I have used my home made version for about 35 years now, with no ill effects. I think the factor to keep in mind is to avoid percussion: that is, to be sure to keep the rifle butt firmly placed against the vertical element of the device designed to receive it, so that it doesn't crash into it when the rifle is fired.

In high school physics class we were shown that if you dropped a metal plate with an egg placed on top of it, the egg would shatter when the plate struck the floor. However, if you taped the egg to the plate, so that the impact with the floor was simultaneous with both the plate and the egg, then the egg would remain intact.

This means that the normal bench rest technique of holding the rifle only loosely when firing the shot is a bad idea. The rifle must be held firmly and pulled toward the rear.

My primary interest in using my gadget was to provide a platform from which to chronograph loads and to zero my magazine rifles. I don't recall using one for my double rifles, but I may have at some time. The accuracy I obtained was quite satisfactory, however.