Being a muzzleloader shooter I know a little bit about the relationship between bullet lenght and weight and twist rate. Generally, the longer a bullet is for a given caliber, the faster the twist needed to stabilize it.

I'm now looking into buying a .22-250 target rifle, but I'm confused about the inconsistancy in twist rates between different rifles shooting .224 diameter bullets. The M16A2 rifle that I was issued in the Marine Corps had a 1 in 7" twist which was adopted to better stabilize the newer 62 grain bullets. The A1 variant had (correct me if I'm wrong about this) a 1 in 10" twist for the older 55 grain bullets. Researching .223 Rem target rifles I find that most have 1 in 12" twist, and most .22-250's have 1 in 14". Yet, all these rifles shoot the same .224" bullets in the same weights (and lengths).

The only explanation I can think of for the difference between the .223 Rem and the .22-250 are the .22-250's higher velocity. Doing a little math I figured (based on the data provided for both rifles on Hornady's website) that a 55gr bullet with a mid-power .223 Remington should be traveling at roughly 3000fps. Out of a 1 in 12" twist bore the bullet would be rotating 3000 revolutions per second or 180,000rpm. At 1 in 7" the same bullet would be rotating approx 308,500rpm. A mid-power .22-250 with the same 55gr bullet should make approx 3500fps at the muzzle. In a 1 in 14" bbl that would be about 180,000rpm. So my conclusion would be that the optimum twist rate depends on bullet velocity as well because the bullet rpms would be similar.

Is this a correct conclusion? If so, why would the M16A2 need so much more twist than an ultra-accurate target/varmint rifle, since even with a slower 60gr bullet a 1 in 7" twist would produce a heck of a lot more rpms (60gr at 2800fps, 1 in 7" equals about 288,000rpm)?

Mike


Gun control means putting the second bullet through the same hole as the first- Ted Nugent
NRA Member
Muzzleloading- Once you go black, you'll never go back!!

But the SS 109/M855 5.56mm NATO green tip ammo is only 62 grains, which then sound only need about 1 in 10", so why did they use 1 in 7" and overstabilize the bullet? Seems like there would be no improvement in accuracy and a big loss in velocity.

Mike


Gun control means putting the second bullet through the same hole as the first- Ted Nugent
NRA Member
Muzzleloading- Once you go black, you'll never go back!!

"But the SS 109/M855 5.56mm NATO green tip ammo is only 62 grains, which then sound only need about 1 in 10", so why did they use 1 in 7" and overstabilize the bullet? Seems like there would be no improvement in accuracy and a big loss in velocity." While it may be true that a 1/7" twist is not absolutely required to stabilize the SS 109 bullet, bear in mind that it is bullet length, not weight, which determines the twist necessary, and this bullet is a long boattail with a steel core or insert designed for pentrating body armor and helmets. It will do this better if it is spinning faster, even though a little velocity and some accuracy might be sacrificed for overstabilization!!

Keep yore powder dry!!