AR-15, 20" barrel, Gov't profile twist rate question

The two standard twists the govt has used are 1:12 for the old M193 Ball and 1:7 for the M855 Ball. All the other twists are boutique and not standard.

Sorry - about the HBAR. It's not bad except it destroys the balance of the rifle. Even the A2 bbl profile destroys the balance, so maybe that's not so big an issue with you. The A1 and older profile had a superior balance where the gun was easier to shoot instinctively. The newer profiles have it distinctly muzzle heavy so it's harder to hold it level with the pistol grip. YMMV

Overstabilization is harmless in most circumstances. In theory it reduces accuracy, but not so as to matter outside of benchrest games. It can overstress thinly jacketed varmint bullets and burst them in midair - which is why a 22-250 is barreled 1:12 or 1:14 - but that doesn't matter for your stated usage.

Government profile is describing the exterior shape of the barrel. A blank can be turned to those dimensions irrespective of its rifling.

My Colt 1/7 twist govt profile barrel shoots almost everything just fine... just not the super low grain bullets (under 50 grains).

Bushmaster should have some 1/9's w/ govt profile though...

Bushmaster has 20" GOVT uppers in 1:9.

The HBAR is a manufacturing expedient. Nothing wrong with it, really. But it does not handle like a .gov profile A2. Then again, the A2 barrel doesn't handle like an A1 until you chop it to say 14.5". This allows you to give up some velocity.............reducing fragmentation range of the already heavier and slower M855.............which is a good thing because..............

Anyway, 1:7 does fine with just about everything. For almost all purposes 1:9 is fine, unless you want to use M856 or 80+ gr boattails.

The HBAR and Gov profile barrels really aren't that far apart, so that you won't likely see a significant difference in accuracy, controllability, etc. Now the Diemaco LMG is a different horse.

Sam

I don't shoot 3-gun so I can't really accurately comment on what will work there.

I do shoot high power though. There's a few problems with what you want to do. First, you say you don't want to shoot any heavy 70+gr match ammo. You're putting yourself at a disadvantage using anything less than the best you can. You'll be getting poor/questionable feedback from using inferior ammo. In addition if you ever shoot a full course match, some ranges and clubs won't let you shoot the lighter stuff at 600 yards - it's dangerous for the guys down in the pits. Lighter bullets get shoved around easier by the wind, your shots could be going all over the place, hitting the berm, other targets on different firing points, hitting the target frames and ricocheting, etc. A 69 Sierra/Nosler or a 68 Hornady should make it to 600 yards on a dead calm day, any wind though and you'll be in trouble.

You could stick the government profile barrel for starters, but if you ever get bit hard by the high power bug you'll want a non-chrome lined HBAR profile barrel with a 1:8 or faster twist rate. Chrome lining isn't exactly consistent in its application, so it's one more variable for inaccuracy. Plus shooting the typical long line loads out of an AR15 that it needs to be competitive (read - heavy bullets humming along pretty quickly), you may start cracking/eroding the chrome lining in the throat and chamber, which will definitely affect accuracy. The HBAR profile will help dissipate heat better and will be more consistent with POI as the barrel heats up, but I've never shot a rifle with a gummint profile barrel across the course so it may work just fine. HBARs are a little muzzle heavy, but that can be balanced out with weight in the buttstock. You'll also want a free float tube - if you're gonna shoot it as a service rifle, you'll need an NRA/CMP legal style float tube that fits under the standard A2 handguards. Sling tension can significantly affect POI on a non-floated AR15 no matter how thick and heavy the barrel is.

Also, I saw that you're shopping for an A3. This could be a problem depending on who's rear sight you use. Most A3 rear sights lack the elevation adjustment needed to make it to 600 yards without fiddling with the front sight. If you're gonna shoot it as a service rifle, RRA makes a carry handle with their NM rear sight on it that's supposed to have the elevation in it to make it to 600 without any need to lower the front sight.

That said, whatever you've got or can get, take it out and shoot it. Don't let me discourage you from high power, the most important thing is to get out there and shoot. Find a local reduced course match or clinic style gig and go shoot.

Bushmaster has 20" GOVT uppers in 1:9.

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Yes, 1/9 is Bushmaster's standard twist. 1/7 twist is special order from them.

I have a 1/9 twist Bushmaster Govt. profile and I love it. It's exceptionally accurate and has a nice balance. I'm not sure if they offer it as a complete upper off the website though. However, you can call them, order the barrel and they'll put one on an upper for you.

I have a couple 1/7 twist barrels, but I prefer the 1/9s. My 1/7s don't shoot 55gr all that accurately. However, my RRA and Bushmaster 1/9s shoot them just as well as my Colt 1/12.

Mine has an Hbar, seems heavy but I have gotten used to it. How do I figure what the twist rate is, or is it standard for that barrle, and if so, what is it?
1980's model Colt, in new condition. Less than 500 rounds shot through it.

I have some 20" 1 in 9 twist government profile barrels on order with Bushmaster.
It is a six week wait for barrels, and about everything else, from these guys so be warned, order well before you need the stuff.

