Gas Tubes

[M4 Colt]

i hope this is the proper place for this.

i built up another AR upper a few days ago (my 37th) and i thought i would toss this out there, i am very strict about this proceedure, when i install a barrel i torque the barrel nut so there is an open notch to run the gas tube thru, but i do not stop there, i make certain the tube is NOT touching the "wings" of the B. nut, i insert the tube and pin it in place, then i run the bolt/carrier in to check if there is any rubbing of the key to gas tube when there is no metal to metal contact, i consider the job complete.

i wanted to add this because i have seen some gas tubes blow out at the B.nut due to rubbing, it takes about 5,000 rounds give or take a 1,000, then also i have seen gas tubes that were worn sharp as a razor on the key end, those that do this also have some failure to lock up and the F.A. must be hit/bumped.

if this is old hat to some, sorry, but there are always new folks wanting to get started and i hope this helps.

[Gunfixr]

I make no claims of expertise in assembling ARs (I've assembled far more FALs), but the few uppers I've put together I've done the same. I've even had to tweak the tube just a little to get the carrier gas key to slide over it without binding and/or shifting the tube around in its hole through the receiver. I use a pin punch to hold the tube in place and the carrier w/o the bolt so I can see what I'm doing and then double check after proper pinning and BCG assembly. That way I don't over use the roll pin or hole in the FSB.

[AR15barrels]

when i install a barrel i torque the barrel nut so there is an open notch to run the gas tube thru, but i do not stop there, i make certain the tube is NOT touching the "wings" of the B. nut, i insert the tube and pin it in place, then i run the bolt/carrier in to check if there is any rubbing of the key to gas tube when there is no metal to metal contact, i consider the job complete.


That's a very good practice to follow.

I have a 0.180" drill blank that I use to visually align the two ears up with the cloverleaf cut.
By setting the pin just inside the receiver and leaving it resting in the barrel nut groove, it acts like an indicator.
While final tightening the barrel nut, I watch the pin until it indicates straight with the barrel.

Failing to properly align the barrel nut causes gas tube wear like this:

http://ar15barrels.com/tech/gas-tube-wear.jpg