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Jan 22, 2010
Western Washington
First off, I don't do this for a living so I'm going to ask a daft question or two.

Then, my black rifle experience is FAL and STG-57 variants with a dash of G3. I never have cared for the mouse gun, but I guess it has its place.

I recently acquired several .308 barrels for who knows what, but I noticed something about all of them. I have several raw machined AR/LR lowers to play with so here we go.

The gas port is drilled into the groove not the land.

The barrel extension is correctly timed, and the locating pin is exactly where it belongs.

Test headspace was perfect.

This was on a 16" an 18" and a 20" .

So how in the wide world of sports do you time the threads in such a way that the barrel extension makes proper torque, the index pin and the gas port line up, the feed ramps line up, AND the gas port is not drilled into a land?

I have a desire to make a few odd chamberings from blanks and all I know that I can get right is the gas hole. The method and the fixture isn't that important other than I can get it about 9 times out of ten if Murphy isn't in the shop. It's basically a borescope on the inside and a punch/index on the outside.

Have pity on the old guy because I know there is some voodoo I'm missing here.


Cast Iron
Dec 8, 2012
St. Louis, MO
Take this with a grain of salt as I've only done a couple of AR barrels (5.56), but I've found that you have a good half inch or so (depending on the twist) of range in the barrel where the gas port can fit nicely in the groove. Instead of a dimple. scribe a line. Now you thread for the extension then set back the shoulder until the pin lines up with your scribe mark. You shouldn't need more than 0.0625 to get it on your line. If you laid it all out correctly, your gas port will now be close to the middle of your scribed line--close enough for government work.