A Colt 603 XM16E1 or a 609 XM177E1 chrome BCG. Added upgrades: Colt gold extractor spring and bushing, a McFarland 1 piece gas ring, and an extractor O-ring.
Why? If I was really going into harms way, that's what I have in my shit hit the fan rifle. (my Princess, Ole Painless, Patches, Safe Queen) If I have to make a choice, that's it.
Not that long ago, they were available, the supply has dried up. Today they are almost the "unobtanium".
The hot ticket for the younger generation is to buy the Colt M16 chrome slab-side carriers and have your local machinist add/cut in the forward assist notches. Same function and result at a reasonable cost. -W