How do i keep my step brother away from my airsoft guns?

i bought an airsoft gun in the past few weeks and when i got home from school my step brother had been shooting it and what do you know it refuses to fire bbs! i knew my step brother had been firing it because he flat out told me he did. so next im going to go to week im going to walmart to get a new thats $45 bucks and i dont want him firing it or even being around it because this ones automatic and it most of all i plan on using it in wars this summer so how can i keep him away from it he is 22 im 15 he stays home i have to go to school everyday i dont want to be mean and say BACK OFF or something like that because my dad and his mom just got married a few weeks back and i dont want to seem like an a** i also wouldn't mine finding a new hidin every few days