Surefire Hi-cap mag (x5) and a slightly modded M4 (heavy barrel and non-standard gas tube.)
A regular gas tube would melt into a pile of molten goop after that.
Surefire has done some mods to the design to hopefully get better reliability in more ARs (they were originally designed around Colt M4s.) Unfortunatly this means the release date is pushed back to the end of June.
It's popped up in a few places around the blogosphere this morning, but it's too cool not to mention. I mainly like it because my Dark Heresy character has the 40k equivalent. (Started with a "Roth Pattern Lasgun," which is a bullpup, and payed the Thrones to have it upgraded to a Long-las. Yes, I'm a geek.)
On sale at Amazon. I tend to keep a couple of the major calibers in my range bag, and .223 type ones on board my ARs (plenty of room in an A2 stock, and the Magpul ACS stock has just enough room to cram one in.) It's nice for blasting a little CLP down the bore and following up with the snake.