My mini did the same out of the box (bought about 6months ago from justpaintball), it got worse over time so i tightened up the bolts that held it in, they loosened up again and this time I've used loctite on them and hopefully it'll stay firm this time, though I've yet to play since.
Know a couple other mini owners who have had this too.
Know a couple other mini owners who have had this too.