Convertible Sniper Rifles?

The Barrett Mk.22, which the marines are adopting, will fire .338 Norma Magnum, .300 Norma Magnum, 7.62mm and, .300 Winchester Magnum. The idea is that they don't have to carry different rifles, just one, with extra barrels.

I don't know how that will work in reality. At what point do they decide which barel and ammo to take? I presume that they're not going into the field with all of that stuff, but one never knows with the Crops.