In my experience, most Marlins that have not been used much will nip off the tip of your thumb when you shove the rounds in, even the .30-30.
One thing that helps is to shove the round in by straightening your thumb somewhat, and puttin the very top point of your thumb on the back of the cartridge (instead of putting the flat part of your thumb on the back of the cartridge). By doing this, the flat side of your thumbnail slides straight along the side of the receiver, and there is nothing to get pinched in the loading gate door. Takes a little practice, but pretty easy to get used to.
Hope this helps.
Mannyrock