Correct bullet length

[Dollar Bill]

A fired case won't hold the bullet and a resized case usually has too much neck tension to get an accurate reading: The bullet gets jammed in to the lands.

Red's method works well. You can also take an old case, resize it and cut a slit in the neck to the shoulder. That will allow the bullet to seat in the case accurately. As pointed out, that's against the lands so set your seating die .010 deeper.

All that said, I have 2 Garands and neither shoots any better with jacketed bullets seated any longer then magazine length. I have some cast bullet loads that do better seated long but they aren't powerful enough to cycle the action anyway.

[Hefights]

For an average or unmodified M1 Garand in an application such as plinking or casual shooting by an average shooter, there are other factors more important for accuracy. In this application, as stated, caution is in order. Trying to seat off the lands can lead to trouble with function or opposite effect on accuracy, especially if one happens to be less experienced.