The ROI on an M16 A2 (that's a .50 cal above), since 2004 has been 50% and as you can see below, the price has risen on a slow but steady basis. No asset class has held its value better than the machine gun in the past 25 years - not stocks, bonds, gold or real estate.
The value of civilian-owned machine guns has gone up steadily since 1986 when the government banned automatic weapons, but they allowed several hundred thousand to remain on the market. So the supply is constrained and actually falling, as the supply dwindles due to attrition. The current price of a 30+ year old M16A2 is right around $25k, which is amazing since you can buy a brand new semi-automatic AR15 for as low as $1,000 over the internet. The AR15 is identical except that the M16 is fully automatic so when you pull the trigger it spews bullets whereas the semi-auto AR15 requires you to pull the trigger to release each round. Both weapons use the same capacity magazines. Buying guns over the internet has become so commonplace that last week some guy bought a tv and got a SIG 7.62mm instead. Of course he wasn't the least bit thankful for having inadvertently received the Cadillac of assault rifles. If it were me, I would have been like a kid on Christmas Day - my only thought would be how the hell do I explain to the wife about the tv?