Does that mean when it is NOT open season for big game that it is OK to target shoot on public hunting grounds in PA where not otherwise prohibited by some posting?
No. The prior law had allowed target shooting where it was safe to do so, but the Game Commission never really approved of the practice. But the current law explicitly prohibits it outside of established PA GC ranges.
And even at GC ranges you need a hunting or trapping license, or a permit from the GC.