Regardless of the when the heavier bullets were made, they were not made for use in the M14, you even point out that the 172 grain bullet was designed for the M1 Garand. The M14 was designed around the 147gr T65 and the 150gr M59, that's what the design specs called for. In fact, the military eventually went back to the 147gr bullet in the form of the M80 cartridge because they found that it performed better than the 150gr bullet - kind of like the rifle was designed to work with a 147gr bullet isn't it.
You keep talking about bullets that were used years after the rifle's design phase and development tests were finished, and that's different than what I'm tying to say.
I keep trying to point out that the M14 rifle was actually designed around a specific cartridge and bullet, the T65. The only bullet that was used in the T65 was a 147gr FMJ, they did not have any concern for what other ammo would be used in the rifle during the design and development phase so the mechanics of the rifle were initially DESIGNED for a 147 bullet in a slightly different case than they have now. Later, after slight modifications to the mechanics and the chamber, they settled on the M59 cartridge with a 150gr as the standard military issue but even that changed and they went back to a 147gr bullet (the M80 cartridge) because it performed better - probably because the bullet matched the original design better, kinda supports my claim that the rifle wasn't built for heavier bullets, yes, they can work in the rifle (I prefer 165gr and higher for hunting and long range shooting), but the rifle simply was not designed for their use.
Yes, other bullets weights were and are used, yes, a proof load was used and like all proof loads they use a heavier bullet for that cartridge but all proof loads are meant to be used ONE time as a test, not for regular use. I'm not talking about fringe ammo that was used for testing or specialty circumstances, the cartridges used under those circumstances are not considered to be the everyday ammo that will work in the rifle without undue pressure or wear, the heavier bullets are used to perform a purpose without regards to any damage that they may do to the rifle, the military has lots of money and they can repair the rifle as needed, I can't. I have a limited budget and can't take the chance of ruining barrels or blowing up receivers.
The current standard issue ammo for the M14 is a 147gr ball round for standard infantry use because that's what the rifle was built to use. The military uses that bullet weight because after millions of rounds they've found that that bullet weight works best for general duty purposes. They also issue a special long range 175gr SMK for the snipers (and even they can have a hard time getting a steady supply of the 175gr ammo in the field because the military just doesn't make that many of those cartridges) and competition shooters. The current long range ammo used by snipers is the MK 316 MOD 0 which uses a 175gr SMK, it replaced the old M118LR cartridge that also used a 175ish grain bullet because the M118LR cartridges - are you ready for this? - were blowing up the rifles in the heat of Afghanistan - hmmm, imagine that, exactly what I've been trying to say, the heavier bullets create forces that wear out the rifle quicker. Yes they can be used but you have to be aware of the potential problems that they can create and do what is necessary to compensate as best as you can. Oh, and here's a fun piece of trivia for the M14 slamfire experts, the MK 316 MOD 0 uses the dreaded Federal F210M primer that supposedly blows all M14s up with slamfires.
