A slam-fire is a rare occurrence in an M1 Garand, but it can happen, as it did to one American Rifleman contributor. Here's how he discovered a potential cause.
Never experienced a slam fire in 5-6 years of continuous use of a Winchester Garand in competition. Most of the ammunition I used was hand loaded by yours truly. Only cause of slam fires that I was aware of was improperly loaded ammunition, that is where the primer was not "bottomed" in the primer pocket.