Barrel twist rate and bullet weight questions

[Lasers]

So I built a 10.5" AR. Here are the specs:


Barrel Length: 10.5"
Gas System: Carbine Length
Barrel Profile: A2 Style
Barrel Steel: Cold Hammer Forged Chrome Moly Vanadium
Barrel Finish: Phosphate
Chrome Lining: Yes, bore and chamber
Muzzle Thread: 1/2-28
Chamber: 5.56 NATO
Twist Rate: 1:7
Barrel Extension: M4
Diameter at Gas Block: .750


Aero M4 lower with Aero LPK and Larue MBT-2S. (doubt that matters).


Here's the thing. I've read up and down about a 1:7 twist loving heavier projectiles. You can find threads on this very site with me actively trying to secure all kinds of variants to test.

I've now loaded and test fired many types from 66gr to 88gr.

I did ladders with my first batch and found she seemed to like 77gr with 24.4 grains of powder pretty well.


So went to reload some plinking rounds. Hornady 55gr FMJ, nothing special. I load them up with the 24.4 grains, since that's what I was set to.

For fun, I go ahead and load up three of each at the same grains: 66gr, 69gr, 77gr, 88gr

At 50 yards, all the heavier ammo was giving me 2-3" groups! But that 55gr plinkers I loaded, 5 shots under 1"....


Has my reading been wrong? Maybe my powder is off?

At the same time, maybe I'll just stick to 55's then?! I need to go back out and run it to 100 and see if the results are consistent, this is just not what I anticipated from reading.

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[FordPrefect]

Don't take my word as gospel on barrel twist, but from what I've read, the heavier bullets require a faster, 1:8 or 1:9, twist to stabilize them.

The AR that I use for CMP has a 1:9 twist and I'm using 75gr bullets. I can get 0.5" groups at 200 yards off a bench with this setup.

My "plinking" AR is a 1:7 that I use 55gr bullets in and I'm pretty sure I'm within 1" groups with that. I've never really tried to get it any better as it's just a "fun" gun.

Just my 2¢ worth, YMMV.

 

[Knute]

I too have read quit a few positive opinions about heavier 5.56 bullets but for the M4's 14.5" barrel length with the 1:7. Supposedly even some of the teams are using 75gr "heavy" bullets in their M4 carbines but I'm not sure about SBR like you're running.

My 14.5" carbine shoots real good with 75gr stuff using just a red dot but thats only my opinion. I haven't done any real testing.

Im sure there are plenty of SBR drivers on the forum with better experiences to share...thanks for sharing your results

 

[Lasers]

I'll get my sheets out and post pics and the rest of my data. Might as well keep a log here for the next guy trying to make a good round for his (or her) 10.5".

 

[DizzyJ]

55 and up to 80 or so should do well out of a 1:7 twist. Lighter bullets can shoot very well out of a faster twist. It's a try it and see thing. Your shorter barrel will probably not cause the lighter varmint Bullets to disintegrate. A long barrel with a fast twist can however be detrimental to accuracy with light Bullets.

 

[P7M13]

JBM - Calculations - Stability

www.jbmballistics.com

 

[flashpan]

Listen to your barrel

 

[Certaindeaf]

So I built a 10.5" AR. Here are the specs:


Barrel Length: 10.5"
Gas System: Carbine Length
Barrel Profile: A2 Style
Barrel Steel: Cold Hammer Forged Chrome Moly Vanadium
Barrel Finish: Phosphate
Chrome Lining: Yes, bore and chamber
Muzzle Thread: 1/2-28
Chamber: 5.56 NATO
Twist Rate: 1:7
Barrel Extension: M4
Diameter at Gas Block: .750
Gas Block Type: Geissele .750" Super Gas Block, Pinned to Barrel
Muzzle Device: A2 Flash Hider
Receiver Material: Forged 7075 T6
Receiver Type: M4
Handguard Type: Geissele 9.5" Super Modular MK14 M-Lok Rail
Bolt Carrier Group Included: No
Bolt Steel: N/A
Bolt Carrier Profile: N/A
Charging Handle Included: No
Overall Length: 19"

Aero M4 lower with Aero LPK and Larue MBT-2S. (doubt that matters).


Here's the thing. I've read up and down about a 1:7 twist loving heavier projectiles. You can find threads on this very site with me actively trying to secure all kinds of variants to test.

I've now loaded and test fired many types from 66gr to 88gr.

I did ladders with my first batch and found she seemed to like 77gr with 24.4 grains of powder pretty well.


So went to reload some plinking rounds. Hornady 55gr FMJ, nothing special. I load them up with the 24.4 grains, since that's what I was set to.

For fun, I go ahead and load up three of each at the same grains: 66gr, 69gr, 77gr, 88gr

At 50 yards, all the heavier ammo was giving me 2-3" groups! But that 55gr plinkers I loaded, 5 shots under 1"....


Has my reading been wrong? Maybe my powder is off?

At the same time, maybe I'll just stick to 55's then?! I need to go back out and run it to 100 and see if the results are consistent, this is just not what I anticipated from reading.

