When I was a kid and my dad was assigned to the USMC Equipment Board at Quantico I used to hang out around the Ordnance School there. One day they had a bunch of 1903 receivers that were in the Low Number range (they were SA's below 800,000) and one of the instructors did a demo on them with a ball peen hammer and the hit the receiver ring with a good hard rap. Several simply broke with one strike. For years I kept those ring tops with the serial numbers but eventually lost them. So, yes they do break easily, not all but some. As for me, I do not shoot my low number rifles, far too nice to risk destruction when there are lots of safe ones out there to shoot without any need to worry. At that time they were also using Johnson Rifle receivers for the students to learn how to do milling cuts, the dumpsters used to get really neat things thrown in them at the end of the day.