That means I can fire 7.62 (well probably. the gun is marked 7.62 and I got it from a reputable gunshop.) But there is extra space in the chamber up front. The cartridge for the 7.62x51 and .308 are nigh identical on the outside dimensions. The big difference is the wall thickness of the brass (and heh. if any gunsmith ever catches me saying something wrong, TELL ME, so I can correct myself and correct my words and not lead some reader astray.) . Military 7.62 as thicker brass. When fired the cartridge expands and this thick brass can expand into the gaps no problem. It's tough. But the thinner commercial brass in a .308 expands and may expand TOO much if there is too much wiggle room. In a chamber with tighter tolerances MADE for .308 (which all 7.62 made to date fit fine) a commercial .308 doesn't have as far to wiggle and the brass won't split or crack or separate. If a brass case has a failure the gun will have issue when the next round goes in on top of cartridge fragments. That would be bad.
Also, the thinner brass means more powder can be crammed into a .308, allowing for higher pressures. I'm not worried about pressure issues in a robust Garand receiver.
Will I stop using .308? Probably not. I will try to buy higher quality .308 and only use stuff that is factory new. I will NEVER reload .308 and fire it out of the current Garand, as reworking brass over and over only makes it thinner and weaker. I might load virgin .308 brass, but still seek thicker brass. For stockpiling I will get 7.62 only from here on out, and probably shoot off the spare .308 if possible. I will not cut corners and buy cheap .308 to save a few pennies a round when target shooting.
In the future I will eventually re-barrel the Garand with a higher quality chrome barrel. Probably from Fulton Armory. When I do it will be a real .308, or a .30-06 and restore it to original.
I did, with help, a bit of online research, hoping to find a bit more info and maybe some encouragement. Here are some results:
This one stressed the 2 rounds are not the same, and cautions against firing .308 rounds in 7.62 chambers.
This one is a good article and is the basic source info for the above article.
This one gives a history of the 7.62 cartridge and mentions that even amoungst the various manufacturer, here, and amoung our allies, there are variation in just that one round. They're ALL different.
This one is just a forum discussing headpsace issue on the M-1 Garand. I link to it just to give you the forum datum.
This is from the same forum and details the proper procedure on how to use a headspace gauge. Check it out. I am going to see if I can list it here.This is another forum with a few threads on the topic at hand.
This one is a thread that goes back and forth on the topic. One opinion says yes, fire it interchangeably, the next says it's gonna blow up in your face, yes it is, no it ain't, yes, no. But it's another datum point.