Well, since your pistol has a Geissele 9.5" Super Modular MK14 M-Lok Rail handguard, 55 grainers with little beards on them should be best.

 

[Kruel J]

Don't take my word as gospel on barrel twist, but from what I've read, the heavier bullets require a faster, 1:8 or 1:9, twist to stabilize them.

The AR that I use for CMP has a 1:9 twist and I'm using 75gr bullets. I can get 0.5" groups at 200 yards off a bench with this setup.

My "plinking" AR is a 1:7 that I use 55gr bullets in and I'm pretty sure I'm within 1" groups with that. I've never really tried to get it any better as it's just a "fun" gun.

Just my 2¢ worth, YMMV.

You're backwards. 1:9 is SLOWER than 1:7. Heavier projectiles do like faster twist rates.

1:9 is good for 40-62 grain projectiles.
1:8 is good for 55-77 grain projectiles.
1:7 is good for 68-90 grain projectiles.


These are ESTIMATES. YMMV.

 

[ferret427]

Barrel length won't affect the spin rate of your bullet, only the velocity once it leaves the barrel. A 20" barrel where the twist rate imparts a 250,000 rpm spin on a bullet will be the same as a 2" barrel with the same twist rate.

In addition, keep in mind that while for most projectiles, the weight of a bullet is used when talking about twist rates in regards to stabilization, it is projectile length that matters. Since most projectiles are lead cored, a straight comparison by weight is simple, because we typically refer to a particular bullet by how many grains it weighs. An increase in weight means an increase in the length of the projectile. This changes when using something like a copper monocore projectile, where for a given weight, you will have a longer projectile requiring a faster twist rate.

 

[edzz]

FWIW
Barrels have been known to have been mismarked.
Not saying that's the case however it's easy to confirm the twist rate.

 

[Medic!]

Velocity.

 

[DizzyJ]

Barrel length won't affect the spin rate of your bullet, only the velocity once it leaves the barrel. A 20" barrel where the twist rate imparts a 250,000 rpm spin on a bullet will be the same as a 2" barrel with the same twist rate.

Is not velocity part of the equation for spin rate? Bullet RPM= MV X 720/twist rate (in inches)
The faster the velocity, the higher the RPM of the bullet. A 20" barrel is going to net you a lot more velocity than a 2" barrel for real world comparisons of rifle ammunition.

Found this as well. Probably does a better job of explaining the relationship in regards to stabilization of projectiles.

You are being redirected...

 

[Hueco]

You've got more experience in this than me...

Having said that, seems to me that twist rate is an almost useless number without barrel length. The faster a bullet travels through the rifling, the faster it gets spinning, no? So a 1:7 10.5" barrel will spin a bullet slower than a 1:7 20" barrel simply due the higher velocities of the longer barrel.

If the heavies like a slow twist long barrel, then it stands to reason that a shorter barrel with the same twist would not get them spinning fast enough to stabilize. You have to find the sweet spot between too slow and too fast bullet RPM.

I could be very wrong here and hope someone corrects me if I am. But my current understanding would indicate that, if a 20" 1:7 barrel plays very well with a 75gr projectile, then that same bullet out of a 10.5" barrel would need a faster twist to get it spinning at the same RPM out of the barrel as it would out of the long barrel.

This would be why the lights work out of the shorter barre with the faster twist - because they can't get to over spinning like they could out of a long barrel.

Am I wrong? Did I just eff up my understanding of bullet flight dynamics?

 

[baker3gun]

+1 dizzy.

The spin rate in rpms is a function of the speed with which the projo is forced through the twist.

If your muzzle velocity is 5 fps, the spin rate will be much lower than the same projo fired from same barrel at 3000 fps.

 

[Kruel J]

Interestingly enough, it's still spinning at the same full revolution per inch.

Meaning a 1:7 barrel fires a projectile that spins 1 full revolution in 7 inches of travel. The faster the velocity, the faster it spins one full rotation, but still at the same distance of 7 inches. My head is spinning now......

 

[DizzyJ]

Interestingly enough, it's still spinning at the same full revolution per inch.

Meaning a 1:7 barrel fires a projectile that spins 1 full revolution in 7 inches of travel. The faster the velocity, the faster it spins one full rotation, but still at the same distance of 7 inches. My head is spinning now......

But the rpm will be faster which is largely what stabilizes the bullet.

Edit:
Which I guess is in a way what you said.

 

[baker3gun]

General comments:

A 1:7 is a faster twist than a 1:9. 1:7 means that the twist completes one full revolution in 7 inches.

Heavier longer bullets require faster spin rate to achieve and maintain stabilized flight. If you don't spin them fast enough, they can be wobbly.

Muzzle velocity is affected but not determined by barrel length.

Hypothetically (DO NOT DO THIS) you could have an AR with a 6" barrel and fire a 55gr fmj at 3000 fps by using a hot load of very fast pistol powder. The problem is that the pressure required to achieve that performance is way out of the design spec for the barrel and the primer and the brass and the bullet used in the platform. Something would blow up.

