If you can use fine sandpaper a pair of pliers and a hammer and punch you can tune up the metal mags to be reliable, all they need is some fine tuning to be reliable with 2 3/4” shells. Take the mag fully apart, remove any burrs or rough edges with 600-800 wet dry sandpaper. I had to shorten the mag spring with my pliers/cutters by a coil or two to match the 1 metal factory mag it came with, I bent the mag lips so the shells were held snug but not tightly when loaded in the mag. This stopped them from popping out, lips too wide and the shells pop out, lips to tight and you get some feeding issues.
That ultimately was all I needed to do but I went a couple steps further, I dimpled the nose of the follower upwards with a punch and hammer. The dimple sits under the nose of the shell, it’s gives a higher angle (1/8”-1/4” higher) to the shell as it comes off the follower eliminating nose diving and jams as it hits the feed ramp and enters the chamber, this really eliminated any fail to feed jams I had. I then took the mags apart and with my bench polisher I buffed and polished the mag lips and followers and lightly lubed them before I put them back together.
All in all it took me 1-2 hours max and my mag fed grizzly loads, feeds and shoots any 2 3/4” shells I throw at it. I wish I could get it to feed 3” shells better but that’s just a limitation of the mag I’m going to have to live with, I used it grouse hunting last year and it didn’t skip a beat. The biggest issue with the metal mags, especially the aftermarket metal mags is they’re over sprung, roughly machined, and the angle of the follower is a bit low. Tweak those issues and you’ll have a gun that works quite reliably, I will say if I was in the market for a mag fed pump action I would just buy an 870dm. It seems like Rem came out with a good mag design, which is the mag fed grizzly’s real flaw. The rest of the gun is no less reliable than the tube fed grizzly, thankfully I like to tinker so tuning mine was not an issue.