I measured the barrel front lug length on my full size as about 11.93mm and on my compact as about 11mm. Interesting what that <1mm of difference will make. The lug on the full size barrel can fit fully inside the slot in the compact frame, but since it is free to move slightly when the locking block unlocks, it can be just far enough forward where the lug gets tapped by the inside of the slide recoil spring shroud when moving back as it does under recoil. With the correct compact barrel, I am not able to make the slide recoil spring shroud and front barrel lug contact inappropriately at all.