I finally broke down and purchased a 9mm upper for my shorty AR. This is a Bushmaster SBR lower with a 7" Rock River 9mm upper attached.

(Click to Supersize)

Unfortunately, this one won't be getting to the range for a little while. I was told I didn'a need to purchase the 9mm hammer. Unfortunately though, the notch in the AR-15 hammer is catching the 9mm bolt and preventing it from closing. (see the upper left of the AR-15 hammer)

I know there are hammers out there that will supposedly work with both 9mm and .223, but I'll likely just install the 9mm hammer and keep the lower in that configuration. I already have a .223 lower and don't really see the need to shoot my .223 SBR upper any way.

(Logic would say that I could just use the shorty .223 upper on my other .223 lower. But since it's not registered as an SBR, the ATF doesn't see it that way.)

For those of you who are wondering, I purchased the upper from Adco. At $485, it's a pretty good deal considering it came complete with the 9mm bolt. I didn't come with a front sight as pictured, however.