There are AR-15 pistols. Really, it boils down to a lack of a stock, having a pistol grip and a short barrel. It is all about cosmetics. Personally, I have little interest in a 5.56mm pistol. That is due to its short barrel. A pistol to me is simply a last resort prior to hand-to-hand combat. My choice in that case would be a M1911 due to it’s knock down power.
There are also AK 74 ‘pistol’.
Which at least don’t have the stupid looking buffer tube sticking out the back...
http://www.tombstonetactical.com/catalog/arsenal/slr106-58-slr-223-pistol-20r/
And don’t forget you can get Thompson Encore/contender types in .223
Not that I’d want any of them.
A pistol is helpful in fighting your way to your RIFLE!