Each of said walls defining {TAIL ~ a secondary receiver chamber} sized to receive {HEAD ~ a magazine cartridge} of rounds a shorter length than otherwise provided for in a rifle 
Each of said walls defining { TAIL ~ a secondary receiver chamber } sized to receive { HEAD ~ a magazine cartridge } of rounds a shorter length than otherwise provided for in a rifle
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH