New build. BCG won't retract fully. Gets stuck when about 2/3 retracted.

I have a situation I am unable to diagnose on a new build. I have a new lower pistol build and I am attempting to attach an upper. The upper fails basic function testing. I can't pull the charging handle all the way back without forcing it. If I pull past the point of resistance then the BCG gets stuck and doesn't return forward. I have to use the forward assist to send the BCG back into battery. When I try the same upper on another lower, everything works fine. When I try a known good upper on the problem lower, I experience the same issue. So the issue is definitely with the new lower. Any ideas? I don't see any new wear patterns or drag marks on anything. Any ideas on where I can start troubleshooting? I'm kind of stumped.

Check to see if your receiver extension is too far in or too far out. The bolt carrier may be binding up against the buffer retaining pin.

I assume you have a standard pistol buffer tube and a standard carbine length spring and buffer.

First, give it a good overall check for straightness by holding the lower (with upper removed) at arms length and sight down the top of the buffer tube across the receiver. You should be able to notice any cast left or right by doing this.

With the upper still removed, and the buufer and spring in place, slowly push the rear of the BCG against the buffer, pushing the BCG into the extension. It should go in and out smoothly nearly to the carrier key without noticeable drag, Take into account the resistance/upward tilt as the bottom of the BCG goes over the hammer. Since the upper receiver is not holding the BCG in alignment with the receiver extension the BCG is somewhat free to move left or right. If you get any resistance here, it is mostly likely a defect in the tube.

Take the buffer and spring out, reattach the upper, reinsert the BCG, then run the action. It should allow you to cycle all the way to the rear. The BCG should fall forward when you hold the muzzle down, if it doesn't, give a sharp downward movement to snap it forward. We are looking to see it there is any rubbing/extra effort indicating the BCG is rubbing against the tube as you bring the action to the rear.

If that is the case, it indicates that the bore of the upper receiver and the bore of the receiver extension/buffer tube are not aligned.

From what you describe, I would guess that the receiver is slightly out of spec where they threaded for the receiver extension.

The hammer may not be fully cocking, causing drag on the bottom of the carrier.

More details would certainly help. Things like parts used, etc..

Does the boltlock function test ok?
Is the hammer function test ok?
What’s up with the buffer retention pin & spring?
Is there a burr or damage to the charging handle or groove it slides in?
Does a different BCG give the same results?

If you can't tell/see what is wrong then eliminate variables. Try removing the action spring/buffer. Try removing the hammer.

Sent from my XT1565 using Tapatalk

So Clang Clang, what did you figure out?

Hey guys, thanks so much for the suggestions and info. Apologies for not checking back in sooner. I'm a .gov contractor and have been getting crushed at work this week. It's 0300 and I'm just finishing up now. Gotta be back at 0800. I haven't even been into my safe since I first posted. I'll finally have some time to trouble shoot, post some photos, and respond to everyone over the weekend. Scouts honor. Thanks fellas!

Welp, I finally found some time to get back into the safe and trouble shoot tonight. Life has been BUUUUSY.

So it looks like the BCG is definitely rubbing on the hammer, as Todd K alluded to. I'm pretty sure the hammer is cocking fully though. It is a KAC carrier on an 80% lower, with a Rise Armament RA-535 drop in trigger pack. I used some crayola water soluble marker to color on the bolt face like so:

https://i.imgur.com/3oPZEBSl.jpg

After cycling the action once, I reopened the rifle and this was the result:
https://i.imgur.com/RbPSQ7Ql.jpg

So it's definitely rubbing. The question is, how much rub is OK?

EDIT: The RA has two tension screws that protrude downward from the trigger housing and push down against the floor of the trigger pocket. I suspected I might have slightly overtightened them (which forces the entire trigger unit upwards from the lower), so after a quick loosening (1/2 turn of each screw), the BCG cycles much, much more cleanly. It's not quite as slick as a factory KAC or BCM, but it's pretty good.

Should I gently relieve the leading edge of the bottom of my carrier with some emery paper? Just to put a subtle round on that edge so that it slides over the hammer better? Not sure if that's a wise idea or not. I'm asking because I'm almost positive that doing any work to the hammer is definitely a bad idea.