Rifle Twist rate question....

[PrimalScream]

Im looking to purchase a AR-15 in the next few days pending a little more research. Im down to the last consideration before purchase and could use a little "enlightenment" on twist rates. I understand the concept that for lighter bullets in the 45-55 range will stabelize in 1 in 12 twist rate and that the heavier 75-80 grain bullets will be better in 1 in 7 twist rate barrels.

I prefer heavier bullets in this caliber. I think that the bulk of my shooting would involve 62-69gr bullets, however I might want to run some as heavy as 75 through it. If I got a 1 in 7 twist barrel it would stablize the heavier 75 gr bullets but how would it preform with 62 and 69 grain bullets?

I would greatly appreciate any feed back from those knowledgable in this subject!

 

[dakotasin]

62-69 would work fine. it wouldn't be optimal, so it is possible you'd sacrifice a tiny bit of accuracy, but nothing to get uptight over.

 

[PrimalScream]

Okay so your suggesting the 1 in 7 barrel will still do a decent job shooting the 62-69 grain bullets and allow me the option of the heavier 75's....

 

[dakotasin]

yes, but if you are going to focus on 62-69's, your best bet is a 9-twist.

personally, i don't like bullets heavier than 60 grains in a 223. just sacrifice way too much velocity, and on a windy prairie dog town at 400 yards... well, velocity is a good thing.

anyway, if you will focus on 77's and occasionally go to 62's 'just to see', 7-twist is the way to go.

 

[45crittergitter]

My 8" bolt gun puts both 75 and 55 grainers into 1/2 MOA or so.

 

[Red Tornado]

I asked this about a month ago, see this thread http://www.thehighroad.org/showthread.php?t=158429

Gungoesboom and others had some great info and pointed me to some great links. Hope it helps.
RT

 

[Zak Smith]

1:8" twist is sufficient to stabilize up to 77gr SMK's.

 

[PrimalScream]

Thanks for all of the replys and helpful info. Thanks especially for the link to a previous thread for which I somehow manage to miss reading before I posted.

 

[vanfunk]

In my exerience the 1/7" twist is the most versatile. There is no discernible accuracy penalty with using the faster twists and lighter bullets (I don't believe a bullet can be "overstabilized"; it's either stable or it isn't...). Very, very light varmint bullets may break up upon exiting the bore, so I wouldn't recommend the thin-jacketed 40 grain projectiles. My 1/7's handle 45 grian loads just fine, and of course the 77's are great, too. More and more AR makers seem to be coming around to the versatility of the 1/7" twist. IMO any accuracy differences between 1/7 and 1/9 twist barrels with bullets in the 50-70 grain range is due to variables other than twist rate.

HTH,
vanfunk

 

[GunGoBoom]

Okay so your suggesting the 1 in 7 barrel will still do a decent job shooting the 62-69 grain bullets and allow me the option of the heavier 75's....

Yes. I'd get the 1 in 7 if I were you. As long as you're not planning on using varmint style loads in it (35-50 gr bullets). 55s should do ok. 62s and up will be fine. A 1 in 9 twist won't work as well with 75/55/80.

 

[Crosshair]

Just adding my $.02. My 22-250 has a 1 in 12 twist. I can push a 55 grain bullet to 3800 fps in my handloads. Is there a way to figure out how fast a 60 grain (or heavier) bullet would have to be moving to be spinning fast enough to be stable?

 

[Jim Watson]

Any handbook load will stabilize a 60 grain flatbase spitzer in a .22-250.
My Ruger 77V has only a 14 twist and loves 'em.

I don't know how much heavier your 12 twist could go. Probably not much except for the 63 grain Sierra semipointed or 70 grain Speer roundnose.

 

[DMK]

IMO any accuracy differences between 1/7 and 1/9 twist barrels with bullets in the 50-70 grain range is due to variables other than twist rate.

I agree. Different rifles, sometimes even of the same make and model, just prefer certain types of ammo.

Just last weekend, I tested two different 1/9 ARs with heavy bullets. One was a 16" RRA and the other a 20" Bushmaster, both chrome lined, government profile(ie. lightened) barrels. I shot Black Hills 55gr, 68gr, 75gr, and 77gr at 100 yards. Neither is a match gun, but both shot the 75gr just as tight as 55gr. The 16" shot the 77gr about the same and the 20" shot it with about a .5" larger group. Neither shot the 68gr. well at all. I need to try some PMC and Federal 69gr that I have but forgot to bring.

 

[Crosshair]

Thanks Jim Watson. When I get around to it I will get some 60 grain bullets and try them out.