[freeman4797]

Hello,

I am new to rifles and just ordered my first AR 15. After researching that, I landed on the fastest twist rate, 1:7 as I plan to shoot heavier bullets. My reasoning for this is the charts I read showed the heavier bullets flying faster and more EFP.

My question is, now that I am researching long range shooting, why are these even (much) heavier bullets spun on slower twist rate than the really light 223 rounds? I would have assumed that you would want to spin a heavier bullet even faster, but when looking at 308, 1:10 seems to be the fastest.

 

[gpark09]

Need to consider the speed of the projectile also.

 

[freeman4797]

Can you please elaborate? Why would an ever so slightly slower moving projectile also need to spin slower? What is the con of it spinning faster?

 

[Gunnn]

Hello,

I am new to rifles and just ordered my first AR 15. After researching that, I landed on the fastest twist rate, 1:7 as I plan to shoot heavier bullets. My reasoning for this is the charts I read showed the heavier bullets flying faster and more EFP.

My question is, now that I am researching long range shooting, why are these even (much) heavier bullets spun on slower twist rate than the really light 223 rounds? I would have assumed that you would want to spin a heavier bullet even faster, but when looking at 308, 1:10 seems to be the fastest.

The longer the bullet the faster the rate of spin for what ever caliber rifle this is to stabilize the bullet at longer distances

 

[freeman4797]

The longer the bullet the faster the rate of spin for what ever caliber rifle this is to stabilize the bullet at longer distances

If I understand correctly, you're saying a 223 bullet is longer than a 308, so it needs to spin faster because it is longer?

 

[Gunnn]

If I understand correctly, you're saying a 223 bullet is longer than a 308, so it needs to spin faster because it is longer?

No as the bullet gets heavier the longer it will be example a 52 grain .22 cal.bullet is shorter than a 75 grain .22 bullet and the longer the bullet the faster the twist needs to be to stabilize the bullet especially at long range if the rate of twist isn't fast enough the bullet will begin to wobble or maybe even tumble you can look on the berger website they can explain better than me

 

[freeman4797]

No as the bullet gets heavier the longer it will be example a 52 grain .22 cal.bullet is shorter than a 75 grain .22 bullet and the longer the bullet the faster the twist needs to be to stabilize the bullet especially at long range if the rate of twist isn't fast enough the bullet will begin to wobble or maybe even tumble you can look on the berger website they can explain better than me

I already understand that, my question is then why is twist rate slower for the heavier 308 bullet and faster for the lighter 556 bullet? By that logic i would think 308 barrels would have a fast twist, but they are 1:10 where most 556 are now moving to 1:7 even though the bullet is smaller and lighter.

 

[Gunnn]

I already understand that, my question is then why is twist rate slower for the heavier 308 bullet and faster for the lighter 556 bullet? By that logic i would think 308 barrels would have a fast twist, but they are 1:10 where most 556 are now moving to 1:7 even though the bullet is smaller and lighter.

It's caliber specific the take for instance the new 6.8 western (270) is 1 in 7 so you stabilize the the 175 grn bullets or heavier where the old 270 Winchester I think is 1 in 12 the centrifugal force of the faster twist helps with longer bullets it doesn't seem to matter in reverse 1 in 7 will be ok for lighter bullets

 

[Harper]

It's caliber specific the take for instance the new 6.8 western (270) is 1 in 7 so you stabilize the the 175 grn bullets or heavier where the old 270 Winchester I think is 1 in 12 the centrifugal force of the faster twist helps with longer bullets it doesn't seem to matter in reverse 1 in 7 will be ok for lighter bullets

You're only correct to a point saying lighter bullets are okay for a faster twist. I'm not sure that I'd recommend shooting a 40gr .223 bullet @ 3800 fps from a rifle with a 1:7 twist. The bullet would spin so fast that it might come apart in mid flight.

And to freeman4797 ...... you're almost comparing apples to oranges in trying to reconcile that twist rates of the .223 to the .308. The heavier .223 bullets need the faster twist to remain stable. The .308 doesn't. Look at the .50 cal BMG. Common twist rate there is 1:15. You simply cannot compare twist rates of different calibers.

