hello Olaf, and good spot!
O-oh, i think we risk opening up a can of worms here, as the subject of to which pressures one can safely load one many of the classic old european cartridges if it is done for strong modern rifles and actions and with good brass etc has been discussed vigorously on here before. And i dont recall any consensus being reached.
I actually myself tried to create a single thread to collect the ideas and opinions on the matter here:
Pressure limits in older european calibers, - do they make sense with modern actions and brass?? But even then i dont think a final conclusion that most could agree upon was reached.
And bringing it all back home to this very thread, didnt
@harrygrey382 state that he might load his 7x57 all the way up to early to mid 60 ks ? (Edit: see post #31).
But back to Jack, - Jack was a reloader, and i think a good few of his loads have been known to be "on the hot side", and not just for the 7x57 but for his 30-06s and 270s as well. This doesnt mean that they were unsafe, in his actions or rifles of course.
As mentioned i am not sure many, Jack included, has much access to pressure testing when building up loads back then, so i could imagine that Jack took an outset in what data he could get access too, and then used his experience from there, probably looking for the same typical signs of (over)pressure that many of us still do to this day, when reloading. But that is just a theory of course.
Another element that could help explain the 53 grns quoted, (other than a typo of course), could be that the specific powders and projectiles used back then are slightly different today. Changes can after all happen to a project over 50-80 years, even if the name doesnt change much.
But yes, Jack OConor might also simply have been loading his 7x57 more aggressively than what many manuals suggest today. Another gun writer, Johns Barness is also a big fan of the little 7, and writes about what he believes could be terms as safe modern loads for the 7x57. But tbh Normas quoted 7x57 loads of today seem to be pretty fast, and i assume that iswhen sticking to the cip limits of 50.565 psi max pressure.
So a valid question might also be if there is much need to push the 7x57 loads much beyond that sort of pressure to chase another 50-75 fps and a bit more energy, if the precision, power and speeds are good at 56.5ks of pressure, and the rifle still feels nice and smooth to shoot ? Maybe if one is occassionally open plains or mounting hunting and could need to stretch it a bit at times?
But each to their own, i just hope that whatever path is chosen, all follows good and safe practice.