Can you should modern, off-the-shelf ammo through the CMP M1’s and M14’s or only surplus or handloads? I’ve heard some debate about it with some saying modern ammo is too high pressure and others say it’s fine.
Does CMP have an official stance?
I don't know the CMP's stance but I've had several Garands over the years and have two now. Am also on my third M1A. I'll add a couple of comments. M1As are difficult to scope. If you don't know what you are doing or have a quality mount you will waste a lot of ammo. I tried with my first one and finally gave up. Didn't even consider a scope for my next two.
Ref the ammo, the weak point on both rifles is the operating rod, more so on the Garand than the M1A. With the Garand the oprod extends to the gas port on the end of the barrel. Too much gas - too much pressure - too much recoil and you bend the oprod. This was so prevalent that the .gov did a modification to the oprod so it would bend instead of crack.
The M1A's gas port directs the gas into a chamber to a piston that strikes the oprod. So the piston takes the beating instead of the oprod.
I use a Shuster plug in my Garand. This is an overpressure valve that directs overpressure outwards and away from the oprod. Eliminates any possibility for overpressure. Its adjustable. When I'm firing reloads or commercial I open it all the way which makes the rifle a single shot. Then as I fire I close it until the rifle ejects the spent cases. I do this with any change of ammo.
The last weak point for both rifles is again the oprod. The oprod has a small tab that inserts into a slot in the receiver. There is a small cutout where it is inserted. With use and inattention the tab will wear and pop out of the receiver slot. The main cause of this is shooters pulling it outwards as they chamber rounds and open the bolt. The first inspection I do with either rifle is open the bolt while applying outward pressure on the oprod. If it pops out of the slot you will need another one. They aren't cheap.
I had a M14 in basic and AIT as a 12B Combat Engineer. I also had a M14 in Vietnam for a while but sent it on down the line as soon as I could because it was heavy and I had a lot more firepower with a lot less weight using a M16.
Either rifle is a good one. Anyone is welcome to shoot mine.