When developing ballistics tables and comparing ammunition each round is fired from the same platform. This is something that the bullet manufacturers perform to provide consistant and comparative data. This is usually referred to as a "bench test" where a round is fired through a 27" barrel with a locking bolt. The information gained from that test is extrapollated mathmatically and provided as a "base line". That is just comapring the ammunition. Comparing weapons is often times trickier in that they can either be "bench fired" or "shoulder fired". "Bench Firing" is going to provide that absolute best results for a sterile environment. It is where the weapon is "locked down" and remotely fired. This test is most often used to test the interaction of the bullet with a specific weapon taking into account variables unique to a specific platform: Barrel Length, rifling, action and operation. It allows the weapon to be scrutinized without the external interfearence of a shooter firing from the shoulder. This test is usede to determine the accuracy of a specific bullet fired from a specific rifle. Shoulder Firing allows all of the scrutiny of above with the addition of the functionality of the weapon's sights and the "shootability" of the weapon taking into account the components of the sight system and the sight radius: Eye relief and distance from the rear sight to the front sight.The greater the eyerelief and the shorter the sight radius the less accurate a rifle tends to be. You can also take into account stock design and recoil to effect rapid fire accuracy. Long story short the Garand has a longer barrel, longer sight radius and shorter eye relief. All of these add up to increased muzzle velocity and more functional accuracy.