Sebastian shows a build he worked up.

Here is one of mine

And add $200 for the tax stamp because the barrel (11.5 inches) is below the length blessed by the .gov (16 inches).

Similar to an idea I have been kicking around for a while. The CAR stock is smaller than an M4, which would be good for this type of build, but maybe too much of a good thing. Other changes from the picture: a dual or quad rail gas block, with a pistol light/laser combo mounted underneath, and maybe some sort of BUIS (you know, in case the laser AND the EO Tech decided to head south at the same time).

Try it yourself here.