>>64179947
Ideally you'd want a downward cant of 20 degrees or so but you can't 100% prevent spall from getting back to the firing line. I've been hit once and had two other pieces whiz or hit other people on the line in 15 years of using gongs in this way (picture related)

If you use a bolt like pic related to move the CG of the target forward from its hanging point, it will naturally tilt downward but still ring loudly (which commercial rigidly fixed gongs do not do)