What Twist Rate for a 6.5 Grendel hunting rifle?

[melensdad]

I've gotten word that my new CZ 527 Carbine has shipped to my dealer. Currently its chambered in 7.62x39.

My plan is to have it rebarreled by McGowen in 264 LBC with a fairly light contour barrel to keep the weight down and a fairly short barrel in the 18" or 19" range.

The gun will be primarily used for hog hunting. I'm thinking the bullet weights most likely used will be in the 120 to 130 grain range. Given the roughly 18" barrel length and the likelihood of most bullets being in the 120/130 grain weight range, what would you recommend I use for a twist rate to stabilize the bullets.

[BjornF16]

I ordered a barrel from Pac-Nor with a 1 in 8 twist, as my primary aim is to shoot 129 - 140 grn bullets.

If you are staying in 120-130 range, then either 1 in 8 or 1 in 9 twist should work.

I believe most of AA barrels are 1 in 9; my Satern barrel is 1 in 8.75?

[Whelenon]

Theoretically, the heavier bullets for a Grendel a 1-8 twist recommended and with the lighter bullets the 1-9 twist is recommended.

Ref:

http://www.varminthunters.com/tech/barreltwist.html

[melensdad]

Among other offerings, the choices from McGowen include a 1:8, a 1:8.5 and a 1:9 twist rate.

Which would be the optimum choice?

[adamjp]

1:9 will handle 120gn bullets. Are you really going to shoot a heavier bullet from the Grendel case?

My new barrel is a 1:10 as I know I won't be going larger than the 120gn bullet.

[LRRPF52]

The bullet weights don't really matter as much as the bearing surfaces and velocities you intend to push them. I think 1/8.5 seems like a good compromise, but Bill A. found that 1/9 worked better for some reason after a lot of testing with the Grendel/AR15.

The traditional school of thought is that longer bearing surface pills need a faster twist, which makes sense. My buddy who has a 22" CZ527 Grendel is getting 2650-2700fps with 139gr Scenars on top of N140, so you will be looking at muzzle velocities that are no longer limited by the 50k psi AR15 bolt.

This means a lot of the longer and heavier pills can be pushed at faster speeds, but I think the 129gr SST run at 2600fps would be an awesome cartridge for this. So would the 125gr Nosler Partition, or any of the 140gr hunting pills.

Another awesome bullet I wouldn't rule out would be the 156gr Norma Oryx, which is devastating in penetration and expansion, even from lower velocities. I saw what it can do this past winter on an unbelievably large rabbit, and it left a 4" exit on him out of a 6.5 Swede going pretty slow.

I would consider going with a 20" pipe, but it won't make a huge difference in mv. Looking forward to seeing how your build turns out.

[BjornF16]

Here is a thread where Bill Alexander weighed in: http://www.65grendel.com/forum/showt...est-twist-rate see page 1 for a couple of his responses.

Mind you, he was discussing gas guns specifically. With the bolt gun, the pressure can be raised from the 50,000 psi gas gun limit to approx 57,000 psi.

I am choosing the 8" twist for my gun to give me the option of shooting 130 and 140 grn Berger VLDs and stabilize them.

For your case, either the 8" or 8.5" would be fine. BStokes uses a 8" twist in a 20" bolt gun and has good results shooting 130 grn bullets.

[melensdad]

I would consider going with a 20" pipe, but it won't make a huge difference in mv. Looking forward to seeing how your build turns out.

I'm actually thinking of shortening the barrel a bit, maybe down to 17" and installing one of these muzzle brakes, which adds 2" giving me a 19" overall length.

LINK => http://intlmidway.com/intl/eproductp...eitemid=524840

[BjornF16]

I'm actually thinking of shortening the barrel a bit, maybe down to 17" and installing one of these muzzle brakes, which adds 2" giving me a 19" overall length.

Personally I wouldn't bother (just my opinion)...the Grendel doesn't produce that much recoil. Although, that being said, I have one of Warped's Ti m.b.'s (very lightweight).

