New development. Kinda weird.
My first MCR is still running fine and well past 1000 rounds. Other than the roll pin replacement and tightening the castle nut it's needed no attention.
My new MCR has only a couple hundred rounds though it and today it doubled, which was weird. I didn't see anything obvious so decided to try to see if it would repeat but it didn't and I was at the end of the magazine. Replaced the mag and then it went 2 shots then dead trigger. I looked in the charging handle slot and the hammer was in the fired position. Opened bolt and extracted a live round. Closed bolt and again, 2 shots then dead trigger and hammer in fired position.
I separated upper and lower and removed sear and hammer, surmising that one was defective and as I had replacement AR parts on hand thought I would replace both. Well, the sear looks similar but was slightly different than the AR sear I had on hand. I have had plenty of AR's and replaced plenty of parts and know AR parts from various manufacturers can have slight differences but I wasn't sure the MCR sear actually is a AR sear so I didn't want to experiment. The sear looked fine, anyway.
The hammer looked like it had some wear marks and a bit of a ding on it too. It was close enough to the AR hammer I had on hand so I replaced it. First time disassembling the MCR lower so I had to fiddle with it a bit but once fully assembled I tried it out and I fired a dozen rounds no problem.
Any ways it was weird. Still not exactly sure what was going on as I would think that from where the hammer ended up it should have doubled rather than been left with a dead trigger but I suspect it's due to the firing pin spring tension acting as a buffer from a hammer that went forward without full spring power.
Also, the hammer from my new MCR is different from the old MCR. I assume it's because they are using whatever parts they can source. Pics attached. Old MCR lower with failed hammer beside it
Failed hammer