Sorry for the thread necro, but I had to share this...
So, filing on the mag catch opening on the mag (the tiny few passes I did anyway) didn't work for crap at the range; it still happily ejected the mag every time the bolt cycled. I just the other day got in two more MecGar mags from Greg Cote LLC, figuring hey, maybe the one I have is bad, so let's order two more...they can't ALL be bad.
Well...neither of the ones I ordered worked, either. The gun kept spitting them out every time the bolt cycled. So I started searching for a new mag catch, figuring it was worn down to a nub.
Anyway, I got home from work, and decided to take a look at the mag catch...you know, just in case; before I spend $60 on a replacement, let's make sure I actually need to replace it.
The mag catch actually looked pretty good! I couldn't believe it. So, WITH THE GRIPS OFF, I put a mag in and cycled the action....hey, wait, it stayed in this time. What the hell? I loaded up some dummy rounds (it NEVER worked with dummy rounds, ever)...and cycled the action...and the mag stayed in like it was supposed to. WTH? I scratched my head and examined the thing up down and sideways...then I looked at the left grip and realized two things:
A) The left grip covers up the mag catch leaf spring
B) The incredibly crappy left grip was cut ...well, crappy, because it was a cheapo grip and not original....and the channel that the mag catch spring rides in, was cut too shallow.
Effectively - the left grip was depressing the magazine catch enough to not lock the mag in.
So, I filed out the channel inside the left grip panel a bit...and now, hand-cycling, it works! The mag stays in. I'll take it to the range and verify it...but I think I'm sorted out. I guess I'll find out soon enough
Hopefully that little tidbit helps someone else further on down the line