What's the twist rate on milsurp AR uppers?


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Yoda
June 7, 2009, 11:38 PM
I was at the Tampa gun show Saturday, and one vendor had some old military surplus uppers. They still had the triangular foregrips and 20" barrels, with the front sights way out, well, in front. Only one of the uppers had the forward assist, and they ALL had the old A1 rear sights. He wanted $250, which is a lot less than new ones, IF you can find any.

I bought the only one with the forward assist. I don't know what the twist rate of the rifling is, and it still needs a bolt and charging handle, but I'm glad to have my hands on an upper rather than just have my name on a list to get one in the indefinite future. Besides, I think the older foregrips are cool.

I am, however, not that enthused about the A1 sights. I guess that if they were good enough for real soldiers, they should be good enough for me.

Anyone know what the twist rate might be? The vendor said the uppers were old USAF ones.

- - - Yoda

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ugaarguy
June 7, 2009, 11:52 PM
You did well. The forward assist, and 1:12 twist bbl were a major part of the M16A1 upgrade from the original M16. That's not to say that those other uppers weren't rebarreled to 1:12, or uppers that Colt originally barreled 1:12 for commercial (pre '86) M16 sales (they were still selling slick side upper SP1s into at least the early 80s). Between the FSB and the flash hider the barrel should be stamped something like "C MP 1/12 5.56 NATO". Also, does your upper have a three prong, or a birdcage flash hider?
I am, however, not that enthused about the A1 sights. I guess that if they were good enough for real soldiers, they should be good enough for me.
You can put an A2 aperture in the rear sight, and an A2 post in the FSB. The nice thing about the A1 sight system is that, although it takes more effort to sight in, once it's sighted in it keeps people from easily messing with your zero - unlike the A2 thumb click windage & elevation drums that everyone seems to want to play with.

Yoda
June 8, 2009, 12:21 AM
The only thing stamped on the barrel that I can see is 'C MP CHROME BORE" I don't see anything about a twist rate. I guess I can get a rod, a patch, and a ruler and just measure it. The flash hider is the bird cage type. Bore looks very good at both ends.

- - - Yoda

ugaarguy
June 8, 2009, 01:41 AM
I'd forgotten that twist rate wasn't always marked on older barrels. Based on the birdcage hider, and forward assist on the upper I'm pretty confident it's a 1:12 though. It'll definitely shoot 55 grain and lighter bullets with no problem. Some 1:12s will shoot heavier. I had a Colt SP1 that liked the Remington factory loaded 62gr HPBT Match ammo. Hopefully I've been some help to you.

Yoda
June 8, 2009, 09:10 AM
Yup, you've been a great help. I figure that the upper would be good enough for social work in a Katrina situation, if nothing else. When I get a bold and charging handle, I'll give it a test run.

- - - Yoda

Jeremy2171
June 8, 2009, 01:29 PM
Only a very few early Model 601 ARs were the 1/14 twist....after that they were all 1/12 until the A2 came along.

"IF" (not likely) they were 1/14s they would be almost totalty unmarked with the exception of the Colt proofs on the barrel at the FSB.

1/12 markings can include..
"12"
C MP C
C MP B
C MP CHROME BORE

If it's 1/7 it WILL have 1/7 on it...

You should have gotten one without the F/A as it's a less common variant known as the Colt Model 604 or the "USAF version". The Army wanted the F/A but the USAF opted out on it.

TeamRush
June 8, 2009, 08:44 PM
Dont' need the super fast barrel twist unless you intend on firing the newer long range tracers...

The twist rate is stamped into EVERY military barrel it will look like 1/10 or 1/12 which is shorthand for 1 twist in 10 inches or 1 twist in 12 inches.

If you start seeing military barrels showing 1/6 or 1/7, you might stay away from them...
Most of the older 'Skinny' barrels with the A-1 sights will be 1/9 or 1/12.

Just make sure you aren't buying what looks like a 'Heavy' barrel out between front sight and muzzle,
But is actually a soda straw barrel from front sight to chamber!

Those 'Undercut' barrels are STILL showing up imported from NATO countries we sold M-16's to,
They are usually shot out, and they usually have undercut barrels, ESPECIALLY if they still have triangular handguards on them!

ugaarguy
June 9, 2009, 03:58 PM
Dont' need the super fast barrel twist unless you intend on firing the newer long range tracers...
Really? Most 1:12 bbls won't even stabilize 62 grain M885 Ball (aka Green Tip or SS109), which is a standard US round, and is not a tracer. Federal Gold Medal .223 is only loaded with 69 grain and 77 grain SMKs. A 1:9 should stabilize the 69 grain, but even a 1:9 isn't fast enough for the 77 grain. Federal also loads a 64 grain soft point in the Power Shok blue box line hunting ammo - again, good luck getting a 1:12 to stabilize those. Winchester offers a similar 64 grain soft point hunting load in the Super-X line as well. Remington now offers the Premier Core-Lokt Ultra Bonded in a 62 grain .223 loading which may or may not stabilize in a 1:12 bbl. Hornady loads a 60 grain ballistic tip in the TAP FPD line - again, may or may not stabilize in a 1:12 bbl. Hornady loads 75 grain HPBT as both a match bullet and in the TAP FPD line; per the label on those cartridges 1:9 or faster twist bbl is required. Keep in mind that some other 75 grain bullets are too long for 1:9 twist to stabilize.

