Bullet head & twist rate for MOA accuracy in .30 bore at MV 1950 fps ?

[Gee]

Hi,

Advice requested:- Bullet head & twist rate for MOA accuracy in .30 bore at MV 1950 fps ?

Noting approximately that the best short range .30 bore ideal for accuracy is 110-125gr flat base jacketed bullet at a MV of 2,950 fps and a 1-17 twist, I am seeking to maintain at least MOA accuracy whilst supersonic, but from a reduced MV of 1,950 fps. Assuming zero wind, I seek to keep accuracy by increasing the twist to a faster rate than 1-17 to maintain the same bullet rpm stability at the lesser muzzle velocity of 1950fps.

Testing at 50 yards, with 123gr Lapua FMJ at a MV of 1950fps has seen at best 1 inch groups using 1-12 twist, and worse with 1-9 twist. This is in seeking to maintain MOA accuracy in a reduced load 308 Winchester, where rifles with twist rates at hand for testing are 1-9, 1-10 and 1-12.

And also to achieve MOA accuracy in a 1-10 twist heavy barrel 30 bore limited in muzzle velocity due to the small case capacity of a 30 Carbine chambering.

These are all heavy barrel bolt guns, shot cross decking a 5-25 power Schmidt & Bender scope.

Gee

 

[Catfish]

How do they shoot in the rifle with the 17 twist? I have had fairly good luck with accuracy with reduced loads in a rifle that shoot the bullets well at higher velocities.

 

[Gee]

Hi Catfish,

This link tells all about 30BR in 1-17 twist:-http://www.6mmbr.com/30br.html
I don't have that twist. I am looking for advice on what type and weight of head and rate of twist will give MOA accuracy in reduced loads at 1950 fps, if it is at all possible ? Being reduced loads in 308 Winchester (AW308) and full power loads in 30 Carbine heavy barrel 1-10 twist high end bolt gun.

Very disappointing to squeeze off a round at 50 yards in testing, and see the 123 gr Lapua head barely hold a 1 inch group at 50 yards at a MV of 1950 fps. Sure, 2400 fps and a 155gr SMK 1-12 twist will shoot a one hole group at 50 yards. But I can't push the 30 Carbine that fast even with a 123gr head.

Intrinsically I don't wish to believe that just because one is using a lower velocity, that accuracy has to suffer, assuming no wind and velocity to target remains supersonic.

Given that the 300 Blackout is being made with 1-8 twist barrels being forced to shoot supersonic light heads, in this twist optimised for 200 to 240gr subsonic, it must give give a pretty poor supersonic accuracy compromise.

More testing on my part. Just would be nice if someone had already obtained MOA in light 30 cal loads.

Gee

 

[jo191145]

Not a direct answer to your question but I have a 30BR cut with a standard reamer that is a 10 twist barrel 16" long.
No idea why someone built it but I had my reasons for buying it.

Surprisingly enough with 115 Bergers I seriously doubt that tube could shoot over 1 moa no matter what you do to it.
I've only had it as low as 2400fps and its capable of shooting 2's and 3's at that speed.

 

[MGYSGT]

Are you shooting the 30 carbine out of a M1 Carbine with standard sights? If so, 1" at 50 seems good to me. Or have you built a bolt rifle in 30 carbine. My M1 Carbine shoots those 110 gr Serria HP pretty well with H110. Really don't know the MV though.

Tom

 

[Gee]

Its a a grip cocking hence manually operaged AR-15 with a heavy stainless barrel in 1-10 twist, 30 Carbine chambering. I'll attach a photo when I've figured out how.......am writing this from an iphone. Have great hopes for better accuracy with 115 gr Speer hollowpoints, can' wait to test. Perplexing that the 123 Lapua FMJ has not performed well in any full or reduced loads in any 308 Winchester 1-9, or 1-12 twist.

Gee

 

[wapiti25]

The quality of your barrel and how "good" was it fitted and chambered will have a large effect on the accuracy. The auto loader guns required different fitting of all the parts because of the dynamics that go on during firing. You may want to go to a smith that has a reputation for producing accurate auto loaders.