How do I figure what the twist rate is, or is it standard for that barrle, and if so, what is it?

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Push a cleaning rod through with a very tight fitting patch. Measure the distance for one complete revolution.

1 rev in 9" = 1/9 twist

9 revs in 1" = 9/1 twist. Fastest I have ever heard of is 1/6. Which is 54 times slower.

You probably aren't going to have a problem going with either 1/9 OR 1/7, with the exception of shooting the heavier bullets, above 69 grains. If you ever think you're going to get into shooting those, definitely get the 1/7.

I have a CMMG 1/7 20" govt profile barrel. Shoots 55gr bullets just fine. Even Wolf shoots repectable in my gun.

Remember this is an AR and AR's are pretty easy to work on. Once you have the proper tools, barrels can be swapped out pretty easily. So you can get a 20" Govt profile barrel, try it and install a DCM barrel later without buying the entire upper.

ZM

Push a cleaning rod through with a very tight fitting patch. Count the number of revolutions for 1".
9 revs in 1" = 1/9 twist

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Sheesh. Don't screw him up!


1 revolution in 9"

Measure how long for one revolution. Just mark the cleaning rod.

Geez, why not just look on the barrel up by the flash suppresser.
All barrels I have ever used have the twist rate and other pertinent information marked there.
This is a whole lot easier than counting patch revolutions,,,,,,

If the gun in question is going to be used for spraying during 3-gun, then typically people choose the cheapest ammo that works in their gun. That's usually not any kind of heavy match grade bullet. That means 1/9 is fine.

Also, for CMP or Highpower matches, if the range you shoot at, or the competitions will be 300yards or less, then the 69gr bullet is ideal, and the 1/9 spins those just fine. Especially in a 20".


1/7 only really comes into play if you want to be able to shoot 75gr or heavier bullets. And do so reliably (that means in the cold weather too). Some 1/9 can do the 75gr, others can't - and of the ones that do, most fail in the cold. Longer the barrel, the greater the chance the 1/9 will work with the 75gr. For example, 16" is iffy, 20" is better, 24" usualy stablizes them every single time.


If 55 and 62gr ammo is the choice, no need for 1/7. And the 69gr SMK is always there for competition purposes out to 300yards.

1 revolution in 9"

Measure how long for one revolution. Just mark the cleaning rod.

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Yea, that's right. Sorry. That's what I get for posting while at work, talking on the phone.

I'll fix it...

I have two AR15s, one with a 20" Government Profile and 1:7 twist and the other with a 16" HBAR and 1:9 twist. I can't tell any difference between the two using 55- and 62-grain bullets. I would highly recommend the Government Profile barrel in a 20" upper, as it is a full pound lighter than an HBAR in the same length. Actually, my 20" rifle is probably a shade lighter than my 16" HBAR.

Hmmm... My 1:9 Bushmaster will not stabilize 62 grain bullets. It seems like it should, but it sure didn't two weeks ago.

Nil, seriously consider the 1:8 or 1:7 twist barrel. You might need it.

Hmmm... My 1:9 Bushmaster will not stabilize 62 grain bullets.

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That probably has more to do with the fact that it was 62gr frangible than the fact it was 62gr. i bet your Bushmaster will stabilize 62gr M855 with no trouble at all.

Barth is right. The whole issue with stabilizing a bullet has to do with the bullet's length, not its weight. Since the diameter of the bullet cannot change, when weight goes up, length increases - obviously.


That's why a pure lead 62gr bullet is much shorter than the SS109 bullet used in M855 military ball. The mild steel penetrator of the SS109 is lighter than lead, so to get to a weight of 62gr, it must be longer than an all-lead 62gr.


Tracers are very long for their weight, because the material they are made of is lighter than lead. That is a key factor in why the military went with 1/7. To make sure they can stabilize those long tracer bullets, even in cold weather. Their requirements are different. They need everything to working in everything - everywhere.


But for most purposes, 1/9 should be fine - and I'm a big 1/7 advocate, but from the perspective that 1/7 gives you a more effective platform due to the use of 75 and 77gr loads for defensive purposes. Not necessarily the #1 concern for a sporting rifle.

Most of the 3-gun guys I've talked to, who also do other kinds of action shooting with their AR's - have a Dillon progressive and source the cheapest powder (data powder), and the cheapest bullets (55gr bulk, usually military pulled) etc...to create affordable, high quantity blasting ammo that is as accurate or better than military surplus.

Frangibles, tracers, heavy match grade, and other types are more rare, and usually more expensive. Not the primary choice of guys who are shooting competitively, which implies higher volume shooting. Not a whole lot of need for a 1/7 if you are always buying ammo by the case, and either 55gr or 62gr which is the vast, vast majority of the available ammunition out there ...


Overall, having a 1/7 can't hurt - but in some cases, it can help. Depends what you want.

I'll tell everybody this,,,,
If a whole bunch of Black Hills 77 grain Military ammunition ever hits the civilian market as surplus I will seriously consider rebarrelling one of my rifles to 1/7.
Until that time comes, I for one am going to stick with the more versatile 1/9.