Good idea ... but, and I hope I am not teaching granny to suck eggs ... do not use steel Bbs anywhere near a steel target. They will certainly bounce back.
Years ago I used some old Meccano to make a moving target, powered by a 12v motor (Meccano?) and a 12v train controller. Worked quite well but a bit unwieldy. I only used it for small paper targets (4" x 4"?).
Cheers, Phil