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