I wore out a couple Garand barrels shooting on a military team. We used 7.62 NATO versions with 1:12 twist barrels from Springfield Armory in New England. These barrels had .3075- to .3079-inch groove diameters and would shoot .3082-in. diameter Sierra Match Kings and military .3086-in. diameter M2 172-gr. FMJ boattail as well as Lapua's 185-gr. .3092-in. match bullets very accurate.
These barrels were broach rifled and a new broach made larger diameter grooves. When the broach wore down from use, groove diameters got smaller/tighter. A rifling broach looks much like a tap used to thread holes but the rows of teeth match the rifling pitch. Each set of teeth would be a few ten-thousandths inch larger than the ones in front. An excellent, accurate and fast way to rifle barrels. But making good rifling broaches required the finest tools and machinists around. Military shops making match grade M1 and M14 rifles air gaged these barrels and used the tightest ones for competition.
Both 30 caliber and 7.62 NATO Garands with standard service barrels with somewhat larger groove diameters from .3080- to .3085-in. shot the military match bullet better than Sierra's. These barrels also shot the .3092-in. diameter Lapua 185-gr. match bullet very accurate.
If you can slug your barrel with a .310-in. diameter lead ball pushing it down the barrel with a semi-flexible rod then measuring it with a calibrated micrometer, use bullets a few ten-thousandths inch bigger for best accuracy. Weights should be 170 to 180 grains for the 1:10 twist barrels; lighter ones typically get spun too fast. If your barrel's on the tight side, Sierra 175 or 180 grain HPMK's may shoot the best.
The standard service bullet used when the Garand was developed was the 172-gr. FMJ boattail developed in the early 1920's for long range machine gun fire. In the late 1920's, it replaced the 150-gr. bullet first used in 1906 for the M1903 Springfield. But recoil with that heavier bullet was a bit much for use shooting accurately standing up (offhand) as judged by the generals running the Army and Marine Corps. So in the late 1930's, that machine gun bullet (also loaded then in .30-06 match ammo) got designated the M2 bullet and replaced by the original 150-gr. flat based version designated as the M1 bullet.