Horst wrote:isn't the biggest issue that overall it needs to have 16" length?
That is the law. The barrel must be 16 inches in length. To have a shorter barrel, one must go the Class 3 route and pay a tax and have it registered as a short barreled rifle. This also brings into question ammunition choice. The military uses the M855 round which uses a 62 grain bullet. Tests have shown that while the 62 grain bullet performs well in a 20 inch barrel, it's range and energy drop off precipitously in shorter barrels. The tests results between a 20 inch barrel and a 14 inch M4 barrel are startling. Tests have also shown greater wound channels generated by 55 grain bullets which tend to yaw after striking an object. This is due to higher velocity when striking an object. For those reasons I use the former M193 55 grain round in all my rifles.
One way of getting around the 16 inch limit is to build a "pistol" and install a "wrist brace" on the buffer tube.
http://www.brownells.com/handgun-parts/ ... 65542.aspx