Don't go with mild, but use T1 and a minimum of 3/8" thick.
No rifle closer than 50yds if using 3/8" thick, for closer use 7/16" or 5/8", but that's starting to get expensive.
FWIW use 10 yds as the minimum for pistol, because I've been hit by ricochets at closer ranges. 20 yds too.