OP - it's generally bad practice to find a charge for one bullet and then just use the same charge for significantly different bullet weights. This is especially true when you find a load for a light bullet and just use that same charge for a heavy bullet because it will cause excessive pressure. The general rule of thumb is use less powder as bullet weight goes up. Also, keeping the same charge across multiple bullet weights pretty much never results in optimal performance for all the different weight bullets.

Hueco - twist rate can still be useful info without knowing barrel length because barrel length is not the only factor that affects muzzle velocity. You also have bullet weight and lubricity and burn rate and charge weight. But you're on the right track. Barrel length is definitely factor, just not the only factor.

In the 5.56 or .223, a 1:9 twist is fast enough to keep a 69gr projectile reliably stable out to 600 yards.
If you shoot long distance, heavier bullet is better.
This is why 1:8 and 1:7 twist gained popularity. People wanted to play in the 80 or 90gr neighborhood, and 1:9 just didn't reliably deliver at long range at those weights.

OP has a short barrel. He's not going to shoot it at 600 yards. He can't get a heavy bullet to go fast enough out of a short barrel to remain stable at long range. But, he can make it fast enough to be stable and avoid keyholing at 200 yards. And, his short barrel can't make a light bullet go fast enough to create a spin rate that is detrimental to the bullet, so he's good to go.

If I had to guess based on the info provided, OP discovered a truism: that this cartridge likes 55gr and will shoot it well across a wide range of velocities. It's like 130gr bullets out of a .270. Just sweet.

OP's gun is primarily a fighting weapon. (As opposed to hunting or target comp).
The barrel is short.
Fighting is a short range sport.
Even with a 1:7, it will be difficult to get the heaviest available bullets to shoot precisely at 150 yards because you don't have enough barrel to push them fast enough to make them really happy and quiet.
But I would bet that you can get some medium weight bullets to do pretty well out to 150 or maybe even 200.
And you already know that 55 is great.

FYI - I set steel plates (18x30") regularly at 400 and 500 yards when i was running a 3-gun match here. My son and I both regularly hit those targets with 55gr bulk Hornady fmj's fired at 2850fps from 16" 1:9 barrels. With iron sights. In front of many ST witnesses.

Good luck OP. Nice gun!

 

[808hondacrguy]

From AR15.com

 

[baker3gun]

You've got more experience in this than me...

Having said that, seems to me that twist rate is an almost useless number without barrel length. Addressed in previous post.

The faster a bullet travels through the rifling, the faster it gets spinning, no? Correct.

So a 1:7 10.5" barrel will spin a bullet slower than a 1:7 20" barrel simply due the higher velocities of the longer barrel. Correct as long as the other variables (bullet weight, lubricity, burn rate, and charge weight) are the same.

If the heavies like a slow twist long barrel, No, the heavies typically require a faster twist than the not-heavies, and do better with a longer barrel
which allows a slower lighter charge to get up to maximum safe speed which produces flattest trajectory and greatest momentum for carrying energy and speed as far as possible downrange before the dreaded subsonic transition.

then it stands to reason that a shorter barrel with the same twist would not get them spinning fast enough to stabilize. Unless you find a safe load that can push them fast enough through the short barrel to spin them fast enough to stabilize. But, generally, your statement is correct.

You have to find the sweet spot between too slow and too fast bullet RPM. Correct. To slow = not quiet and stable. Too fast = exploding or otherwise unstable flight.

I could be very wrong here and hope someone corrects me if I am. But my current understanding would indicate that, if a 20" 1:7 barrel plays very well with a 75gr projectile, then that same bullet out of a 10.5" barrel would need a faster twist to get it spinning at the same RPM out of the barrel as it would out of the long barrel. Correct but there are limits on twist rate. Best understood by imagining a 1:1 twist rate. If you blast a bullet into such an extreme twist barrel, it will slow down the acceleration event which will increase the pressure, and it will also catastrophically damage the bullet in all kinds of different ways.
Wrap your mind around those forces, and then just extend them linearly out to nominal twist rates. The slower the twist, the sweeter the journey through the barrel, but the less stability you get once the bullet exits the barrel. So we are back at finding your previously mentioned balance.
In my experience, 1:6 is stupid fast spin and pretty rare. I've never heard of 1:5.
Also - in your example above, if you want to be exhaustive, then you have to say that a faster twist from the 10.5" is one way to skin that cat, but the other way is to increase speed by increasing charge weight, if there is any room left to do that without causing excessive pressure or giant fireballs of unburned powder.

This would be why the lights work out of the shorter barre with the faster twist - because they can't get to over spinning like they could out of a long barrel.
Kind of. It would take a very long barrel and a big charge to spin a 55gr .223 out of a 1:7 fast enough to harm the bullet.
And, since light bullets are so fast in small bores, even if you do run into problems, you can always slow the bullet by 100 fps to solve the problem and still have a bullet that is screaming fast.

Am I wrong? Did I just eff up my understanding of bullet flight dynamics? no.

I should have quoted this earlier instead of just rambling. See red text above. HTH.