For anyone attempting to build a current M4 or M16 clone:

Just came across a review for Carolina Laser Works who produces UID labels for firearms and all you need to do is to provide them with your serial # and the appropriate Cage Code for the manufacturer (i.e. Colt's is 13629) and the Part Number for your rifle (i.e. older Colt M4s were assigned the "9390000" part number, whereas most recent and current M4A1's have the "12972700" part number).

Chad from Carolina Laser Works may be able to assist in tracking down Part Numbers if you are unable to get in contact with someone at the manufacturer.

I ordered from them on Tuesday online ($10.00), was shipped Wednesday for $1.00, and in my hands today (Saturday).

I don't have an M4 clone, but thought it was a nice little cosmetic add-on to my rifle. The quality of the UID label is some sort of thin metallic material, definitely will be able to withstand the elements, CLP, oil, etc.

CAROLINA LASER WORKS: https://carolinalaserworks.ecwid.com/