Another option might be to use a SLED, which is a single load "clip". They are used for single loading the Garands and remain in the gun after firing and have to be manually removed. Never tried to use one with more than one round though, but it might work. If not, then maybe have the clips modified to remain in the gun like the SLED does after firing. That way you just load three rounds into the rifle, and the bolt locks open on empty, and the clip stays put.