Now that you've done extensive testing (2 different guns, many different magazines, etc) on Blazer, try a different ammo. Hope you find a solution soon.
Just a blue sky question: Is #3 mag factory HK?
That. Only issue I ever ran into with my USP was with a knock off magazine.
FWIW, I'll shoot anything through my USP, but I avoid crap ammo for the same reason I avoid crap gas in my turbo car or crotch rockets: sure it works, but I'm not going to get optimal performance, in addition to greater wear and more fouling.