I highly recommend only buying a small number of third-party magazines to test, rather than buying a pile only to find some sort of problem, later.
While I don't regret my purchase, all of the 10-round AGP magazines I bought have a tendancy to "feed low", jamming the front-middle of the shell into the bottom of the lower feed ramp. Essentially, all my mags seems to need some adjusting of the feed lips to achieve a steeper feeding angle. Last time I was at the range, I'd shot 150 slugs out of one AGP mag with no failures, so they're great for the range. Any time I let them sit loaded, locked into the S-12, the first round jams against the bottom of the feed ramp consistantly! Thankfully, this does not occur with the 5-round magazines I own, which have fed reliably every time.
So, treat those third-party mags as great for the range or as a product needing a little TLC before being used for potential self-defense!