It is the magazine that came with the gun. It wouldn't click into placeproperly at times (would come out if you pulled it a little), but I was always able to get it clicked in good before going out to where I couldn't just pull it out. Anyway, this time I pulled my gun up to my treestand, only to find the magazine was gone. Must have fallen out en route to the stand.

I will see what the gunsmith has to say. Thanks for the replies!