand what im tryin to spit out here. Merry Christmas all, and thanks for the help!
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Your a bit long on twist. By atleast two inches
 
hunter-4-life said:
Ok here it goes. I know every rifle has a different ROT.
Nope, lot's of rifles have the same twist.

I also know that the ROT affects how lighter or heavier bullets fly. Ex some rifles don't have a fast enough ROT to stabilize heavier bullets and others are too fast for lighter bullets.
You are only half correct with this. Heavy bullets tend to be longer than lighter ones. The length of the bullet in relation to it's diameter is what effects it's optimal twist rate, not the weight. Berger actually recommends a twist rate for each bullet they sell, very handy.I just corrected this (Thanks Eagleye)

Ultimately what im asking is what bullet weights would be "ideal" in my 308 win with a 24" 1-12" twist barrel? It shoots sub moa with 150's but id really like to load some 208gr A-Max's for some longer range fun, but if my ROT isnt fast enough to stabilize the bullets it wont be very accurate at 400+ yards.
Bullets in the ballpark of 168-178 will likely be ideal (175 smk, 178 HPBT, 178 AMAX). The 208 A-Max might not work well with your twist. You probably need a 10" twist for that. It never hurts to try though if you are curious.

Hope you guys understand what im tryin to spit out here. Merry Christmas all, and thanks for the help!
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Just for fun one day I tried some 105 Amax's in my 243 Win that required a faster twist than I had. Best groups were about 4.5" at 100 yards and the worst wouldn't stay on the target. It was really amazing how poorly they performed. I was glad I did the test because I learned first hand how important twist rates are. This rifle shoots very well with other bullets.

I hope this helps.
 
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"The length of the bearing surface of the bullet is what effects it's optimal twist rate, not the weight". Quoting feetfats

Not quite correct....It is the overall length of the bullet in relation to it's diameter, not the length of the bearing surface [which is the part of the bullet in contact with the lands in the barrel]

Dave.
 
Eagleye said:
"The length of the bearing surface of the bullet is what effects it's optimal twist rate, not the weight". Quoting feetfats

Not quite correct....It is the overall length of the bullet in relation to it's diameter, not the length of the bearing surface [which is the part of the bullet in contact with the lands in the barrel]

Dave.
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OK, maybe I am wrong.
Later on I will go back and research this again but I was pretty sure it was bearing surface.
Do you happen to have a link? I think I had read about this from Berger.

The learning never stops :)
 
feetfats said:
OK, maybe I am wrong.
Later on I will go back and research this again but I was pretty sure it was bearing surface.
Do you happen to have a link? I think I had read about this from Berger.

The learning never stops :)
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Bearing surface has an effect on pressure - that is where you saw the reference.

Twist relates to length of the bullet.

Imagine trying to spin a bullet like a top and have it stable on a table. A short fat bullet (like a 180 gr 45cal) might spin ok. But a long skinny bullet, like a pencil, would have to be spinning at very high RPM to be stable.

The OP made reference to being stable after 400 yards. If a bullet is not stable, it is bad from the muzzle and gets worse. But if it shoots well at 100 yards, it actually gains stability as it goes down range. the RPM does not slow down much, but as the bullet slows down there is less air compression, and less RPM is required in a less dense medium.

So, it you want to see if you have adequate twist, shoot a 100 yard group.

My experience is that 1:14 shoots u to 180 Sierra MK, 1:12 shoots up to 200 Sierra MK.
 
Ganderite said:
Imagine trying to spin a bullet like a top and have it stable on a table. A short fat bullet (like a 180 gr 45cal) might spin ok. But a long skinny bullet, like a pencil, would have to be spinning at very high RPM to be stable.

The OP made reference to being stable after 400 yards. If a bullet is not stable, it is bad from the muzzle and gets worse. But if it shoots well at 100 yards, it actually gains stability as it goes down range. the RPM does not slow down much, but as the bullet slows down there is less air compression, and less RPM is required in a less dense medium.
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That actually makes a lot of sense!
I didnt know about the lesser air compression issue, although I had heard that a bullet will "settle down" after traveling some distance.
I heard it compared to a toy top that wobbles at first, then calms down and stays stable.

I recall reading a test the US Army did for AP ammo.
As bizarre as it sounds, they found AP ammo penetrated slightly MORE at 100Y, than at 25Y
At 25Y, the bullet struck the test plate slightly sideways, while at 100Y, it hit straight on.
 
tootall said:
That actually makes a lot of sense!
I didn't know about the lesser air compression issue, although I had heard that a bullet will "settle down" after traveling some distance.
I heard it compared to a toy top that wobbles at first, then calms down and stays stable.

I recall reading a test the US Army did for AP ammo.
As bizarre as it sounds, they found AP ammo penetrated slightly MORE at 100Y, than at 25Y
At 25Y, the bullet struck the test plate slightly sideways, while at 100Y, it hit straight on.
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The "settle down" has nothing to do with twist or velocity. Barrels whip when fired and the bullet wig wags for awhile. This was well documented in Mann's "The Bullet's Flight" (published 1909). He tested by shooting through a series of targets in 25 yard intervals. The first 3 targets showed egg-shaped holes.

For this reason, if you want to do a more accurate test of bullet performance on a medium, shoot it at 100 yards. Any closer and the bullet will be hitting it a bit sideways. (This refers to rifle bullets. I don't recall if he tested pistol bullets and I can't find my copy of the book as I write this.)
 
Ganderite said:
My experience is that 1:14 shoots u to 180 Sierra MK, 1:12 shoots up to 200 Sierra MK.
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X2!
I could shoot 190gr Hornady HPBT bullets in my 1:12 twist 308Win Sportco 44 rifle up to 500m (max at my gun club) with great accuracy and not even hint of understabilisation.
At the same time my handloaded ammo (for "bush" hunting) for my 1:12 twist 308Win hunting rifle had 200gr Nosler Partition bullet that was stable and accurate at 300m when I tried it.
OP.... the 308Win will give you all that it can do with 1:12 rifling twist.
 
Ganderite said:
This was well documented in Mann's "The Bullet's Flight" (published 1909). He tested by shooting through a series of targets in 25 yard intervals. The first 3 targets showed egg-shaped holes.

and I can't find my copy of the book as I write this.)
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Scribd.com has a copy!
 
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