I had the same problem just a couple weeks ago. here's some advice i got. it turned out to be all mags for me but after a lil filing of the trigger guard it's taking all mags 100% and they drop free!
Check that the trigger guard is not the problem.
Make sure it is not related to the upper or bolt catch. If not,mark all over a mag with white chalk and insert it fully. Remove the mag and see where the chalk is removed. File those areas inside the magwell that correspond to the areas that the chalk was rubbed off. Take 5 swipes with the file,then repeat the above til it fits well.
Generally takes less than 3 minutes.
cpermd
Before you start grinding on your lower have you tried using different mags??
I have 2 Oly lowers and 1 Bushy. The Oly's are both tighter. I was using Center Industries mags and they were really tight in both Oly rifles.
Then I bought some Okay mags and they fit perfectly. They slide in and drop free easily. Also Thermolds fit great for me too and drop free.
Maybe this will help you out and keep you from messing up your mag well if you don't have to.
NightHawk
Wow, it is a Center Industries mag that i'm using. Only mag i have at the moment.
And it is getting caught on the trigger guard. Before i start filing the trigger guard i'll get my hands on some other mags and see how they work.
Thanks for the help guys.
Stone