Just because it says 50 rounds doesn't mean you can't fill it up with less.
I have purchased some 68 round M4 mags and if I so choose to I can fill it with 30 rounds (if I so desired).
Also note that with a 30 round mag you're not going to get 30 rounds out of it. I know for the M4 at least the last 4 BB's don't feed because there isn't enough tension in the spring in the mag to push the last 3-4 BB's out.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE