A bud of mine had an out of battery slamfire in his Garand with cast bullets. I believe it was due to oversized bullets and requiring the action to crush the bullet to fit the throat. If there is an interference fit with the cartridge, lets say fat case, long case, or bullet seated to the lands, the bolt has to crush that case into the chamber. When the bolt stops before the firing pin tang is held back by the receiver bridge, that free floating firing pin is hitting the primer at the highest velocity in the bolt cycle, and the lugs are not in battery.
Whatever you do, make sure the case closes without any resistance. Most cast bullets are larger than the bore and some people like to have the cast bullet touching the lands. This is fine in single shots or bolt rifles, but it is very dangerous in a Garand.
If I'm not shooting, I'm reloading.