PCC 9mm Fox Trot Mikes lower - doubling issue

I have a 9mm PCC that I just recently assembled using a Faxon 16" barrel and Fox Trot Mikes lower assembly from Brownells. It has been doubling here and there. Yesterday about 3 times out of 70 rounds. Today about the same average. Trigger is mil spec 5.5 pounds so I don’t think I’m somehow bump firing it because it has done it from the shoulder as well as the hip. I traded out the trigger group thinking maybe something was not quite right about it. Still does it. I traded out the firing pin spring for one that is a lot stiffer, still does it. ANYONE have any ideas? Since it is blow back operated, I figure that a possibility might be slightly high primers and the energy of the bcg going forward is setting them off on contact. But I’ve never had any issues with HAVING high primers to my knowledge. I guess that would be the next think I look for.

It can’t be a high primer. I just checked all fired rounds and there was a firing pin indentation in all of them. High primer ignition doesn’t get firing pin indentations in them.

Have had similar issue with 2 of my 9mm AR’s. They ran great and ate any ammo. Then I tried running range grade ammo. Never again. Even squibbed one and fired one behind it. Was lucky that barrel and chamber so thick. Didn’t damage gun, but put some debris out between receiver halves.