I've been looking at different light options for awhile now to put on my AR. Seeing all these pic threads and everyone with their Surefires, I obviously thought I should be looking at Surefires. Anyway, being that I am a college student, the prices kinda scared me. I started looking around and found a video review for a 4sevens Quark 123 Turbo I think it was called. Anyway, the guy in the video shows how he's been testing it on his AR for awhile, shot hundreds of rounds, shot an AK with the light, and even shot a shotgun with the light mounted. It survived all those tests as well as other tests.  It's $75 and has a maximum output of 230 lumens. In the specs it looks to be a great light but it is not mentioned as a weapon light or recoil proof or shock proof. I'm assuming it's not listed as such because it has not been tested as a weapon light.

My question is would a well made light like this one be okay to use for a weapon light? If not, what differences are there between a light that can be mounted on a weapon and a light that is a hard use light that has never been tested on weapons?

I don't have an absolute answer for you but let me offer you the advise that if you go the route of a non-weapon light and you want to mount it then you would be wise to look at an LED offering.  Now, the other option is just learn to use a good hand held light with your weapon,  A very good light designed for this is the Surefire Z2 while not cheap (about $100) it is certainly cheaper then say a M962xm07!

Also look at used lights as I have a couple Insight M3 and M3LED lights that I bought cheap.  I think I only paid $30 for the last M3 and it sits on a shotgun.  I have an M3Led mounted on a Glock too even though my primary HD Glock has a newer and better Surefire X300.

You don't need to have the best if you can't afford the best but looking at what you can get used can allow you to have a capable option that will not bust your budget!

Examples:

http://www.ar15.com/forums/topic.html?b=7&f=89&t=795320
http://www.ar15.com/forums/topic.html?b=7&f=148&t=800384
http://www.ar15.com/forums/topic.html?b=7&f=25&t=800355

Thanks guys

The biggest difference between a "weapon" light and handheld is that in a weaponlight, the batteries are "captured", so that recoil doesn't allow the batteries to slam into the spring/reflector/module in the lighthead.  Repeated pounding can break components in the lighthead.  There's usually a spacer, machined stop or rubber insert that limits the forward movement of the batteries, while still maintaining contact with the spring or contact.  Of course, and LED will survive repeated shock better than an incandescent (solid state vs. filament), and weaponlights are usually designed with housings that have built-in accommodations for mounts of some kind.

That being said, there are thousands (or more?) of Surefire 6P's and other handhelds being mounted and used without failure every day.  I think the move from incandescent to LED was probably the best improvement for durability, allowing weapon use.

Personally, I'd look for something a little more "utilitarian" than the Quark.  All the blinking, flashing, and other "features" are cool, but all you really want (or need) in a weaponlight is enough light to illuminate your target.  You can find Surefire 6P's and, as Aimless suggested, Elzettas in your budget range, which are proven lights that many use for weapon mounting.  They won't signal aliens to land, but do a good job at lighting crap up.