PDA

View Full Version : AR-15 Colt Carbine Upper


wey97
January 18, 2010, 12:36 PM
I have an older Colt 20" A2 that I want to get a new carbine upper and stock for. I've been looking at the Colt uppers and notice they're 1:7 twist and chrome lined. I've been reading that chrome will be less accurate and the 1:7 twist may be a little too high pressure for performance ammo. Is this correct info? I've seen a lot of 1:9 with chromoly.

I wanted to keep it Colt but not if it's going to be a severe impact on my accuracy. I don't think it would be that severe due to Colts reputation.

Bartholomew Roberts
January 18, 2010, 01:45 PM
The major underlying factor in accuracy is the barrel itself. When you start talking match-level accuracy, chrome-lining causes a very minor decrease because of the difficulty in applying a uniform thickness of chrome to a barrel. Having said that, a good quality chrome-lined barrel can be sub-MOA.

The upside of chrome lining is less corrosion, better reliability, and marginal increase in barrel life (assuming you need high levels of accuracy).

the 1:7 twist may be a little too high pressure for performance ammo.

Some people have reported jackets spinning off with very lightweight (40gr), thin-jacketed, very fast varmint bullets in 1:7 twists. I've never seen it myself; but the lightest bullet I've ever put through a 1:7 is the Winchester White Box 45gr JHPs.

I've seen a lot of 1:9 with chromoly.

The average 1:9 chromemoly barrel for sale out of kit shops is probably less uniform than your Colt barrel, even without the chrome lining.

wey97
January 18, 2010, 07:55 PM
The major underlying factor in accuracy is the barrel itself. When you start talking match-level accuracy, chrome-lining causes a very minor decrease because of the difficulty in applying a uniform thickness of chrome to a barrel. Having said that, a good quality chrome-lined barrel can be sub-MOA.

The upside of chrome lining is less corrosion, better reliability, and marginal increase in barrel life (assuming you need high levels of accuracy).


The average 1:9 chromemoly barrel for sale out of kit shops is probably less uniform than your Colt barrel, even without the chrome lining.

That's all good to know.

Some people have reported jackets spinning off with very lightweight (40gr), thin-jacketed, very fast varmint bullets in 1:7 twists. I've never seen it myself; but the lightest bullet I've ever put through a 1:7 is the Winchester White Box 45gr JHPs.

I would say that more than likely I won't be shooting anything lighter than 55gr.

My only other concern would be wear and tear on a carbine from the higher gas pressure than say a mid-length or full-length. I seriously doubt I'll ever put enough rounds through it for that to be a factor anyway :)

wey97
January 19, 2010, 01:57 PM
While we're on the topic of Colt uppers, I've found the LE6940CK monolithic upper and the LE6920CK standard upper. I believe these are available to the public even though they have the LE # or am I wrong?
Are these the only two uppers currently offered by Colt?

Is the handguard on the monolithic part of the upper or is any of it removable? Any other differences besides the sights that aren't obvious?

p99guy
January 19, 2010, 02:24 PM
you say you have a 20" A2 type....thats allready a chrome lined 1/7 barrel.
-if its the old A1 barrel under a A2 round handguard, chrome lined 1/12 twist.

wey97
January 19, 2010, 04:25 PM
you say you have a 20" A2 type....thats allready a chrome lined 1/7 barrel.
-if its the old A1 barrel under a A2 round handguard, chrome lined 1/12 twist.

I've read there should be no problem with a 1:7 on a 20" because of the decreased gas pressure involved. But I was concerned because of issues with a 1:7 carbine barrel and the higher pressure.

Now I know the chrome lining won't affect my accuracy a noticeable amount to me.

wey97
January 20, 2010, 07:13 AM
It's been brought to my attention that I may have been a little unclear on my first post. If there was any confusion, I should have said "I've seen a lot of 1:9 with JUST chromoly." or a chromoly barrel with no chrome lining versus a chromoly barrel with a chrome lining.

Still interested in knowing if anybody has tried the Colt LE uppers.

RockyMtnTactical
January 20, 2010, 07:16 AM
chrome will be less accurate and the 1:7 twist may be a little too high pressure for performance ammo.

Not exactly correct. Chrome lined barrels can decrease accuracy slightly, but they don't always and it is usually not a drastic difference.

1/7 doesn't affect the pressure of the ammo. It should work just fine unless you want to shoot really light bullets.

wey97
January 20, 2010, 09:15 AM
1/7 doesn't affect the pressure of the ammo. It should work just fine unless you want to shoot really light bullets.

You're right, I eventually made myself more clear.
My only other concern would be wear and tear on a carbine from the higher gas pressure than say a mid-length or full-length. I seriously doubt I'll ever put enough rounds through it for that to be a factor anyway :)

On the twist and performance ammo, I may have been thinking about a 6.8 discussion on fast twist being a factor on performance ammo.
Glad I went 5.56 by the way!