im not sure but could it be possible to keep the door open with an object. the first door by the shooting range auto shuts so if you could close that you could do the start of the chair skip and go to end of gun range.

i just read the rules and am not sure if that counts as the gun range glitch

I need to update the rules, glitches or skips aren't allowed for Gun Range runs. Also I haven't tried yet to get a chair stuck in a door but I think you should be able to, but I don't know if you could fit.