I think what I want is a 20" carbine size varmint barrel. 50gr to 70gr bullets inside 250 yds.I doubt there is any need over 69 SMKs.
I think 7-8 twist is too tight. Muzzle piece not needed, but a thread cover is ok

I have a Rock River upper receiver and bolt assy. ready to go. I would want the CMP TRO hand guard.

1-10 is ideal for 50 or 55 grain bullets.

If you are going to shoot 69s, you probably actually want a 1-8.

I've fired 50 gr VMAX through a 1-8 and it was very accurate.

what do you mean when you say 'carbine size'?


With a 20" barrel, you want a rifle length gas system.

quote:

Originally posted by Hamden106:
I think what I want is a 20" carbine size varmint barrel. 50gr to 70gr bullets inside 250 yds.I doubt there is any need over 69 SMKs.
I think 7-8 twist is too tight.

Uncertain what "carbine size" means. Gas should be rifle length for a 20" barrel. If you mean barrel profile, then the typical carbine-type-profile works. Don't go with a pencil profile barrel, as its accuracy will likely suffer as the barrel heats and cools.

At my altitude, 69 SMK bullets are stabilized by a 1:9 twist. At sea level a 1:9 twist should still be fine for 69 grain loads, but likely nothing heavier.

I've shot Hornady 40 and 50 grain Vmax loads with an 18" 1:7 twist barrel with very good accuracy. Faster twist barrels shoot light bullets well, if the bullets are made well.

My most accurate barrels have 1:8 or 1:7.7 twists. They shoot 40-77 grain loads just fine. I recommend 1:8 twist.

By "carbine" I mean not a bull bbl.

I've shot Sierra's from 52 gr to 69 gr with great accuracy out of 1:9,

both guns did not like 75 gr,

both are Rifle length 20" barrels, button rifled vintage SGW barrels

quote:

Originally posted by Hamden106:
By "carbine" I mean not a bull bbl.

I have two Larue Stealth barrels, they are a medium heavy contour and both of mine shoot both light and heavy bullets very well.

If you were looking to go lighter than that, the Larue PredatAR barrels are a skinnier contour that are reputed to still shoot very well. But I have never owned one.

There's really no downside to a 1:8 twist. A quality 1:8 will shoot any bullet weight well.

According to the ballistician Bryan Litz, spinning a bullet faster than needed for stabilization at the ranges and under the environmental conditions it’s being fired at has the disadvantage of increasing the adverse effects on precision that any defects in the bullet will have. I.e., if the bullet isn’t made perfect with no variations in the core or jacket as compared with another bullet, a faster spin rate will increase the effects of any imperfection.

That said, I believe it’s generally accepted that for a 69 grain lead core bullet the rifling twist rate should be 1/9" or faster. And in practice if a faster rate might have some tiny effect on precision and accuracy, that’s nothing compared with a bullet’s becoming unstable in flight before it hits the target. According to Litz’s book Ballistic Performance of Rifle Bullets, Third Edition, the 69 grain Sierra MatchKing requires at least a 1/9.5" twist “for optimal performance.” Bullets that are heavier (longer, actually) may require faster twists.

quote:

Originally posted by sigfreund:
According to the ballistician Bryan Litz, spinning a bullet faster than needed for stabilization at the ranges and under the environmental conditions it’s being fired at has the disadvantage of increasing the adverse effects on precision that any defects in the bullet will have. I.e., if the bullet isn’t made perfect with no variations in the core or jacket as compared with another bullet, a faster spin rate will increase the effects of any imperfection.

Thank you for the information. I assume the first part of your post was in response to my deleted post. I deleted my post when I noticed the age of the OP. It must have been before you made your submission.

For the sake of discussion and expanding my knowledge: are these effects commonly seen with 5.56/.223 1:7 twist barrels with 55gr bullets?

quote:

Originally posted by DuMO:
For the sake of discussion and expanding my knowledge: are these effects commonly seen with 5.56/.223 1:7 twist barrels with 55gr bullets?

I can’t answer that based either on my experience or research.

From a practical standpoint, though, I doubt that would be much of concern. Litz says that benchrest shooters who are striving for maximum precision at (usually) relatively short ranges, use flat base bullets because their shorter length as compared with boattail varieties permits them to use slower twist rate barrels. Those shooters, though, are unlikely to use the cartridges you mention for their sport because there are many others that are more suitable.

For shooting at long ranges and striving for maximum accuracy (hitting a specific target), though, shooters usually use other cartridges as well, but if they used something like a 223 Remington, they’d want heavier (longer) bullets for their stability at long ranges, but particularly for their resistance to wind drift which is a major factor in things like Precision Rifle Series competitions. For top tier bullets the advantages of slower twist rates are probably overcome by those factors, plus the best quality 0.224" bullets are (as far as I know) almost always heavier and longer. I doubt there’s much demand for premier 55 grain bullets in that diameter.

I don’t want to overemphasize the disadvantages of faster rifling twist rates, and I don’t believe Litz does either. My comments were because I’ve run across a lot of misinformation about twist rates, it can be useful to know one of the technical disadvantages of an unnecessarily fast rate and what it’s actually based on. For years I’ve read about bullets’ being “overstabilized” by faster rates, and that never made any sense to me. Litz’s discussion does.

Someone will probably come along and point out that fast twist rates, high velocities, and thin bullet jackets can actually result in bullets’ disintegrating in flight because they are spinning so fast. That’s primarily been reported by shooters firing 0.22 caliber “varmint” bullets at very high velocities.

Last, for best wounding effects with military cartridges and nonexpanding bullets, the lower stability imparted by slower twist rates is generally accepted because the bullets upset and start tumbling faster when they hit their targets. That’s why the original 5.56 loads with 55 grain bullets at higher velocities from slower twist barrels as were used in Vietnam were considered by some to have better wounding effects than loads like the M855 from 1/7" bores. Of course in military combat not all bullets fired hit enemy soldiers directly, and fast, light, unstable bullets aren’t ideal in all situations. Plus we sometimes want greater penetration than light bullets produce.

And last, last, the question about wounding effects is, “Better than what?” Heavier 5.56mm load bullets have reportedly gained much favor in US military circles because they provide the advantages of heavier bullets but at the same time destabilize quickly in flesh because of their longer lengths.