If you have a chronograph with a proof channel it will tell
you what your loss per/foot is ,If not I would guess at your velocity
your only losing about 8 to 10 ft/sec.
I had to build a blast shield for mine because the shock wave would
cause an error in velocities on some of the big bores unless I
moved the screens way out.
The blast shield is nothing more than a 2x4 frame with a piece of
plywood 1/2x3'x3' with a 3''x6'' vertical slot at the matching height
of the screens.
The down side is more noise.
Placed 3 or 4 feet in front of the muzzle,this allows you to set the
screens 5 feet from the muzzel.
The only other reason for setting the screens close is that it gives
you a larger window to shoot through.(Multiple targets with out
moving the screens.
I hope this helped
J E CUSTOM