Barrel twist rate vs bullet grain

[thexman]

Not sure where to post, if this is the wrong section, please help to move to the right sub forum.

I am getting a new barrel for target shooting, mainly around 3-700 yards, the factory barrel has 1:11 twist rate for a 24inch long .308.

I am not sure if 1:7 would be too much if I am only going to shoot factory ammo between 150-175gr? It doesn't always mean the higher twist rate, the better stability the bullet would be, right?

Thank you.

 

[308pirate]

A 1-7 rifling is excessive for the bullet weight range you're talking about. It might be necessary if you're going to shoot bullets heavier than 200 grains at subsonic velocity, but that's not what you're doing.

There's a real good possibility that you will have problems with bullets flying apart due to excessive rotational velocity with rifling that fast and supersonic muzzle velocities.

Pick any twist rate between 10 and 12 and worry about what really matters.

 

[thexman]

A 1-7 rifling is excessive for the bullet weight range you're talking about. It might be necessary if you're going to shoot bullets heavier than 200 grains at subsonic velocity, but that's not what you're doing.

There's a real good possibility that you will have problems with bullets flying apart due to excessive rotational velocity with rifling that fast and supersonic muzzle velocities.

Pick any twist rate between 10 and 12 and worry about what really matters.

Thank you, this is what I thought, the factory one is 1:11, so I thought anything close like 1:10 or 1:12 should be OK.

And I guess for 1:7 twist rate, it may need flatline bullet and used for extra long range distance shooting?

 

[rangerlong]

Bullet length relative to caliber determines the necessary twist rate for stability. Distance to target is not relevant.
Even the longest .30 cal bullets at typical rifle round velocities don’t need a 1:7 twist rate.
Excessive spin is undesirable because you will get more spin drift, and groups will open up with cheaper bullets that are not well balanced on their longitudinal axis.

Cartridges like .300 blackout are exceptions with long, heavy bullets and low, pistol velocities, so that’s why they need the fast 1:7 twist rate.

 

[RmeJu]

Try Berger's Twist Rate Calculator. You'll have to input some data about your bullet and conditions, but it should give you a good idea if the twist rate you were planning on getting is appropriate. You're looking to have the stability for your twist rate somewhere between 1.4/1.5 (barely stable) to 2.0 (starting to get overstable).

If you're planning on shooting more than one kind of bullet, that's ok, but you should check them all to make sure they all fit within the sweet spot. You probably won't be able to get all bullets working in one twist rate, since the shorter/lighter bullets probably need a slower twist than the faster/heavier bullets. One of the most important factors in the calculation is the length of the bullet. You can find a relatively complete bullet length library here.

 

[stello1001]

I guess another thing to take into consideration aside from bullet weight relative to barrel twist is MV. The slower your MV the more twist required to stabilize a bullet. This is why you see many .30 cal barrels with twists such as 1:7 or 1:8. Because they're designed to be used more than likely with 300blk rifles shooting subsonics; meaning they will be traveling at a velocity slower that 1100FPS. Because they're going so slow, in this case, a more aggressive twist is required in order for the bullet to stabilize throughout its flight path.

If I'm wrong, someone please correct me.

 

[RmeJu]

One thing I should clarify is that more stability is not always better. You can overstabilize a bullet, which can cause other problems I won't get into here (and I'm sure other members could speak to better than I), but if you use that calculator I linked to in my earlier post, you should be good to go on that front if you don't go over a stability factor of 2.0.

 

[Rant Durden]

I hate to assume, but I assume you reload. To me the point of reloading is to save money (cough cough) and to have options for different loads/rounds. Going to fast or too slow will limit your options in regards to changing things up. If you're good with a limited use, like running solids or just one type of round, go for it. But if you want versatility the middle is likely where you want to be. I do some competitions with an AR, but I just bought an RPR. I haven't used it to compete yet because i am shooting unleaded bullets (California dumbass law thing) to kill squirrels. Is it silly to use a chasis rifle to kill squirrels? Oh yea, but man it's fun and I'll be a better shooter for it. Versatility for me!

 

[thexman]

Bullet length relative to caliber determines the necessary twist rate for stability. Distance to target is not relevant.
Even the longest .30 cal bullets at typical rifle round velocities don’t need a 1:7 twist rate.
Excessive spin is undesirable because you will get more spin drift, and groups will open up with cheaper bullets that are not well balanced on their longitudinal axis.

Cartridges like .300 blackout are exceptions with long, heavy bullets and low, pistol velocities, so that’s why they need the fast 1:7 twist rate.

Thank you for the information.

 

[thexman]

Try Berger's Twist Rate Calculator. You'll have to input some data about your bullet and conditions, but it should give you a good idea if the twist rate you were planning on getting is appropriate. You're looking to have the stability for your twist rate somewhere between 1.4/1.5 (barely stable) to 2.0 (starting to get overstable).

If you're planning on shooting more than one kind of bullet, that's ok, but you should check them all to make sure they all fit within the sweet spot. You probably won't be able to get all bullets working in one twist rate, since the shorter/lighter bullets probably need a slower twist than the faster/heavier bullets. One of the most important factors in the calculation is the length of the bullet. You can find a relatively complete bullet length library here.

Yeah, I tried that calculator before I started this thread.

So If I am shooting .308 caliber with 168 or 175 VLD, BC is between .480-.500, not sure about the bullet length, so I just used the default value and muzzle velocity is about 2600-2700, twist is 7inches, temperature is about 20C/68F, 0 Altitude. I will get stability between 4.1-4.7, seems quite stable. If I change the twist rate to 10 inches, stability drops to about 2.3, and it is still stable.

