So the purpose of this is to illustrate the 90% solution. It requires an intermediate cartridge like the 5.56 or 6.8SPC firstly (a cartridge designed for 16-20 inch barrels), and it's pictured with a standard AR magazine, two components our current military could continue using more effectively from a bullpup design like this.
-Secondly, the fuzzy shadow in the background is of a collapsed stock M4 Carbine as close to correct size as I could manage, showing that even with the 20inch barrel or the suppressed 16 inch (top image), this design is still as compact as the M4.
-Third, I changed it to incorporate user configurable rails on 3, 6 and 9:00 positions.
-The bottom image is of my Designated Marksmen Rifle (DMR) version. Rather than lego systems or common receiver systems, this requires only a barrel and optic change, both of which can be done in seconds by the user. Rails could be bolted to the bottom and a bipod could be stuck on too to round out the role.
I used photographic elements in this and that scope is like 4 grand which is nothing more than overpriced tacticool BS (though the ACOG is plenty pricey). But, a scope that offers something like 1 or 2 to 8 power magnification really does sound ideal for the DMR. That said, a good ole fashion scope and offset irons or dot sight would work just as well.