Nope. Buy the stamp (i.e. pay the "tax") and buy the machine gun. Lots of folks own fully automatic weapons, including REAL assault rifles, machine guns, etc. Not cheap to keep'em fed, but perfectly legal to own (at least by federal law).
The cost of the ammo fades into insignificance compared to the cost of the weapon. The post '86 ban (signed into law by Reagan) has had the effect of driving prices through the roof.