 

[thexman]

I guess another thing to take into consideration aside from bullet weight relative to barrel twist is MV. The slower your MV the more twist required to stabilize a bullet. This is why you see many .30 cal barrels with twists such as 1:7 or 1:8. Because they're designed to be used more than likely with 300blk rifles shooting subsonics; meaning they will be traveling at a velocity slower that 1100FPS. Because they're going so slow, in this case, a more aggressive twist is required in order for the bullet to stabilize throughout its flight path.

If I'm wrong, someone please correct me.

Good to know this.

Just wonder what does MV stand for?

 

[thexman]

One thing I should clarify is that more stability is not always better. You can overstabilize a bullet, which can cause other problems I won't get into here (and I'm sure other members could speak to better than I), but if you use that calculator I linked to in my earlier post, you should be good to go on that front if you don't go over a stability factor of 2.0.

With that calculator, I used the data from Federal's website:

Buy Gold Medal Sierra MatchKing 308 Win Ammo 168 | GM308M

Shopping for the Gold Medal Sierra MatchKing - Learn more about the latest Gold Medal Sierra MatchKing and other hunting or shooting gear at Federal Ammunition.

www.federalpremium.com

Buy Gold Medal Sierra MatchKing 308 Win Ammo 175 | GM308M2

Shopping for the Gold Medal Sierra MatchKing - Learn more about the latest Gold Medal Sierra MatchKing and other hunting or shooting gear at Federal Ammunition.

www.federalpremium.com

Even the twist rate drops from 7inches to 10inches, on a average day, the stability factor is still over 2.0 between 2.1 and 2.7. Does that mean I need to drop to something like 11 or 12?
11 inches is what the factory barrel uses, then the stability factor stays at about 1.9, which is under 2.0, does that mean I'd better sticky with that 1:11 twist rate if I just want to shoot 308 for average factory match ammo?

Thank you

 

[thexman]

I hate to assume, but I assume you reload. To me the point of reloading is to save money (cough cough) and to have options for different loads/rounds. Going to fast or too slow will limit your options in regards to changing things up. If you're good with a limited use, like running solids or just one type of round, go for it. But if you want versatility the middle is likely where you want to be. I do some competitions with an AR, but I just bought an RPR. I haven't used it to compete yet because i am shooting unleaded bullets (California dumbass law thing) to kill squirrels. Is it silly to use a chasis rifle to kill squirrels? Oh yea, but man it's fun and I'll be a better shooter for it. Versatility for me!

Not yet. I am still shooting factory ammo at the moment. Probably will do reloading later.

 

[Deleted member 10043]

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.

bergerbullets.com

 

[rangerlong]

MV stands for muzzle velocity.
Optimal stability is 1.5. You don’t need any more than that.
1:10, or 1:11 would be fine for those bullets in .308 win, and still let you go a bit heavier/longer.
You’re not going to find many factory .308 win options with bullets heavier than ~185gr due to the 2.800” cartridge overall length saami spec they’re all loaded to.
1:7, 1:8 twist is for .300 blackout
1:8.5, 1:9 is for the big 30 cal magnums firing the 230-250gr bullets.
Those twist rates are excessive for .308 win and you could see negative effects down range.

 

[thexman]

MV stands for muzzle velocity.
Optimal stability is 1.5. You don’t need any more than that.
1:10, or 1:11 would be fine for those bullets in .308 win, and still let you go a bit heavier/longer.
You’re not going to find many factory .308 win options with bullets heavier than ~185gr due to the 2.800” cartridge overall length saami spec they’re all loaded to.
1:7, 1:8 twist is for .300 blackout
1:8.5, 1:9 is for the big 30 cal magnums firing the 230-250gr bullets.
Those twist rates are excessive for .308 win and you could see negative effects down range.

Oh yeah, Muzzle Velocity, I should know that means MV?

I think I will go something like 1:10 or 1:11. Not going to shoot anything heavier than 175gr, mainly 150-170gr.

Thank you for the explanation.

 

[stello1001]

Good to know this.

Just wonder what does MV stand for?

Your MV is going to be your muzzle velocity.

 

[W54/XM-388]

If you are going to shoot the 155gr stuff close up, it really works well in the 1:11 and 1:12 barrels from what I've seen.
And those same barrels will still do good on the 175gr.

 

[bschneiderheinze]

1:11 best of both worlds some 1:12’s will shoot 175’s good some 1:10’s will shoot 155’s good. Stick with 1:11 and you should be good depending on MV

 

[supercorndogs]

My factory Remington 1 in 12 shot with the 208 a max.

 

[chevy_man]

One thing I should clarify is that more stability is not always better. You can overstabilize a bullet, which can cause other problems I won't get into here (and I'm sure other members could speak to better than I), but if you use that calculator I linked to in my earlier post, you should be good to go on that front if you don't go over a stability factor of 2.0.

I would worry less about over stabilizing and more about under.

Example: I just ordered a 1:7 twist 25-06. I'll be way over stable pushing 120grn partitions for hunting, at 75*. At 0* I'm safely above 2, but not by much.
With a 1:8 I could shoot blackjacks at 50*, but at 20* they'll be down at 1.2.

Weather and elevation play huge into the stability. If you live where it's 100* temp swings and you may vary from 3-8000+' elevation it's a little difficult to figure out where that "marginal" area is and error to a safe side.

 

[308pirate]

Jesus Christ on a cracker, OP you are seriously over thinking the fuck out of this.

Marine snipers have killed truckloads of people with M40 rifles with 1-11.25 (IIRC) twist barrels from hot ass deserts to humid ass jungles to frigid ass mountains. I doubt they worry as much as you about this.