This military collectible was lost when there was a work-around, either entry in the NFA registry as a transferable SBR or simple conversion to legal length. Numrich Arms aka Gun Parts Corp received a number as parts kits, but the last time I checked more and more of the parts are marked "sold out". Maybe not a big deal to hunters or wilderness hikers since there are the M6 and AR7 in civilian versions, but for military collectors, it means there are few examples of the M4 around. It is like the stamp collectors: why bother with glued paper when there's e-mail?