OK.. I'm not sure how/why kids think that if you join any of our military services you get to pick waht weapon/uniform/tactical gear you get to use. There are some things you can "get away" with, but most times you get issued a standard item.
If you join the Army and earn the right to be a sniper, you will start out with an M24 (Remington M700) and possibly you'll get to the M82. If you get to the elite ranks of SF or CIA or eventually employed by a PMC then you may get a choice since they usually are the groups who test new weapons.
If you join LE and eventually a S.W.A.T. unit, you'd have a little more freedom on the choice of weapon you use, but the catch may be, you pay for it yourself.