I know this is an old topic but it's the first that comes up when you search on Google for pro-mag vs Tapco.
And Since I just got 2 mags of each with my T6 system which honestly has nothing to do with fitment of either mags I figured I should post an update
So Tapco. . . Great quality ! feels great works SMOOTH BUT ! There performance is lacking . . It's a 20rd mag and really only works properly at about 17rds. I have pulled the spring to help with this issue and it does help . . For awhile. . Then you have to do it again. . . Kinda stinks. And this is with BOTH mags so it wasn't a luck of the draw issue. No keep in mind this is with a Tapco mag that came in a zip lock type bag.
And Pro Mag. . Well TONS of online hate for PM but I had to try it since my other 2 Tapco mags had there little quirks and I wanted 30rd mags. . First thing I check was tension . . It was GREAT ! holds all 30 rounds properly and ejects them with no feeding issue. . Then we go to the fitment. . and this is where your going to get frustrated. The back of the mag at the very top section that is the back of the curve overs that hold your rounds in. . well they need to be sanded down JUST a hair. That was it. After that was done they go in well with a little umph needed. The Tapco's still slide in better but meh. . These pro mag's where in zip lock bags as well just not near as nice bags as the Tapco ones. .
So all in all it's really up to you. I will say of the 2 issues it seems that the Pro Mag one is less of a long term hassle. That also being said I had already purchased a full size Tapco fore-grip for my stock and it fit very nice with the 20rd mags but the 30's it just doesn't fit . . . AT ALL. So your either going to need a stubby or what I really like now since the Pro Mag's fit VERY snug is to look into getting the hand grips that slide on your mag's. I think those would be great for the 30's and then that leaves you options for lights/laser/Bi-pod on the front or your Bayo if that is what you already have. . .
So I hope this helps and I really hope this doesn't get deleted. Lots of people search for this topic and really the SKS Boards always pop up on top when I search ANYTHING regarding SKS.
When holding the mag's instead of a fore-grip I will say that the area that is cut out on the T6 stock for the duck bill on the mags is a little sharp so I am thinking of putting maybe some permanent padding on the bottom of the duck bills to avoid getting my fingers dug into.
Thanks a ton ! :D