MCR owners;

Anyone have the the carrier impacting the receiver extension, even with the carbine stock spacer installed?

Is it impacting the tube or the lower receiver rear hoop with the threads?

What buffer tube are you using? If its longer than mil spec you may need a larger spacer or longer buffer weight.

EDIT: Don't own a MCR upper, just thinking what could cause the carrier to travel to far rearward.

Had it happen with my Shrike. I put a quarter in the buffer tube with the spacer and it solved the problem.

Because I hate people that just go "Fixed!" without stating the resolution: I dropped a quarter into it, and it can no longer contact the rear of the upper receiver. Thanks for the tip.

From an engineering/design standpoint, the maximum travel of the MCR bolt carrier, as allotted by the upper receiver, may bottom out (impact) on some receiver extensions due to the variations in how many rotations/threads you utilize. If the lug can hit the rear of the upper receiver, it may hit the receiver extension. The additional spacer can offset the maximum travel of the recoil assembly so either the buffer impacts the bottom of the RE/spring fully compresses before the shuttle feed lug can impact the end of the upper/beginning of the RE.

I guess Fightlite stretched out the tolerance in this area as far as possible, giving the carrier as much room to accelerate as possible to ensure reliability.

Yeah they are notorious for having troubles stripping rounds from links so in sure they want as much travel and momentum on the forward stroke as possible.

When cycling the action by hand, the carrier may appear to stop ahead of the buffer tube. However, at speed, the buffer end will compress and allow contact.

I have not needed to use additional spacers.

Too many spacers will cause failure to strip or cause the bolt catch to not engage.