I have built a 9mm pistol, which I'm waiting on the stamp to come back to SBR. I have a New Frontier Glock mag lower, standard upper, 5.5" barrel, 9mm United Defense Lab bolt (with added weight from factory), carbine/pistol tube with standard carbine spring. I made my own heavy buffer out of cold rolled steel for the extra weight. The buffer is longer than standard, it will clear the mag no problem but is too long to get behind the bolt hold open lever. That is by design, not a mistake on my part. I have no LRBHO on this set up and wanted to minimize the speed of the bolt. I also planned on it being very heavy so that the action would stay closed longer because I will be adding a suppressor 100% of the time. Total weight bolt and buffer is 23.2oz.

in the build configuration I have, I test fired and when I hold it at arms length I get good function out of a standard G17 mag. When I simulate holding as if it were a carbine it fires but fails to return to battery, in fact the buffer hangs up in the tube, then will move forward again and strip a round. Cases are ejected at 3 o'clock, about 2-3 feet from where I stand. This was with Winchester 147gr FMJ.

Again, I want as heavy as I can to keep is closed longer for more quiet when suppressed, but it has to be reliable.

Open to suggestions on what to tweak.