It seems to me that a lot of the indoor ranges in Denver have a flat fee for shooting, no time limit. I'd much rather be able to buy 30 minutes and get in and out. I've been at Firing line waiting for people to leave, when they stop, come out, chat, leave a lane empty for 15 minutes. People don't do that when they have a time limit.

Firing line just asks about your ammo, Green Mountian checks it, so I don't think you can even preload your mags.