OK....I know I am reviving an old post but I am not sure if I am experiencing the same issue as the OP and need some help making my XCR mags work. I am NOT experienced in smithing or altering parts as I've never had to but am trying to learn...slowly.
I purchased a new BCL 102 a few weeks ago with 4 XCR-M mags only as a package deal. Excited to finally get it, I inserted one of the XCR mags but had trouble seating it with the bolt carrier closed. I then locked the bolt back, tried the mag again and was successful. At this point I pressed the HOD to release the bolt but it got jammed along the way.
The picture below is what it looks like...it's almost like it initially gets jammed on the "bullet ramp" of the magazine.
I then pulled the bolt back with one hand, pressed the magazine "bullet ramp" down with my finger and slowly advanced the bolt again. The second picture (below) shows how it got stuck again but it appears it is getting stuck on the top of the mag this time.
That being said, again, I currently know ZERO about smithing....any advice?