One of the guys uses that Magpul insert thing for PCC matches. It seems to work well and it isn’t expensive.
I had an older 9mm upper conversion from CMMG. I tried 3 different magwell conversion blocks before I found one that worked reliably. And it was quite expensive. It was the Colt magazine conversion. The problem I had were with the Colt magazines. I bought good ones but when the second from last round was fired the last round would come out of the magazine before the bolt came back. The bolt would hold open like it was the last round and there was a loose round in it. It was annoying but didn’t jam. I gave up on having a 9mm PCC as I just wanted something like an SBR in 9mm but obviously I can’t have one here.
Fast forward a bit and I bought a CMMG upper in 300 Blackout. That upper has worked flawlessly. My lower is another story. When I adjusted my trigger a little too light and when it got a little dirt in it after about 40 rounds it would go full auto.
I finally had to back off the adjustment.
I have a lower looking for an upper. I tried the 9mm conversion I mentioned above but I just was not impressed with it. I was not also impressed with the 9mm cartridge in a 16” barrel carbine. But now with the Banshee upper conversion with those ARC magazines I think I know what I want now. And since I am moving to a free state an SBR and a can looks like are in my future. The day I move I am going to apply for my tax stamp. My nephew wants me to go with him to the tactical PCC class he goes to. It’s at an indoor range so has to be a PCC. I was thinking a dedicated PCC instead of the AR conversion since I don’t like those magwell conversions. This certainly makes things more to my liking.