I think RK has nailed it for you. IIRC, the closest that Colt made was the R6721, which only had a 16" barrel. This was the tactical carbine model. However, there were only 134 made before the ban and many went to Colt employees so they are very expensive, about $4K. However, I think that the R6721 is still made for LEO's only. If you are interested in a 16" barrel you might check with Colt. I assume that they would sell you an upper since it is not a restricted item. Otherwise, your best bet is to build it as RK says.