Just when you think you've seen every possible modification to the AR weapon system, another one comes along.  This one is particularly interesting as it's the invention of Mack Gwinn Jr., ex special forces and founder of Bushmaster, one of the leading companies building the AR platform today.  His new company, MGI (MG Industries) has brought the AR to it's next iteration as a completely modular weapons system, able to easily be converted between many calibers from .22 rimfire to .50 Beowolf.

Hydra in .45 caliber

The heart of the Marck 15 (weapon system is a modular lower receiver with a replaceable magazine well, and a quick change barrel and bolt.  This allows quickly converting the rifle to any number of calibers, my favorite being the  .45 acp. This changes the carbine into the spiritual successor to the grease gun, and actually uses original magazines.  The gun isn't much bigger or heavier than the original, especially with the stock collapsed.  Besides being handy, it has a much better chance at being reliable also, the original I had in Vietnam had a disturbing tendency to empty the mag at the slightest bump.  Seems to be an interesting take on the AR-15.