I used to fire on an indoor 22 range much the same as you describe. ISTR
that the 45deg plate was backed with sand to deaden the noise. Also, we shot
through small openings in a wooden wall. This served three purposes. It
deadened the noise to the shooters. It stopped splinters flying back at us
and NO ONE was allowed through the door in the wall while there were loaded
guns on the range.

John