The proof was only for 144gn NATO ammo nothing else. Not for 308 150gn or larger. These rifles are not 308 proofed. Lithgowdid also convert some No6 Mk1s the same action as the No1 Mk3* but they all failed. The range shooters with No4s could send their rifle to Lithgow for conversion to 7.62 NATO. There was a failure rate with them as well. Not all rifles came back from the proof firing. So if you have a No4 7.62 conversion use only 144gn NATO ammo or similar handloads.