Swagger is the more skilled of the two in both CQC and ranged combat. This means that anytime he fires he has more of a chance of hitting. Honestly I can't see him missing. So even if Payne is more durable he is going to be getting shot more so it all works out.
Truth is Swagger being a trained sniper and all Payne may never see him coming.
Originally posted by Rogue Jedi
Swagger is armed with an assault rifle, not a sniper rifle and they are in a restrictive space, his stealth will be an asset, but not much of one.
You can snipe with out a weapon specifically built for said purpose. According to an online dictionary the definition of snipe is..
1. (Military) (when intr, often foll by at) to attack (a person or persons) with a rifle from a place of concealment
Though even this definition is a bit restrictive as even a sniper who has been spotted ( not in concealment ) can still snipe and the work can be done with a weapon other then a rifle.
In other words sniping is an action. Some thing done, not the weapon used.
As for the environment being confined. It is more so then say the woods, but it is still more then enough space to get hidden and wait out your opponent. Which is a possibility given Payne's guns blazing approach and Swaggers patience.