I bought an original about 1 year ago and haven;t fired it.
I belong to several gun groups on line and posed the question about modern ammo versus military ball ammo in Garands and a number of responders told me hwat you said. A great many others said that the Garand wasn;t desinged to handle the kind of higher pressure commericial ammo available today.
I do know the Greek government still manufacturers military ball ammo for these things and there are suppliers on the net where you can get the stuff.
I had some modern commercial ammo from Winchester and contacted them directly about the safety of using it in an M1 Garand and a M1917 .30-06 caliber WW1 bolt action.
They told me it was safe in the original WW1 bolt action but not in the Garand.
I think that it would be wiser to avoid modern ammo in a Garand, and if you absolutely had to use it, use the lightest weight bullet available - I think its 150 or 160 grains. But it would appear you are still taking a chance on damaging yourself and/or a WW2 piece of history if you use modern ammo in it.
I wouldn;t based on what Winchester told me in an e-mail communication.
It is not really modern vs military ammo. It is bullet weight and pressure.
http://www.inetres.com/gp/military/infantry/rifle/30_ammo.html
You can buy modern ammo that is similar to military although it is still more expensive to shoot. I’m cheap, I shoot the Greek stuff - preferably in the clips - have a ton of them around the house.