The issue sounds like it's the mags, not the rifle...
If a mag is pinned @ 5 such that the 5th round 'just' fits in, then there isn't any room in the mag for the 5th round to move down into the mag a little bit when the mag is inserted into the rifle with a closed bolt...
It's why you see guys posting that mags need to be pinned carefully @ 5.5 rounds... This provides that little bit of room for the mag to be seated on a closed bolt, but doesn't allow a 6th round to be loaded into the mag...
Cheers
Jay