The main reason for opening the flash hole for primer-only powered loads was to keep the primer from backing out and tying up the cylinder.

This was to be done only with primer powered wax or rubber bullets.

As pointed out already, it is extremely dangerous to do this with any load containing powder.

I have shot .38's with as little as 1.0 Bullseye and normal flash holes with no trouble, other than a stuck bullet every now and then cause of the light powder charge.