How to convince my parents to buy me an airsoft gun for christmas?

im 15, and christmas is pretty much the only time i ask my parents to buy me things, but this airsoft gun is 300 dollars. i already have one that i bought from my friend, but it is not that good, and getting my *** kicked in airsoft wars has been a monthly event. they are convinced that if i get it, i will never use it, and they don't want the BBs in the backyard, although i do not plan to use it in my yard.