I can't place an M4 Sand on the weapon rack, the game tells me it's not compatible.
Also, while modding it with Trader Modding And Improved Weapon Building, it keeps prompring me to build the gun even though it is already built (might be related, might not, thought I'd report it nonetheless).
Edit: I tried with another separate M4 Sand. This time the second issue is not present, but I still can't place the weapon on the weapon rack.