Help With Proper Twist Rate

10 June 2019, 05:55

Dutch44

I just slugged and measured my buddy's H&H .450 3 1/4 BPE double hammer rifle. Bore measures .447 and grove diameter is .4519 to .4520. Twist rate is 1 in 28 inches. I'm thinking a 275 to 300 grain lead projectile. I don't have the Greenhill formula so can anyone tell me if I'm in the right ballpark with the proper weight with this twist rate? The barrels or barrel flats doesn't give me any details. Thanks for any help.

10 June 2019, 06:08

45 2.1

Ballistic performance
Bullet mass/type Velocity Energy
270 gr (17 g) Lead copper-tubed 1,975 ft/s (602 m/s) 2,340 ft⋅lbf (3,170 J)

310 gr (20 g) Lead solid 1,800 ft/s (550 m/s) 2,240 ft⋅lbf (3,040 J)

325 gr (21 g) Lead copper-tubed 1,775 ft/s (541 m/s) 2,280 ft⋅lbf (3,090 J)

350 gr (23 g) 2,150 ft/s (660 m/s) 2,340 ft⋅lbf (3,170 J)

365 gr (24 g) Lead solid 1,700 ft/s (520 m/s) 2,340 ft⋅lbf (3,170 J)
Source(s): Barnes,[1] Kynoch[2] and Taylor.[3]

10 June 2019, 07:35

Geargnasher

350 grains or lighter will stabilize well in a 33" ROT or longer at any velocity the rifle is capable. That gives you a wide margin, so I wouldn't worry about stability unless you start bumping up near 400 grains.

15 June 2019, 14:57

The Dane

Length not weight is the key of finding max bullet for a given twist.

30 January 2021, 18:20

RJM.@Jax.FL

Dutch,

Did anyone ever get you the Greenhill formula.?
Note:
There is a school of thought which holds that The constant value in the formula varies with velocity. So the twist rate formula when using Black Powder is different.

I don't have my notes handy but will dig them up if you still need them

quote:
Originally posted by Dutch44:
I just slugged and measured my buddy's H&H .450 3 1/4 BPE double hammer rifle. Bore measures .447 and grove diameter is .4519 to .4520. Twist rate is 1 in 28 inches. I'm thinking a 275 to 300 grain lead projectile. I don't have the Greenhill formula so can anyone tell me if I'm in the right ballpark with the proper weight with this twist rate? The barrels or barrel flats doesn't give me any details. Thanks for any help.

31 January 2021, 03:02

Fury01

The whitworth of civil war fame fired a 530 grain bullet in a .451 bore of 1-20 twist quite well. I would expect a plus .002 over groove bullet of modern flat nose profile of 450-500 grains will work just fine.

31 January 2021, 03:25

Fury01

https://wikimedia.org/api/rest...2fa21cc8fe7e6654d156

Just google it. Copy paste on this website is shall we say problematic.

13 February 2021, 19:37

RJM.@Jax.FL

quote:
Originally posted by RJM.@Jax.FL:
Dutch,

Did anyone ever get you the Greenhill formula.?
Note:
There is a school of thought which holds that The constant value in the formula varies with velocity. So the twist rate formula when using Black Powder is different.

I don't have my notes handy but will dig them up if you still need them

quote:
Originally posted by Dutch44:
I just slugged and measured my buddy's H&H .450 3 1/4 BPE double hammer rifle. Bore measures .447 and grove diameter is .4519 to .4520. Twist rate is 1 in 28 inches. I'm thinking a 275 to 300 grain lead projectile. I don't have the Greenhill formula so can anyone tell me if I'm in the right ballpark with the proper weight with this twist rate? The barrels or barrel flats doesn't give me any details. Thanks for any help.

===================================
Greenhill Formula (simplified)

L=150 x D x D
___________
T
(that is, 150 x D squared divided by T)

L is the bullet length
The number 150 is a constant used by Greenhill and works well at velocities in the vicinity of 1500 f.p.s. or greater. At 2800 f.p.s. the constant can be changed to 180 with good results.

For Black Powder velocities use 125 as a constant instead of 150

Miller Formula:
https://en.wikipedia.org/wiki/Miller_twist_rule