Malfunction was bolt locking back with one round remaining in the mag.  This was during a carbine course and it was sporadic at first, but then eventually was happening pretty consistently and with a variety of AR mags.  Some of those mags were verified as good (NHMTG with Magpul followers) and others were newer, but known quality mags (Lancer AWM and BCM USGI with Magpul followers).  The lower was a fairly new Colt OEM with all stock parts and maybe 150 rounds through it. 

Initially suspected grime had gotten in the lower, causing the bolt catch to stop the bolt prematurely.  Cleaned the lower and when I tried inserting a mag with one round in the mag, I noticed that it would push the bolt catch up.  That there wasn't much spring force on the catch.  Assumed something wrong with the plunger or spring, so ordered replacements.  Also checked other lowers that I had and all had good spring force on the bolt catch except for my LMT lower (about 6,000 rounds). 

After the replacement plunger and spring came in, I removed the bolt catch from the Colt and the plunger wouldn't come out.  I could depress it, but it wouldn't pop free as I expected it would.  Tried scraping around the plunger, tried applying lube, tried applying some super glue gel and a skewer, etc.  Nothing worked.  When I removed the bolt catch from the LMT lower, same thing.

I read as much as I could on the topic of stuck bolt catch plunger and nothing.  I resigned myself to that the plunger would need to be drilled out.  Took it to a LGS and while the staff had seen that, it wasn't common.  With the Colt lower, they eventually got the plunger free by sort of tilting it at an angle.  Apparently, the material on the bottom of the hole had gotten deformed and formed a slight lip that prevented the plunger from coming out.  I assume that was from the bolt catch hitting that area.  Thinking back, I noticed that that area of the lower was silver, which I assume was from the material being deformed there.  Once the plunger was removed, you could feel a slight lip that was formed on the bottom of the hole when you ran a dental pick over it. 

Then onto the LMT lower and that tilt trick didn't work.  I had to file the area where the lip had formed while holding the plunger down.  Since the LMT lower has a much higher round count than the Colt, I assumed that the material deformation was more severe.  I ended up rounding the edges of that bottom area on both lowers, tried to smooth it out as best as I can and put the plunger and spring back.  I haven't tested the lowers yet, but I believe that will fix things. 

I had not heard about the deformation of the lower being an issue.  The LGS staff that I spoke to said that while he's seen it, it appears to have been with lowers manufactured in the last 4-5 years.  I bought the Colt in 2016 and the LMT around 2012.  I had not noticed the LMT lower malfunctioning though.  And now I'm curious to check the bolt catch on my other lowers, one of which is a BCM that has maybe 5,000 rounds through it.  Though the bolt catch on that lower seems to still have positive spring force on it.  So it's not like these are crappy brand lowers. 

Anyways, just sharing what I thought was pretty interesting.  I guess it's somewhat reassuring that the malfunction didn't affect the basic operation of the gun, in that it could still shoot. 

Interesting.

I had the stuck bolt issue when the pin sheered off in the forward assist. Got a new one from the manufacturer and replaced with the small punch. Good as new.

Apparently, the material on the bottom of the hole had gotten deformed and formed a slight lip that prevented the plunger from coming out.  I assume that was from the bolt catch hitting that area.

thanks for the insight.  now I'll have to check my AR lowers to see if there's any signs of this.  thanks for taking the time to write it all out and post.

thanks for the insight.  now I'll have to check my AR lowers to see if there's any signs of this.  thanks for taking the time to write it all out and post.

Sure.  No worries.  That's what forums are for.  To share information and hopefully it can help someone who is encountering the same thing.

I wouldn't worry about it too much.  Just if you notice the bolt catch being a little "light", then something to check out and pretty easy.  Just don't lose any of the small parts.  When I was chatting with the LGS staff after getting one of the plungers out, I was fiddling with the plunger and spring and the spring went flying. Luckily I found it again though.   

Been having the same problem w/ my G19 AFTER  I put in an extended slide lock.
 It's getting better tho

Wow, I wasnt surprised when you mentioned its a new Colt lower, but for the LMT to have issues is a kinda surprising.

Wow, I wasnt surprised when you mentioned its a new Colt lower, but for the LMT to have issues is a kinda surprising.

Really?  I think of LMT, BCM and Colt in the same light in terms of quality.  I'll get around to checking out my BCM lower soon, at least to check it out. 

I've heard of some other brand lowers having issues with the plunger getting stuck due to burrs or other tolerance issues, but this was different. 

Feedback on another forum was wondering if buffer and spring weight played a role?  I wouldn't think so, but who knows.  The LMT has an H2 buffer (I think I put that back in there) and SpringCo blue buffer spring.  The Colt has the A5 system.  That said, I know many folks run the same Colt setup with the A5 systems and didn't have this issue.  And the deformation in the LMT was worse, but it wasn't malfunctioning like the Colt. 

Chalking it up to a learning experience and thankfully something that's an easy fix.  And the malfunction wasn't something that shut down the gun.  So while not good, you still had a gun that shoots. 

Shot the Colt lower yesterday after filing of the hole for the bolt catch and so far ok.  Didn't have any bolt lock backs with a round left, but I also never had that with this particular mag before.  I was using a 10 round PMag since I was zeroing the gun from the bench.  Hopefully won't see it any more.  At least if I do see it on other lowers, I know it's a quick fix.