Fellow C&R firearms enthusiast Oelund has undertaken a rather extreme experiment.  He has given us permission to display the results here:

Some of you might have heard the term “Garand-Thumb” or “M1-Thumb”. That is the name for when the bolt of an M1 Garand slams closed on your thumb. A Garand-Thumb often occurs when you try to close the bolt on an empty magazine. To close the bolt on an unloaded M1 Garand you need to push down on the magazine follower before the bolt can close, and if your finger doesn’t get out of the way in time it’ll be caught.

Often people will get a Garand-Thumb because the bolt has not been pulled fully to the rear. When the bolt is to the rear it is locked in place by the bolt catch and won’t be released until you relieve the pressure on it.  However if you don’t manage to get the bolt all the way back it might rest on the magazine follower instead of the bolt-catch, and it might slam shut without warning.

The best way to prevent a Garand-Thumb is to operate the gun with your right hand, having the palm of your hand flat against the side of the gun and the back of the hand resting against the charging handle, as seen on the picture above. This will prevent the bolt from closing while your thumb is still in it’s path.

Here is a clip of me purposely giving myself a Garand-Thumb, recorded at 1200 frames per second.  As you can see, the bolt was resting on the follower and released as soon as I touched it with my thumb.

Not being completely satisfied with the previous clip, I recorded one more. Don’t try this at home (unless you are recording it)

Here is what the clip looks like in real time, to give you an idea of how suddenly the bolt can slam shut.