The hole in the paper.

The "punched" hole from the hollow point slug will usually be smaller in diameter than the bullet diameter. Keep that in mind if you trying to precisely measure groups by deducting the projectile diameter from the outside edge to outside edge width of the group. Shoot a match type flat nosed pellet next to a slug and compare the hole sizes.

Hoarding hint: Those little circles of paper can be dug out of the slug or collected from the ground, saved and used as confetti on New Year's Eve!
 
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