The as issued Garand as provided by the CMP is more then capable of shooting good scores at 200 yards, the distance the Garand Match is fired. The 10-X ring is approx 2 MOA.
Ammo is furnished by most CMP GSM Garand Matches if they are sanctioned matches ran by a GSM-MI. This ammo will be the M-2 ball ammo (149-150 grns). With a match barrel you don't gain a lot by using this ammo.
If your rifle has a bad barrel then a new "non match" barrel may be installed.
The first thing you want to do is take your rifle out and see if it shoots, you may not have a problem at all.
The ideal of the CMP Vintage Rifle games, or "Spirit of the Games" keep this shooting program from developing into a "high priced" game High Power Rifle has become. Its about "MARKSMANSHIP" now how much money you can pump into a rifle.
Marksmanship not gimmicks.
Instead of trying to circumvent the spirit of "as issued" I recommend you attend a CMP Vintage Rifle Clinic, taught by GSM Master Instructors. They are low cost, or no cost at all. You will come away with a better understanding of marksmanship with the Garand (or other vintage military rifle.)
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071