salukipv1

Does twist rate affect velocity?

ie would a 165gr bullet coming out of a .30cal 1:11 be faster than say a 1:10 ?? Assuming same barrel length, same cartridge, etc...

or is it negligible?

stapher1

Yes, it does. A slow twist will be faster than fast twist. Think of a bolt with fine or course threads. The more the bullets turns in the barrel, you get more drag and a loss of speed. I don't know how much difference there is between the rates you have listed but i think it wouldn't be that big.

Nomercy448

I've actually heard this both ways. One guy says that the more turns in a barrel, the more drag, so the slower it goes. Another guy says the more turns in a barrel, the more drag, so the more pressure it builds against the progressive powder, so the FASTER it goes.

Honestly, I worry MUCH more about bullet stabilization than muzzle velocity. I can lob a 300grn bullet from a .45-70 a LONG ways out there and still make a hit, only travelling 2200fps at the muzzle. As long as my muzzle velocity is consistent from shot to shot, I wouldn't necessarily care if it's a touch slower, or faster, due to a different twist.

stapher1

Well i was also told that you can stablize a heavier or lighter bullet than your twist rate can shoot by using a heavier or lighter powder charge, but that seems too iffy to me. I've found that the listed velocity ratings in reloading manuals are wrong, but more of a ballpark number. I only believe what my chrony says.

Nomercy448

I didn't state that very clearly in my last post. I was trying to point out that I'd rather have a stable bullet at 2200fps than an unstable one at 4200fps.

Of course, muzzle velocity is directly linked to rotational velocity (RPM's) by the twist rate. My point was that it's much better to have a muzzle velocity that properly stabilizes the bullet weight I'm using than to just maximize my muzzle velocity.

I might be able to gain 100fps by using a 1 in 12" twist instead of a 1 in 8" twist, but a 1 in 12" won't stabilize a .223rem 68grn bullet nearly as well as a 1 in 8"... So even though the 1 in 12" barrel might need a little less dope, it isn't worth anything if my group opens up from 3/4" to 2".

Like you said, it's possible for a guy to bump up the power charge just a bit to use a heavier bullet in a slower twist, or to back off the powder to slow down a lighter bullet in a faster twist, but it really only works for a bullet weight that's on the fringe of your twist rate, meaning a guy might be able to push a 77grn bullet hard enough to stabilize well in a 1 in 12" .223, but it's not going to let a guy run a 90grn Berger. You're right, it's a little iffy, it works for a little ways, but at the end of the day, I don't like to depend on my powder charge (read chamber pressure) to make up for a poor choice of bullet weight for my barrel.

All the stars have to allign: powder charge, chamber pressure, bullet weight, muzzle velocity, caliber, barrel length, and twist rate all need to be in sequence to make it work, and as you well know, even when those stars all allign, all of the stars in the neighboring galaxy need to allign, and then all of the stars in the galaxy across the street have to allign too... These are just the short list to find a starting point, and beyond that is usually experience, skill, and a little luck, not just calculation.

Nomercy448

I definitely agree, if the bullet can take it, I want it spinning as fast as I can. I've never personally pushed one hard enough to frag out of the muzzle. I've ran down to 40grn bullets in a 1 in 8" .223 AI and haven't seen one pop yet (crappy accuracy from excessive throat jump, should have seen that coming ).

I suppose I could believe an over-spun bullet would foul more than a "normally" stabilized bullet, but I don't know that it'd be significantly MORE difference in fouling than the difference between a poor quality barrel, or too soft of bullets, etc etc.

Pawildman

[quote=Ridge Runner;3782429]well IMO an overstabilized bullet is a myth, if the bullet jacket can stand the rpms, you'll need a micrometer to see the difference, In my 8 twist 223, I've shot anything from 50 to 80 gr. the 55's shoot better in it than they do in my 9,10, or 12 twist 223's now is it barrel quality? don't think so the 12 twist is a lothar walther, the 8 is a harris.
some of the 9 twist remmy 204's shoot 32 gr v-max's incredibly well. my 8 twist 6.5 gibbs does way better than good with 95 gr v-max's. (I always twist a rifle one step faster than I know I need to shoot the bullet that I've planned for.)
RR[/quote)


..... Good point.