They come with a 25 round mag.
When we got ours I thought it was pinned to ten rounds because I could barely get 11 rounds in it.
I had the same problem with mine,disassembled it and then put it back together. It seemed to ease up a little, but has been getting easier and easier to operate the more I use it. It seems that the follower catches where the metal upper body mates with the plastic lower. With a little bit of wear it seems to sort it's self out.