 

[freeman4797]

You're only correct to a point saying lighter bullets are okay for a faster twist. I'm not sure that I'd recommend shooting a 40gr .223 bullet @ 3800 fps from a rifle with a 1:7 twist. The bullet would spin so fast that it might come apart in mid flight.

And to freeman4797 ...... you're almost comparing apples to oranges in trying to reconcile that twist rates of the .223 to the .308. The heavier .223 bullets need the faster twist to remain stable. The .308 doesn't. Look at the .50 cal BMG. Common twist rate there is 1:15. You simply cannot compare twist rates of different calibers.

Ok on the part that apples to oranges, but I guess I just want to know why a much heavier bullet actually spins slower? When looking at 223 by itself everyone says heavier bullet must go faster, so why not true for really heavy bullets?

 

[LensWork]

Hello,

I am new to rifles and just ordered my first AR 15. After researching that, I landed on the fastest twist rate, 1:7 as I plan to shoot heavier bullets. My reasoning for this is the charts I read showed the heavier bullets flying faster and more EFP.

My question is, now that I am researching long range shooting, why are these even (much) heavier bullets spun on slower twist rate than the really light 223 rounds? I would have assumed that you would want to spin a heavier bullet even faster, but when looking at 308, 1:10 seems to be the fastest.

Here are a couple of charts and calculators that may help in understanding twist rates and how caliber, bullet weight, sectional density, bullet length and velocity all affect the recommended twist rate and in-flight stability:

JBM - Bullet Length List

www.jbmballistics.com

JBM - Calculations - Stability

www.jbmballistics.com

 

[Slowndow9]

Hello,

I am new to rifles and just ordered my first AR 15. After researching that, I landed on the fastest twist rate, 1:7 as I plan to shoot heavier bullets. My reasoning for this is the charts I read showed the heavier bullets flying faster and more EFP.

My question is, now that I am researching long range shooting, why are these even (much) heavier bullets spun on slower twist rate than the really light 223 rounds? I would have assumed that you would want to spin a heavier bullet even faster, but when looking at 308, 1:10 seems to be the fastest.

To truly understand ballistics you should read the Dept of the Navy manuals on long range gunnery.
They explain twist rate as it applies to weight,speed,and mass of a bullet. And the effects of spin on stability and drift. And evn the effect of the rotational speed of the earth if your projectile is airborne for that long...

 

[djauofd]

Here is the 30,000 ft overview. Things two major factors that affect stability are weight of projectile and overall length. Because thirty caliber bullets have more volume per cross sectional area a bullet of equal length will take lest rotational force to keep it stable. Therefore 308,s will take less twist relative to length than a 5.56. All of this because volume goes up by the cube while surface goes up by the square. Berger has a stability calculator. You can go there and play with some bullets. You’ll get it then.

 

[freeman4797]

Here is the 30,000 ft overview. Things two major factors that affect stability are weight of projectile and overall length. Because thirty caliber bullets have more volume per cross sectional area a bullet of equal length will take lest rotational force to keep it stable. Therefore 308,s will take less twist relative to length than a 5.56. All of this because volume goes up by the cube while surface goes up by the square. Berger has a stability calculator. You can go there and play with some bullets. You’ll get it then.

Awesome, thanks. I'll start there.

 

[USNCPO]

Hello,

I am new to rifles and just ordered my first AR 15. After researching that, I landed on the fastest twist rate, 1:7 as I plan to shoot heavier bullets. My reasoning for this is the charts I read showed the heavier bullets flying faster and more EFP.

My question is, now that I am researching long range shooting, why are these even (much) heavier bullets spun on slower twist rate than the really light 223 rounds? I would have assumed that you would want to spin a heavier bullet even faster, but when looking at 308, 1:10 seems to be the fastest.

Sir, I believe you have it backwards. A 1:7 is faster than a 1:10 twist rate. 1 is one complete revolution in 7 inches of barrel length. 1 is one complete revolution in 10 inches of barrel length. A twist rate of 7 is completed before a 10, thereby faster.
1:7 the bullet completes 1 full rotation in 7 inches.
1:10 the bullet completes 1 full rotation in 10 inches.
Hope this helps.