55 gr in an AR?

[txhoghunter]

Ok so I constantly see people saying that a 1 in 9" barrel will have a hard time stabilizing a bullet weighing more than 69 gr. Now this thread is not to argue that at all, what I am wondering is if a 1 in 9" barrel will overstabilize a 55 gr projectile. My question here comes from the fact that Remington's SPS varmint in .223 has a 1 in 12" twist. I am thinking that the 1 in 9" will stabilize a 55 gr just fine, but the 1 in 12" will do better and is used because varmint rounds usually do not weigh any more than 55 gr.

Thanks for the input, and sorry if this question seems a little on the no-brainer side, I am brand new to the AR world and the stability question just keeps coming up.

 

[kwelz]

Is it Monday already?

For the 5 billionth time, YES.

1/7 works fine for 50 grain up through 80+.

Bolt guns need the slower twist because people may run ultra short varmit bullets through them.

 

[harryheadshot]

the 1in12 should shoot the 55's fine

My ar's a 1in7 and it shoots 55grains, to 75grains great I'm getting better groups with the 75gr hand loads, but the hornady 75gr bthp from the factory stay tight too. Sorry man I meant your 1in9

 

[harryheadshot]

the 1in9 should shoot the 55's fine

My ar's a 1in7 and it shoots 55grains, to 75grains great I'm getting better groups with the 75gr hand loads, but the hornady 75gr bthp from the factory stay tight too.

 

[NCsmitty]

You can under stabilize the longer bullets in a slow twist, but you won't over stabilize a 50-55gr in a 223/5.56 with a 1in9" twist. Often the 55gr will shoot very well in that twist.



NCsmitty

 

[Al Thompson]

55 grain shoots just fine in the 1/7 too. My 1/9 twist rifles shoot 75 grain bullets just fine as well.

 

[rbernie]

Overstabilization (over-spinning the bullet, e.g. too high of an RPM) is an issue in 223 only with the lightest of varmint bullets in the 40gr-50gr region. The high rotational speed of a 1:7" rifling twist can cause them to disintegrate in flight. A 1:9" twist generally can shoot even the light varmint bullets just fine and I've never seen any 55gr bullet of any construction that was too light for my 1:7" barrel, much less as a 1:9" barrel.

Since I don't need the light range (40gr-50gr), I tend to prefer fast twist rate barrels (1:8" or 1:7") that do just fine with 55gr but also allow me to shoot 75gr fodder equally well. A 1:12" barrel will be optimized for 40gr-55gr and will not do well with much ammo above 55gr-62gr.

 

[FlyinBryan]

The high rotational speed of a 1:7" rifling twist can cause them to disintegrate in flight.

i keep hearing this but i just have trouble believing it.
maybe im just dense, but it seems to me that if the bullet can survive 50000+ psi in the bore and not turn to a molten drop of copper and lead, it can certainly survive getting a little dizzier.

 

[kwelz]

FB, same here. I have seen videos where people say bullets have spun apart but it just seems strange to me. Of course we all know that when you apply the wrong type of force to something it can easily damage it.

 

[rcmodel]

in the bore and not turn to a molten drop of copper and lead,

It doesn't come apart in the bore, because the bore is holding it together.

Believe it can happen in the air after it exits the bore though, cause it's true.
Been there, done that with a 1/14 22-250 years ago with 50 grain Sierra Blitz bullets designed for the .222 Remington.

I have never done it with a fast twist .223, because I have never owned one.
But I have no doubt at all it can happen.

rc

 

[wally]

Quote:
The high rotational speed of a 1:7" rifling twist can cause them to disintegrate in flight.

i keep hearing this but i just have trouble believing it.

Believe it. I had a bunch of 50 gr JHP "varmit" bullets, I don't remember the exact brand, but the reloading manual suggested not using them in a Mini-14 because its twist was "too fast".

I loaded them anyways and got good results at 50 yards and great effects on water jugs, but when I moved to 100 yards wasn't even hitting the paper! Leaving the gun on the sandbags and looking through my spotting scope I'd see little "puffs" in the air at about 75 yards.

If you reload, buy some and verify it for yourself with a fast twist rifle.

 

[benEzra]

Quote:
> The high rotational speed of a 1:7" rifling twist can cause them to disintegrate in flight.

i keep hearing this but i just have trouble believing it.

Out of a long barreled varmint gun, a 40-grain .223 bullet can exceed 3800 ft/sec.

http://www.shootingtimes.co.uk/guns/226321/223_Remington_cartridge_review.html

If the twist rate is 1 rotation in 7", that means that the bullet exits the muzzle spinning at over 6500 revolutions per second (3800 ft/sec times just over 1.7 rotations per foot), or roughly 390,000 rpm (!).

If you run those numbers through a centripetal acceleration calculator (2.78mm radius, 390,000 rpm), you will find that the strain at the surface of the bullet is just over 470,000 g's, if I figure it correctly. Copper is reasonably strong, but stuff a thin shell of copper full of lead and subject it to half a million g's, and it most certainly can fail---and if it fails, it disintegrates most dramatically.

Since centripetal acceleration/centrifugal force is exponential based on the rotation rate, slowing the rpm's causes a big drop in the strain. If I figure it correctly, a 1:9" twist drops the spin rate of that 3800 ft/sec bullet to 304,000 rpm, which reduces the strain by almost 200,000 g's (to 288,000 g's). A 1:12" barrel drops the spin rate to 228,000 rpm and the strain to "only" 162,000 g's.

This is more critical with longer barrels, because the velocity (and hence the rpm) is lower for shorter barrels. Cutting back to (say) a 16" barrel instead of 24" drops the velocity and rpm considerably, so a shorter barreled carbine can get away with a faster twist rate than a long barreled varmint rifle can.

 

[Sky]

Wow Ben good post!!

 

[Freedom_fighter_in_IL]

i keep hearing this but i just have trouble believing it.
maybe im just dense, but it seems to me that if the bullet can survive 50000+ psi in the bore and not turn to a molten drop of copper and lead, it can certainly survive getting a little dizzier.

I've seen more "sonic puffs" more times than I would care to admit from my .220swift. Over spin can and does exist as ben Ezera's post explained so well. The forces from spin can be MUCH greater (stress wise) on a bullet than the initial forces from chamber fire. Plus another thing you have to remember, only 50% (+-5%) of chamber pressure is exerted on the bullet. The other half is exerted on the bolt face of your rifle.

 

[txhoghunter]

Well, we can just go ahead and call Ben the RPM master. Thank you very much Ben as you have answered my questions more than thoroughly.

Sky said it best: wow. just wow.

 

[husker]

Was most excellent. Great data for a new-B like me. Re Loading that is.