My airsoft gun uses 8.4v 1200 mah and i was wondering, Can i use like 1500 mah>?

Question

With out damaging it?

Answer 1

As long as the voltage is the same, you can use it. It will last 125% longer.

Answer 2

Yup. The mAh rating of a battery is a measure of the amount of power stored in it or, to put it another way, how long it will last. The higher the mAh value the longer it will last.

The voltage rating V determines how fast it turns your motor and therefore how high a rate of fire you get.