Does anyone know what FOV I should set the camera in Max too to see what a weapon would look like when being used in-game in a FPS.

The field of view is normally of 90 degrees in games.
The only exception I can think of from the top of my head is AvP where the Aliens have a wider range, probably 120 degrees.
In any case, most games allow you to change that, but assume 90 degrees as default.

Some games have narrower views too. Soldier of fortune 2 is 80 degrees. Battlefield 1942 is about 70 degrees. [o.o]

My bad, I didn't know about that. That's kind of weird, although it would make sense when you are, say, piloting a plane or something fast; to simulate the tunnel vision effect.

Heres an interesting link to get some dimesnions and FOV from Halo:

http://www.cortana.org/HEK_Tutorial/references/general/halo_player_stats.html

I think the games i mentioned did it because at 90 degrees theres some obvious distortion at the edges of the screen, due to how coordinates are projected. I think it's an entirely cosmetic decision by graphics worshippers. It's also annoyingly justified because in some cases it can increase framerate by a couple of fps.

I personally prefer 90 degree fov. I could tolerate 80 degrees, but 70 degrees feels too much like tunnel vision. Sometimes in bf1942 I cant tell whether i've zoomed in with a weapon with no scope or not.

I've also heard of people getting 'Quakevision' from higher fovs like 90 degrees - it's a temporary loss of depth perception from not having depth on screen, but I've never experienced it myself.