[quote=Adam Stinson]Thanks for the info guys! Well, how about this?

Can I put 20ga reducers in my 12ga (the chamber inserts... not the full length tubes)? Would it be okay then? It seems like the hardness of the shot doesn't affect the bore because of the wad. The biggest issue appears the be the choke. But if I am shooting a 20ga shell in a 12ga barrel, the choke shouldn't affect much, right?

Thanks
Wrong. If some of the pellets get past the 20br cup in 12ga bore damage to interior of the barrel is possible.
Shooting hard shot through choked barrel is equivalent to shooting hard slug or "solid projectile" through the barrel. This is bad idea in guns marked or designed 'NOT FOR BALL'. I hope that helps.