Building a load for 75 gr on 1/9 5.56

[DuGaul]

I've been reading (elsewhere) and learning from folks on this forum about hand loads.

From what I gather a 75 grain BTHP bullet is right on the edge of stability in a 14.5" barrel with 1/9 twist. Ie., on a cold day it might key hole, but on a hot day it might shoot fine.

Since the rifling twist imparts a spin velocity to the bullet based on the muzzle velocity, would it be possible to make a more stable shooting load for a 75 grain bullet by loading it close to the top 5.56 velocity?

On this theory a bullet shot through a 1/7 barrel will come out with the same spin rate as a bullet shot through a 1/9 with 28% higher muzzle velocity.

Example:

1/7 x 2500 fps = 1/9 x 3214 fps


Or would it just be a waste of time, better to stick with the lower weight bullets for a 1/9 carbine?

Thanks for input!

[5pins]

There is no way you can safely get an additional 700fps out of a 14.5 bbl. The max velocity I have gotten out of my 20in colt with the Hornady 75gr BTHP is 2770fps. I don’t think there is any way you can get 3200fps out of a 20in bbl never mind a shorty. Looking at my Nosler manual I see that even the 22-250 with a 24in bbl can’t make 3200fps.

I think you should stick with one of the 68 or 69grs if you want to use an OTM type bullet.

[DuGaul]

That was just an example of the conversion, I wasn't planning to try to build a 3200 fps load.

Your advice is well taken... Thanks!