Can I shoot BBs in a rifled barrel?

Steel BBs in a rifled barrel will erode the rifling over time, gradually destroying accuracy with pellets.

Some dual-ammo guns have polygonal or shallow rifling designed to tolerate both, but they are a compromise. If your gun is sold as pellet-only, do not shoot BBs in it. If you must shoot round projectiles in a rifled barrel, use lead BBs (Smart Shot, Gamo round lead) which are soft enough not to damage the rifling.