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I need to pick up a couple lower receiver extension kits for the Northwest Firearms lowers I'm building and I'm trying to figure out if there's a right way and a wrong way to go in terms of quality or function.

Any input? :)
All of this is fun to do and you can do it.

If memory serves, the new CAR type AR15 rifles have two distinct diameters and group compatibility. One is USGI spec and the other is after market or civilian spec.

Go with the USGI spec receiver extension. Also known as the buffer tube. The USGI sliding butt stock will have more positions. The USGI buffer spring will have ...

Flat grounded ends. Some of the after market buffer springs are just industrial springs wacked off at different lengths. Also use a USGI CAR or carbine buffer. Good stuff.

I think I have this right. A good long day. I am tired. Funny how all the builds were rifles and not carbines. Now things are reversed and all builds are carbines and not rifles.

Some of the greatly improved receiver extension tube lock rings and castle nuts are greatly improved from OEM. Be sure to get stuff designed for the USGI spec assembly.

Consider using locking compound and not staking. If properly done there should be zero staking anywhere in the build. They now make great locking compound ... compounds.

Beware of that pesky barrel gas port diameter. Some barrel makers deliberately drill over large gas port holes. They do this to compensate for lousy builds. Over gassing results.

Humm. Good night. HB of CJ (old coot) Hundreds of builds. Uncounted rebuilds. Long ago far away. Now I am old and cooted. :) :)
Mil Spec vs. Commercial Spec Buffer Tubes - AR-15 for Noobs!

I think this is a basic sheet; some of the newer commercial spec tubes now have a flat back; and some of them have 6-7 positions now, but for sure, there are diameter differences; the mil-spec ones always have larger diameter threads than the main tube, while the commercial ones will have threads at the same diameter as the main tube... the commercial ones would also be usually "pull extruded" and milled only on the bottom of the carbine tube, and sometimes welded rear caps... the military one would always be milled all over to final diameter and so on.

There however are two kinds of carbine tube positions for the most part; the older 2 or 3 position CAR stock, and the newer 6 position carbine stock. The 3 position ones, have a simpler butt piece, you can see this on various Colt carbines and sporters and most importantly on the old CAR 15s.... the later 6 position ones are called the M4 stocks, they're a little bit more complicated looking rear stocks, and have an angled butt piece, as well as slotting on the tube side of the butt piece.

Rifle tubes, there's only one spec I believe....

and nowadays there are also the super compact aftermarket pistol buffers and compact collapsible stock versions with pistol length buffer tubes, but I think those are always mil-spec pieces, though I could be wrong.

CAR15 stock, usually 3 positions, some commercial ones would fit 6 position tubes

Mil-spec M4 stock

either commercial M4 stock or mil spec tube; its difficult to tell sometimes, PSA mark theirs with "Palmetto" or "PSA" or "PTAC"; some will fit commercial tubes, some will not.

Super compact AR15 stock, one example, they generally have pistol buffer tubes, different weight buffers due to the shortened springs.. depending on the brand, are expensive.

Link provided at the top will have pictures of two tubes that are known to be mil spec and commercial, however newer commercial tubes may be identical to mil spec except for thread diameter and main tube diameter, so beware.
Good info above.

I will just add that for the money I like the Strike Industies enhanced 7 position tube. Under $40 with nut and QD end plate with a solid mil-spec fit
+1 to information above.

Choices really come down to:
Commercial or milspec.

Pick one size and base everything around that. It's no fun having each style, make it all the same and be happy especially since there is no benefit of one over the other.

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