My B-I-L is wanting to put a 9mm upper on his rifle lower.

As I read it as long as the barrel is 16" or longer he is legal. Correct?

Correct.

He will need magazine adapter, unless he gets a dedicated 9mm lower which has a mag well specifically made for a Glock mag. I just built one last week. Fun to shoot.

He will also need to replace two pins with stainless steel pins and will need a different recoil buffer.

Which pins? I used standard parts except the lower which has the ejector and mag release in it. And which buffer? I have a standard CAR buffer in mine; the bolt carrier is weighted to accommodate the unlocked, no gas system, blow back operation.

Stainless fcg pins are/were standard on Colt factory 9mm ARs. They're not a bad idea, but you don't need them.

Pistol cartridges (direct recoil operation) usually generate greater hammer velocity in ARs than does the in-line piston design and 5.56. Sufficiently high hammer velocity impact loads fcg pins. Result is pin breakage and/or wear on the receiver fcg holes. Trigger slap is the subjective correlate.

Parenthetically, it is indeed possible to make Stoner's design overcycle with 5.56. An overgassed, worn, pistol upper can generate trigger slap and break pins.

Back to 9mm, to prevent your lower from beating itself to death you want a ramped (beveled) carrier, and/or proper hammer, buffer, etc. My understanding is most makers - including Colt - now ship properly properly ramped bolts. Supposedly DPMS mitigates overcycling via hammer design.

The one I have, has a bolt carrier that has a big steel weight welded into the back of it, I assumed to prevent what you are describing. The bolt velocity does not appear to be more than a gas operated one.
Anyway, even if the SS pin is harder than a standard pin (which are pretty hard), the impulse is still transmitted to an aluminum hole; same result. So far, they are still tight, after 500 rounds.

quote:

Originally posted by dpcd:
The one I have, has a bolt carrier that has a big steel weight welded into the back of it, I assumed to prevent what you are describing. The bolt velocity does not appear to be more than a gas operated one.
Anyway, even if the SS pin is harder than a standard pin (which are pretty hard), the impulse is still transmitted to an aluminum hole; same result. So far, they are still tight, after 500 rounds.

Increased carrier mass is another approach. And agree stainless pins are unnecessary if you pay attention to carrier velocity. FWIW, I feel the same about those so-called anti-rotation pins. FCG holes do not wear because of pin rotation, anyway.