For anyone new to reading this thread who doesn't know what's being talked about: Infiltration is a Total Conversion rather than a simple Mod. It's started from it's own front end, not from within UT, but you have to have UT installed.
Infiltration is a "War" mod. You are on one of two squads of modern-day soldiers with realistic weapons who shoot it out. The squad which wipes out the other squad wins.
You don't pick up weapons, you start with a "loadout" that you select at the beginning of the round. You can create and save several loadouts for use in different maps. Weapon & ammo weight has penalties.
One of several unique features is that you can lie completely prone as well as crouch or stand. The mod tries to be realistic so, for instance, you dont have a crosshair, you aim using the actual sights on your weapon (or you can shoot from the hip with much less accuracy of course). That works suprisingly well.
There is a "last player countdown" to prevent things being drawn out for ever should the last survivor of the losing squad go to ground and hide.
You need to download the Map Pack as well as the Game File.
The current version of Infiltration is is 2.85.3. Version 2.86 is due for release soon
http://www.planetunreal.com/infiltration/