I'm very pleased with my B&T adapter. Well machined, good tight fit, easy to install, mags slide in nicely and fit securely. The XCR mags I'm not overly impressed with. They're kinda stiff and don't feed smoothly BUT they seem to function well. I shot 160 rounds today only had one failure to feed. I did however notice that it is CRUCIAL to have the rounds at the very back of the mag and to really let the bolt/carrier hammer them into the chamber.
One other note, I wasn't able to successfully load these with stripper clips.