AR-15 Bore Twist Versus Bullet Weights?


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TooTaxed
June 22, 2005, 11:17 PM
The ARs seem to have been made with a wide range of rifling twists...my early Colt SP1 Car-15 has 12" twist, and I see rifles with 7" twist. What bullet weights are optimum/work well for each twist, and which bullet weights should be avoided for each twist? I haven't seen this well-discussed in the reloading manuals... :confused:

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Coltdriver
June 22, 2005, 11:28 PM
The original 55 grain .223 bullet was designed to work well in a 1 in 12 twist barrel.

You can always go to a lighter bullet with a 1 in 12 twist.

However a heavier bullet is not likely to stabilize in the 1 in 12 barrels.

I am not sure when the tighter twist rates came in to being for the AR's but I know that 1 in 9 is common. Still works great with a 55 grain bullet.

With a 1 in 9 I am sure you can go up to a 65 grain bullet and a 70 grain bullet will probably stabilize fine.

The funny thing to me about using a .223 with a bullet heavier than 55 grains is that you start to get a much higher arc with the heavier bullet. So, other than for target work, I am not sure why you would want a heavier bullet in an AR or any .223 for that matter.

I reload for a Ruger in .223 for varmint shooting. For my AR I just use the mil spec 5.56 ammo.

Harry Tuttle
June 22, 2005, 11:43 PM
www.ammo-oracle.com

Q. OK, what is all this stuff about rifle twists and different ammo?


Rounds in flight spin for stability because of the rifling on the inside of the barrel.? Depending on how much they spin, they are more or less stable in their flight and therefore more or less accurate.? The earliest AR15s from the early 1960s had a twist rate of 1 complete twist every 14", or 1:14.? This was increased to a twist rate of 1 turn in 12" for the M16, XM16E1, M16A1, and later rifles and carbines.? The current M16A2s and up and the M4 carbines have a much faster twist rate, 1 turn in 7".? The reason for the 1:7 twist is mainly to stabilize the M856 tracer bullet, which is much longer than other bullets.? You will recall from above that the M856 was designed to provide 800 meters of trace out of the SAW.


While the slow 1 in 12" twist is adequate to stabilize the 55 grain M193, it will not stabilize the 62 grain M855.? As a result, the newer M855 ammo will group 1-2 feet at 100 yards, with bullets flying through the air sideways, instead of shooting to about 2" at 100 yards, like military ammo should.


All this has some ramifications for ammunition selection depending on your rifle's rate of twist.


You can also overspin projectiles and cause overstability. This results in the not-so-desirable condition that keeps the nose of the round pointed high, as illustrated below:

You can also spin them so hard they fly apart.? That's rare, but it happens if you are dealing with very tight twists and very high velocities.? When fired at 3200 fps in a 1-in-7 twist rifle, a round is rotating at over 300,000 rpm when it leaves the muzzle.? Light, thin-jacketed varmint bullets (i.e., 40gr Hornady TNT or Federal Blitz bullets) often can't take that much spin and will pull themselves apart.

Jon Coppenbarger
June 22, 2005, 11:44 PM
1-6 to 1-6.5 will work with up to 90's
1-6 to 1-8 will work up to 80's
1-9 will work 77 and below
1-12 55's
my hot loads will drop around 5 minutes from 300 to 600 and my slightly lighter loads are at 7 minutes from the same

444
June 22, 2005, 11:46 PM
Just as a point of trivia: it isn't the weight of bullet that determines the twist rate required to stabilize it. It is the bullet length.

Of course from a practical standpoint, the heavier the bullet, the longer it becomes, so we usually use weight to determine twist even thought this isn't strictly correct.

Jim Watson
June 23, 2005, 09:03 AM
Which is why the Speer Semi-pointed 70 grain .224" shoots fine in my 1-14" Ruger .22-250.

Bartholomew Roberts
June 23, 2005, 09:15 AM
1-9 will work 77 and below

Not always - there are a lot of factors that play a part including quality of the barrel and length of the barrel. I have yet to see a 24" 1/9 barrel that won't stabilize up to 77gr. I've never seen a 20" 1/9 that won't stabilize a 77gr; but I have heard of it.

I personally own a 16" 1/9 chrome-lined Bushmaster HBAR that will not stabilize anything above 70gr well. Quality match ammo above 70gr from Black Hills shoots about 4-5MOA @ 100yds from this barrel while Santa Barbara 62gr SS109 shoots 1.5MOA @ 100yds.

In general, quality match 1/9s seem to have a better record for stabilizing heavy (70gr+) .223 rounds than your normal 1/9 barrel. In an informal survey over at AR15.com about 50% of the people who tried 70gr+ ammo through their 1/9 barrel reported poor accuracy similar to what I described above. Almost all of the 1/9 barrels that did not like the heavy ammo were 16" or shorter in length.

rbernie
June 23, 2005, 10:14 AM
Just as a point of trivia: it isn't the weight of bullet that determines the twist rate required to stabilize it. It is the bullet length. Well, this discussion then also needs to introduce bullet density and bullet velocity into the mix, since they're significant factors. Which is why the Speer Semi-pointed 70 grain .224" shoots fine in my 1-14" Ruger .22-250. This is enabled mainly by the the higher velocity of the 22-250 over the .223. The higher velocity causes an increase in the rotational speed of the bullet, which allows the 1-14 twist to have the same effect as would a faster twist on a slower-moving bullet....

Jim Watson
June 23, 2005, 03:25 PM
Maybe not.
I never got beyond the handbook starting load for the 70 gr Speer in my .22-250; 3000 fps or less. Speer shows loads for that bullet down to .222 at lower velocity yet. That slug is very blunt and short for its weight.

Maybe that is what is being reported for 16 vs 20 inch barrels with heavy bullets; but they would have to be on the ragged edge. I read one ballistician to say that bullet stability was related only to the fourth root of muzzle velocity.

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