just recently my grandpa gave me a model 12 16 gauge and i shot bout a box a shells out of it and then it became very hard to pump shells into the tube and once theyre in they wont come out. believe its the cartridge cutoff? any suggestions

Check th chamber for rust, and burned gun powder.
It sounds like the chamber is rusty, and or dirty.
Cleane it real good, and try it again.
If that don't work have a gunsmith look at it.

JMHO

i had that problem once, the shells had melted just enough to take em outta shape and they wouldnt feed right.