My son just got a M1 Garand in trade and was telling me he had to change the gas plug to shoot commercial ammo.  I've never owned a M1, so I couldn't tell him either way.  Thoughts?

Right.   There is a risk of damaging the operating rod.   Ammo labeld garand safe might also be an option, but I have looked into that in very many years.

Correct. M2 is lower pressure than commercial ammo.

There are adjustable plugs and fixed-volume plugs. It's a personal preference thing.

Thank's a bunch!  I will look for a plug for him.

It's gas port pressure that hurts the M1 Garand.  Brand/powder type affects the gas port pressure.  Two powders can have similar safe chamber pressures but a powder that had  burning rate too slow for the M1 Garand can have gas port pressures too high.

With commercial ammo you just don't know for sure what powder they're using.  It may be safe this time and next year they've changed to a different powder and it's no longer safe.  And they don't tell you which powder they use.

The adjustable gas plug doesn't alter chamber pressure or gas port pressure.  It just bleeds some of the gas port pressure off so that the pressure in the gas system stays in the safe range.

I have one on my "shooter" M1 Garand and still, at this time, only shoot surplus ammo or my reloads with IMR4895 powder.  If I was going to run commercial hunting ammo I'd test it/set it up every year before deer season when sighting the rifle in to make sure the ammo company didn't change the powder to a slow burning powder out of the "safe" range for the M1 Garand gas system.

My opinion is most commercial ammo is fine. High velocity hunting ammo and heavy bullets are what you want to avoid. Both use slower powders that give higher gas port pressures that unnecessarily over gas them. Over gassed, I am more worried I will crack a receiver heel than damage a relatively cheap and repairable op rod.

CMP gives similar advice and says to avoid ammo over 170 some grains. They don’t address lighter bullets running high velocities.

In general there is no hard data what is acceptable or not. The military did not have a spec for max gas port pressure and no one has produced comprehensive data on the ammo mostly used by the military through them. It makes a hot debated topic.  

There is old guys that ran whatever through them or pushed 180+ gr bullets hard in 1000 yard matches. Then there is collectors that want to preserve them and are overly cautious. The later is becoming more predominant.

Excellent info gents.... make me glad I opted for a M1A for my choice of 30 cal. years ago

It's not an issue.  Use a proper greased rifle with saami spec ammo.