If it's getting stuck halfway back, requiring a trigger pull to release, it's most likely the hammer getting jammed on the sear. I had a couple of mk2 crowns doing this and for some reason (my assumptions are the hammer not being adequately hardened, as well as quite a bit of side to side play available to the sear, allowing the sharp side edge to drag on the hammer) the sear had scored the hammer and made burrs. Because the hammer is free floating it can also freely rotate, so they only intermittently bind up.
I also initially thought it had something to do with the mag as it seemingly only happened when shooting but I eventually managed to reproduce it without the mag in the gun.
The problem was initially solved by polishing the hammer and making little spacers to keep the sear in the midline. The hammer in of the guns did eventually require replacement however as it just continued to get progressively more damaged and started to bind again after about a month of heavy shooting. Once replaced it never happened again.
