I'm looking for a flashlight with a 1 inch diameter body (my mount is a 1" mount) with pressure switch. I would strongly prefer the light took CR123 batteries. Would also like the light to have constant-on capability as well, but will consider lights without it.

I bought a FAB Defense Speedlight 3V which was perfect except it has no pressure switch and adding one turned into a massive pain in the ass for multiple reasons. So basically I'm looking for something just like the FAB Defense Speedlight 3V but with a pressure switch. Small, simple, light. 

Don't want anything junky. Willing to pay up to $90. Oh, and this is going on a 12ga, not an AR. What are my options?

If you are mounting it to a pic rail I'd highly recommend the streamlight rail mount 1. Has pic rail mount built in and can be had on Amazon or optics planet for your target price. Runs on 1 cr123 or 1aa, has constant and momentary on and there is a tape switch included.

Yes, it's being mounted on a pic rail. Already have a mount (nice mount too, VLTOR scout mount) so it would kinda suck to have to ditch my mount but I checked out that Streamlight really quick and that's certainly a solid option at a glance. I'll definitely keep it in mind, but I'm going to wait and see if anyone comes along with something else I might like before deciding on anything. 

+1 on the duel fuel (CR123 or AA) Streamlight Pro Tac RM-1 with tape switch.

This light is compatible with any scout light mount also

Oh, that's not what I have. I didn't realize there was something else called a scout mount. You're talking about the Surefire mounts, correct? This is what I have. That's why I was requesting a 1" diameter light, to fit that mount.

Primary Arms compact Gen II weapon light. You can get the light and tape switch for your budget. I've had one for over a year now and it's been great. I have it in a 1" Larue mount.

Light and tail cap come in black or tan.