I think the BUL (and Kimber) have the mag catch slot about .050 higher on the top edge than the SA, RIA and ParaOrd. It's a mod you can do with a flat needle file and some care/patience--just a few strokes and check fit it 'till it will latch in, then a couple more to give a little leeway--DON'T OVERDO IT and take your time.
I've done it to the Para mags from MecGar and haven't had a problem so far.