Odd, unusual, and interesting cartridges chambered in an AR

[Troublco]

Kidicarus13's post about a .357 SIG upper in my 10mm thread got me to thinking...anyone have any odd, unusual, or interesting caliber AR's? There are a few curious ones out there; I used to have a .300 Fireball upper before the .300BO craze hit, and I had fun with it. I don't know if you can really consider a 10mm that way, although I know they are fairly unusual. I'd have to say mine is a .17 Remington upper, with a 24" SS bull barrel and a beast of a tubular free float fore-end. Thing's gotta weigh 8 pounds all by itself, but that gun doesn't move when you fire it. Baby's got back but it sure shoots well.