13.5″ KAC URX 3.1…
The URX assembly is unbelievably light. Tube rails can be very light, but often what lets them down is a heavy steel barrel nut. KAC’s design allows for an aluminium barrel nut. The only downside being the need for an expensive, proprietary KAC wrench. Dytac and Iron Airsoft do their own versions on the wrench. The Dytac one seems to be made of cheese, so if I had my time again I’d go Iron Airsoft (which is available from FCC).
The URX is a two piece design, which also distinguishes it from most other tubes. The rail halves are secured together with a friction fit and a sprung catch at the receiver end. The halves are further secured by two bolts, which can be screwed into any of the threaded holes along both sides of the length of the tube.
Extra screws are available for a more secure fit with all of the panel kits, which are available separately.