Believe it or not, I've never ran into stretched primer pockets yet in this caliber. I suspect holding them to around 38.5k psi isn't much strain on a case who's parent is rated for 56k psi. Now in the case of the 30 Remington AR which uses a necked down 450b case and which I also load for, it has the heavy duty bolt and barrel extension. You can load them up to 55k psi. They do stretch primer pockets if you load them all the way up to their max rating. Getting back to 450b cases. I've gone as far a 5 reloads without annealing. Your "Waterloo" will be case shortening, or at least that was mine. I do not use them once they get below 1.680. The taper crimp works against the case mouth. As it gets shorter, the same die setting will produce less and less crimp tension. That's why you never trim these babies. I sort my used brass by:
A) Case Head Expansion. I have large plastic jars labeled <.5000 (like new), .5000-.5005, .5005-.5010, .5010-.5015 and >.5015. Depending upon the generosity of your chamber cut, you may or may not get .5015-5020 to reliably chamber. Nothing worse than prying a round stuck almost all the way in, out on the firing line. My original loosey goosey Bushmaster chamber would digest .5020 all day long, but the three subsequent barrels I've owned, had spec spot on chamber dimensions and they would not take the .5015-.5020 case heads reliably. One will take up to .5018 if it is clean, but again, why risk it. So, I draw the line at .5015 max.
B) Length. I used to sort them into clear Plano Deep Stowaway boxes with adjustable partitions in .005 length increments. One box for each number of times they'd been fired, then by length. Like 1.680, 1.685, 1.690, 1.695 and >1.695 (like new), but the length was almost always predictable by how many times they had been shot. That being the more times they had been shot, the shorter they measured out at. So, now I sort them just by case head growth. When reloading time comes, I match ones from the particular jar I choose by length in groups of 5, lined up in the reloading tray. I try to keep ones for the same charge weight around +/- .005, IE 1.690-1.695 or 1.680-1.685, that way at least the taper crimp will be consistent within the charge weight group. One less variable to add to the experiment. I've never shot identical loads, except for different case length to see what actual impact it actually had upon velocity SD. Might make for an excuse to go to the range and do some shooting some time. Of course, if you side crimp them, the length wouldn't matter since you're crimping below the mouth. I save my cases that have grown too short for side crimped experiments.
Something to consider: My experience has been that light loads cause more case shrinkage than stout loads. I suspect it is about how hard they grip the chamber wall when fired. The other factor is if you have a loosey goosey chamber. The wider they stretch before being limited by the chamber wall, the shorter they will become. The good news is you get some of that length back after resizing. Try taking a handful and measuring their lengths before and after resizing.
In Theory, there is no difference between theory and practice. In Practice, there is.