There is no doubt that some, probably most, single heat treat (SHT) M1903 receivers are perfectly safe. There is no doubt that a few were very brittle and while they passed proof will blow under certain conditions (not always ammunition or high pressure related).
The problem is that there is no non-destructive test to determine which is which; a proof test might "weed out" a bad receiver, but that will be done when a nice old collectors' item blows. Firing the rifle might also detect a bad receiver by having its pieces perform rapid surgery on the shooter's anatomy, but that too is the hard way.
Some folks contend that there have been no blowups since WWII because none have been recorded. But no one has kept records since the M1903 was taken out of service; there is no central point for reporting failures. I have heard of several and personally know of one receiver broken with a hammer (by me).