My buddy who has a 22" CZ527 Grendel is getting 2650-2700fps with 139gr Scenars on top of N140, so you will be looking at muzzle velocities that are no longer limited by the 50k psi AR15 bolt.

Another awesome bullet I wouldn't rule out would be the 156gr Norma Oryx, which is devastating in penetration and expansion, even from lower velocities. I saw what it can do this past winter on an unbelievably large rabbit, and it left a 4" exit on him out of a 6.5 Swede going pretty slow.

LRRPF52...any chance you can get that recipe for 139 Scenar?...I'm definitely interested in trying the Berger 140 VLD and that should be close. Never considered heavier, but the 156 grn Norma Oryx sounds intriguing...

[Bwild97]

I have a 24" tube with 1:9 twist and it is stableizing 130g NORMA's and 142g SMK's just fine!

[bwaites]

2 things to point out:

1) AA decided on twist after a lot of trial rounds were put downrange. 1:9 worked great in longer barrels, and 1:7.5 in shorter.

2) No one did any testing on bolt guns and twist rates.

Per Bill Alexander:

When we were first working on the Grendel we followed what everyone said about 6.5 calibers and made the barrels with 8 twist rifling. Some barrels would shoot but more often than not the groups were open and the guns were tempremental. One of the very early test rigs had run a 9 twist and it actually shot well despite a less than optimum chamber so out of frustration we ignored conventional wisdom and build some 9 twist units at 24" and 20" barrel lengths. The change in accuracy for the group as a whole was unquestionable. Subsequent production of 9 twist barrels has proven that this is optimum for the caliber.

One of the contributors to the 9 twist is actually the Grendel itself. Pressure limits the ability to slug the bullet to the grooves and the case capacity holds the round to lighter bullets. The 168 grain roundnose nickel steel projectiles are simply not a consideration any more than the 142 and 155 grain SMKs

We have tested barrels as short as 14.5" in 9 twist (not recommended use 1:7.5 this short) with projectiles up to 152 grains and both accuracy and stability are maintained. Equally we have run 9 twist barrels out to 1600 yards (12" group) and the round remains stable and it would appear accurate through the transition velocity. Longer 8 twist barrels often demand velocity reduction for best accuracy

It is our conclusion that the 9 twist offers the best solution for the Grendel for barrels of 20" and longer while the 18" and shorter barrels thrive on 7.5 twist.

[BjornF16]

I have a 24" tube with 1:9 twist and it is stableizing 130g NORMA's and 142g SMK's just fine!

Every tube will have its own "personality"...

Litz's data says the SMK 142 has a "nominal" stability factor of 1.36 and worst case of 1.18 (less than 1.4 indicating marginal stability; less than 1.0 being unstable) with a 1:9 twist. Stability factor improves to 1.72 and 1.49 respectively for 1:8 twist.

His data also indicates Berger 140 VLD has 1.22 nominal and 1.05 worst case from 1:9 twist; this improves to 1.54 nominal and 1.33 worst case for 1:8 twist.

Of course, this is all "theory" and ymmv...

As I responded to OP earlier, the 8, 8.5 and 9 twists would all work for his stated range of 120-130 grn bullets (unless he plans on Berger 130 VLD which Litz indicates has a stability factor of 1.35 "best case"; 1.02 worst case for 1:9 twist)...

FYI: From Litz's book (Applied Ballistics for Long Range Shooting):

Stabiility Factor (SG) using Miller Stability formula:
Best case: 1000' altitude, 100% humidity, 100 F, 3000 fps mv
Nominal case: S.L., 50% humidity, 59 F, 2800 fps
Worse case: S.L., 0% humidity, 0 F, 2600 fps

Stability factor, in theory, improves as altitude is increased...and becomes more important with the longer range at which one is shooting.

(btw, I'm an engineering geek turned fighter pilot...I enjoy the theory, but not as much as the application of firepower ;-)