I'm more of a pistol guy but I've always understood that carbines were rifles chambered in a pistol cartridge.

I'm looking at putting together my first ar-15 with the help of a friend, but when I head to look at different websites they offer ar-15 rifles and ar-15 carbines.  It seems to depend on the barrel length more than the cartridge.

Granted my navy training didn't involve much with the m-16, but I did get to spend some weeks with the corps during my youngster summer and IIRC the m-16 was referred to as a rifle, not a carbine.

What gives?

carbines are just shorter than a standard rifle

for AR15's a 16" barrel is usually referred to as a carbine and the 20" and up are typically referred to as rifles.