Why can't a crosman 1377 shoot BBs?

Okay I just got the crosman 1377 American clic. I love it and think it's a great pick for me I know it's nothing amazing but it's good enough for me. Anyways I was wondering why the ammo for it is only pellets? Would the rifling in the barrel be ruined by a BB or would the BB not fly straight from the Rifling? Also is there any .177 pellets for my gun that aren't lead and are less malleable on impact so I can break gl and stick into trees instead of ricochet.