The twist rate is stamped into EVERY military barrel it will look like 1/10 or 1/12 which is shorthand for 1 twist in 10 inches or 1 twist in 12 inches.
As stated in the post above by Jeremy, many early military barrels did not have the twist rate stamped into them. Further, the US Military never issued M16s / M4s with 1:10 twist bbls.
If you start seeing military barrels showing 1/6 or 1/7, you might stay away from them...
Again, there are no US Mil issued 1:6 barrels (with the move to heavier bullets it's possible that SOCOM is testing them, but nothing that fast has been officially adopted, so you wont see it as Mil Surp). Further, why would you stay away from a 1:7 twist barrel? 1:7 will stabilize all of the new heavier ammo, and still does fine with 55 grain FMJ.
Most of the older 'Skinny' barrels with the A-1 sights will be 1/9 or 1/12.
No, most of them will be 1:12, with a very few, very early ones 1:14. The US Military NEVER adopted the 1:9 twist. 1:9 is purely a civilian twist rate for AR-15 bbls.
Just make sure you aren't buying what looks like a 'Heavy' barrel out between front sight and muzzle,
But is actually a soda straw barrel from front sight to chamber!

Those 'Undercut' barrels are STILL showing up imported from NATO countries we sold M-16's to,
They are usually shot out, and they usually have undercut barrels, ESPECIALLY if they still have triangular handguards on them!
Actually a barrel like that is standard on the M16A2/A3/A4, all of which would have round handguards. The barrel is turned down under the handguards to reduce weight, and to allow an M203 grenade launcher to be mounted to the underside of the barrel. M203 mounting is why M4 barrels have the small cutout forward of the FSB. Full heavy barrels (heavy all the way under the handguard) are almost entirely a civilian thing.

kwelz
June 9, 2009, 05:58 PM
To the OP, please ignore TeamRush like the rest of us are learning to do.

You got a pretty good price on that upper. You will have trouble with heavier bullets however. The 1/12 twist causes bullets to tumble so you are going to loose accuracy with heavy rounds. Try to stay under 55 Grain if you want to get decent grouping.

Yoda
June 10, 2009, 12:08 AM
OK, so it won't stablize the heavier bullets. That will certainly influence accuracy beyond rock-throwing range, but for anything closer, the loss of bullet stability should increase lethality, right?

AND, a 1:12 barrel in the hand is much better than being on another list to get one ... someday.

I'm just thinking it will look really, well, unique with the long barrel and the older triangular foregrip on top of a lower with a collapsible stock, topped off with an adapter and a red dot scope. I may have the only one!

- - - Yoda

benzy2
June 10, 2009, 12:17 AM
If I came across an upper for $250 that didn't look terribly abused I would jump on it and enjoy shooting it. Even if it is worn and doesn't hold paper punching accuracy it should still be accurate enough to enjoy at the range.

ugaarguy
June 10, 2009, 02:01 AM
OK, so it won't stablize the heavier bullets. That will certainly influence accuracy beyond rock-throwing range, but for anything closer, the loss of bullet stability should increase lethality, right?
Good news is there's still plenty of 55gr FMJ on the market.
AND, a 1:12 barrel in the hand is much better than being on another list to get one ... someday.
Absolutely, and you can't beat the price.
I'm just thinking it will look really, well, unique with the long barrel and the older triangular foregrip on top of a lower with a collapsible stock, topped off with an adapter and a red dot scope. I may have the only one!
If you put green A2 handguards on it you could make it look like a Canadian Diemaco C7A2 - http://www.casr.ca/101-rifle-c7a2.htm

Badger Arms
June 10, 2009, 02:07 AM
TRUE milsurp barrels have one of three twist rates:

1-in-14" -- EARLY AR-15's bought by the Air Force. Good luck finding one.
1-in-12" -- Standard until adoption of M16A2. Light contour barrels. Models AR-15 (military, not commercial designation) XM16 (all), M16, M16A1, XM177.
1-in-7" -- M16A2 and beyond, M4 (all).

Al Thompson
June 10, 2009, 03:04 PM
I'd have jumped all over one of those uppers. I think you got an excellent deal. Did you get the guys name? I'd love to buy one.

Dont' need the super fast barrel twist unless you intend on firing the newer long range tracers...
Or anything much heavier than 62 grains or so.. What a buttload of bad information. :barf:

Onmilo
June 10, 2009, 03:37 PM
The uppers you saw are 1-12" twist.
I have seen quite a few of these being offered at the gunshows from excellent/like new to near relic grade.

If you know someone with a throat erosion gage it is best to check these uppers before you lay your money down.
I have seen and examined some that looked great from a distance, but when examined closely were found to be worn out and crudely refinished.

If you already have a bolt/carrier assembly you wish to use in one of these uppers I would strongly recommend you gage headspace the combo to the upper you are interested in buying before you lay your money down.
There is almost always someone at the shows who has headspace gages and throat erosion gages who will be willing to help you out.

If the vendor will not allow you to do these checks, politely pass on the offering and move on.

If you do find a good upper, you will find any .5.56/.223 ammunition with bullet weights of 55 grains or lighter to be excellent choices for this upper assembly.

mongo4567
June 10, 2009, 08:24 PM
Great deal, congrats! I got an A1 upper like that to use with a .22 LR conversion. The 1/12 works very well for this purpose.

Yoda
June 10, 2009, 11:13 PM
For those who asked, I didn't get the vendor's name. He said he got the uppers from a surplus sale of USAF stocks.

- - - Yoda

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