So glad i found this thread !! Had a Colt mag CMMG 9mm carbine get this same issue. Had not shot it much (bought for Home Defense for wife "years" ago), just a few hundred rounds of ball and 3 boxes of HP to ensure we had sufficient familiarity and functionality and then sat it in the closet at the ready. Pulled it out for a shooting session (had recently replaced with a handier/zippier "pistol" version with much easier to load glock mags, don't hate me..) and after about 40 rounds it started locking the bolt back on every shot !! I assumed the spring broke and ordered a new one. When it arrived i tapped out the pin and the plunger just sat there staring at me like a shell-shocked solider in a foxhole, barely any of it sticking out above the lip. I applied a "fair amount" of CLP and let it set then tried all manner of internet suggestions to get it out but no dice. I did notice on the "bottom" lower half of the recess/hole some "silver"... which i assumed was that heavy grease sometimes used on gun parts and will come out when mixed/thinned out with CLP.. but upon close examination, i found it was the raw aluminium showing through. I did not make the connection that the impact of the bolt catch on the plunger's hole "lip" had actually created a burr that extended out into the hole and then prevented the plunger from leaving the hole to push the catch out/down. I had gone back to the internet and found this string and then the lights went on.. the line drawing opened my eyes, that had to be it. Went back downstairs and grabbed the micro file set and used the small file flat on one side but arc-ed on the other, 3 small flicks later, plunger pops about half way up !! Another couple of runs and it popped right out. Pulled it completely out, hit the entire lip (lightly) with the file, flooded it with CLP to wash out any filings, cleaned out the CLP and put the spring and plunger back in.. tested it and BINGO, back in business !! I would have never in a million years thought that the lip had "burred" over time from the bolt catch's impact, just a perfect storm.. might never happen to anyone else, but if it does, i hope they find this before they freeze or heat up or drill out their lower.. 10 seconds with a file just might do the trick !!
