Hello,
I’ve got a 16” MK1 REPR that has given me fits and I’m hoping somebody may have an idea of what to try next. Rifle will fire first round without issue but it will absolutely short stroke, barely eject the round but usually not travel far enough back to even sort of grab the next round. Will do this with all different types of ammo and all different types of magazines. After doing it once or twice in the first five shots, usually will do this two consecutive times on shots 2 & 3, the rifle will begin to run normally. Have tried different buffers and springs with no improvement. Gun has been back to LWRC literally at least 5, maybe 6, times over the last several years (first was for accuracy issues in 2016 and subsequent trips were all for this issue) and i get “it worked fine with our lower” or “need to put factory buffer and spring back in it” when I had only changed buffer and spring in hopes of getting some functionality. Local gunsmith took a look and he doesn’t see anything that jumps off the page. I wouldn’t let him get too deep since I didn’t want to do something to possibly void/alter the warranty. Even being so frustrated with it, I cant ethically sell a gun I know has issues so here I am. So here’s some details:
Has the original 2 postion gas block on it
Buffer springs feels as if there’s some sort of binding up issue at the read of the travel. Nothing overly harsh, but it’s different than the first 2/3 of travel.
Gun has less than 200 rounds through it
I’ve run the gun dry and it’ll do it and run it sopping wet with oil and everywhere in between and it makes no difference.

I don't personally own a REPR but I can say you're not alone in problems with a gen 1.  A buddy of mine has a 18" gen 1 REPR, with the fal-style side charging handle.  3 years ago shooting together it started having cycling issues, not fully going into battery, etc.  The BCG cracked straight-line bottom rear.  LWRC replaced it of course.  Just a couple weeks ago we were shooting together again, and the top of the carrier where the piston rod pushes it (gas key basically but it's a piston gun) broke partially.  Metal pieces went into the lower and dead-lined the trigger.  He also had a different spring and buffer in it than factory.  LWRC had him send it in but no word back yet.  I wouldn't go so far as to call it a lemon yet, but these issues are very strange and seemingly unrelated.  Maybe QC slipped for a time when the Gen 1s were being made and they were being cranked out on overtime or something, who knows.  Haven't heard of anything similar with the newer REPRs.  If I hear anything back about what LWRC says to my buddy I'll pass it along.