The breech block has {HEAD ~ at least one bore} which receives a locking element supported for radial movement in {TAIL ~ the bore} 
The breech block has { HEAD ~ at least one bore } which receives a locking element supported for radial movement in { TAIL ~ the bore }
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL