How should i hold my gun?

There is an airsoft gun i might be getting but i can't decide, my one concern is this - I might be getting a full size replica of a m1a1 thompson machine gun. I don't think my arm will reach all the way around it or at least it will strain my arm to reach around the round mag to the grip, So i was wondering would someone with a real gun hold it by the magazine, i highly doubt it but i don't want to look stupid holding it during airsoft games. Please don't say im too young to be playing airsoft, Don't worry im 13 and it is legal to play at an arena and take my gun. i just have quite short arms and i bet it's hard for some aduts too because of the round (tommy gun) mag. Thank you!