Mag was probably polymer follower, (admitted, I was WAY too excited to strip it
) but the gas piston was dusty/dirty, now that you mention it, and compared to the rest of the rifle was slap dirty. Could very well be a dirty/obstructed gas port causing the short stroke. It would help explain the funny dent in the cartridge rim on the first round, too (I don't see that dent gettin there with a bullet in the cartridge and firing properly. It has to happen on the ejection, right, caught on the rim of the chamber but not enough to jam? Hmmmm).
And I didn't see evidence of half-feed. When it ran it ran. No unfired stovepipes; no chamber jams. What confuses me is that it really seemed the bottom of the bolt face was catching the next cartrige; the second in the magazine; Now that I think about it the short stroke could produce the same effect. The second cartridge could catch the bolt on the way forward if it's not moving as fast as it should be 'cause it didn't go back as far as it should?
When it feeds it chambers fine, (when it feeds) so that enforces the elimination of the magazine as cause, to my mind. It did seem funny, since I've never seen a mag spring that strong!
He called his gas tube loose, but by my standards it wasn't. There was no significant rattle and the piston fit smoothly. I'm guessing since I saw the gas piston caked with carbon he's not cleaned that area as well as he did the receiver/bore. Carrier return spring was definately all the way, no prob there.
And the short stroking could also account for the stovepipe fte's. Also, the piston was bone dry, giving me the idea he didn't give it much attention.
I'll tell him about the gas piston port and let you know if that clears it up...
Ah, my favorite pragmatist...
Somehow I knew someone here would pull me down to earth.
Muchos gracias, bro.