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Posted: 5/3/2004 2:35:44 PM EST
I am looking to replace my ar15 single rail gas block to a 4 rail gas block but they come in dift sizes how can i tell the difference?

heres a pic of whqat im going to be gettin

Link Posted: 5/3/2004 2:59:27 PM EST
Link Posted: 5/3/2004 2:59:48 PM EST
You need to measure the barrel's ext diameter at the gas block.

Link Posted: 5/3/2004 3:03:16 PM EST
is there a guess by looking at it?
Link Posted: 5/3/2004 3:13:23 PM EST
0.750 is the most likely size.
Link Posted: 5/3/2004 3:13:51 PM EST
thats a .750 Diameter Barrel, looks like a Model 1 M-4 upper! or M&A, or Rock River, but it is .750
Link Posted: 5/3/2004 3:16:40 PM EST
I think you really need to measure it. If you don't have calipers, then wrap a piece of paper snugly around the barrel where you wish to measure it, mark on the paper how much is required to wrap exactly once around the barrel (the circumference), unwrap the paper and roll it out flat, measure your mark, then finally divide the measurement by 3.14 to get the diameter.
Link Posted: 5/3/2004 3:32:43 PM EST
[Last Edit: 5/3/2004 3:36:46 PM EST by RAMBOSKY]

Originally posted by Drew-tx: "is there a guess by looking at it?

Drew, don't feel bad, my wife asks me that all the time.

All good advice above. But....zrxc77 can help you find out for yourself. You can always call the dealer you got it from or a friend who reloads will have calipers for you to measure it with.

Link Posted: 5/3/2004 4:24:52 PM EST
You basically have three standard gas block diameters:

0.625" for some early A1 barrels

0.750" for just about every other modern barrel out there today (with a few custom profile exceptions exceptions - Compass Lake Space Gun is one that comes to mind)

0.920" for gas blocks on bull barrels

Yours is 0.750 gas block
Link Posted: 5/3/2004 6:11:19 PM EST
thanks now i think i have to get a 4 rail system sight
Link Posted: 5/4/2004 1:48:49 PM EST
Aren't the Bushmaster Superlites .650 under the gas block?

I assume they make this gas block themselves.
Link Posted: 5/4/2004 1:53:46 PM EST
According to the Bushmaster description on their website:

"Diameter tapers from .670 inches at the chamber end to .600, then steps up to .625 at the sight base, and tapers to .570 from the sight base to the muzzle - similar to the original Vietnam era A1 barrel profile."

So it is similiar to the early A1 barrels.
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