The unmodified Garand can't shoot commercial .30-06 ammo. It was designed to shoot M2 Ball, which is a softer load than the current commercial loads. M2 is different for two reasons: pressure and primers.
First, pressure. The Garand can't handle hot loads like those made commercially today. Modern ammo is designed for bolt-action rifles, which can handle higher pressures. These give greater energy for whacking [fill in large game animal here]. The Garand action can handle those too. It's supposedly stronger than the Springfield M1903, and it reportedly took well in excess of the spec'd chamber pressure in testing to break anything (read: they finally stuffed a case completely full of powder and shot it, causing a crack in a locking lug of the bolt). The weak link is not the action; it's the gas system, which modern bolt-action .30-06 hunting rifles don't have to worry about.
The Garand's action is cycled by porting gas about an inch back from the muzzle to push on the operating rod. This is why the Garand has that "double barrel" look. The lower "barrel" is where the gas piston end of the oprod resides. The other end of the oprod is the charging handle, which is attached to the bolt. So gas pressure cycles the action back, ejects the spent cartridge, than spring tension takes over and moves the bolt forward, chambering a round. Go here in lieu of a thousand more words.
The oprod happens to be a very precisely machined (and thus EXPENSIVE) part of the weapon. It is designed to work with a given pressure at the gas port. Less will not have the oomph to cycle the action. More than that will cause it to bend, breaking the rifle. There are adjustable gas cylinder locks available which supposedly allow you to shoot commercial ammo, but they come with hefty disclaimers (read: it's still your fault if you break the oprod). Thus, if you shoot $120 worth of modern ammo through a unmodified Garand, chances are you will be shelling out another $120 for a new operating rod. This is why Garands need surplus or reloads.
Second, primers. The Garand was designed, as the SKS was, with a free-floating firing pin. The tip of the firing pin will actually tap the primer when the bolt slams home. Because of the hard primers of military cartridges, John Garand could get away with this engineering technique. Commercial primers are softer. This causes a phenomenon known as SLAMFIRE where the weapon goes off as the bolt goes home. VERY VERY BAD JUJU. I'm told this can also happen if you reload with non-milspec primers, but I haven't investigated the mysteries of reloading just yet (though I have my eye on a Dillon progressive press once I acquire the funds).
Capische?
A Mosin Nagant and SKS are aslo in the running.
