Isn't every last base in the game part of the main story line anyway, except a couple of the tiny camps you don't "have" to destroy but have like 5 people in anyway.
Side missions and faction quests have no point at all, haven't played it patched, finished before there were any.
Invincible, or as good as 2 hours in(if that) make it a touch ridiculous, 2 great fights in the game, very early on, incredibly dumb AI, ridiculously accurate weapons and powerful past the first hour of the game. Once you get a gun that actually puts bullets at the crosshair , which is basically the 3rd/4th gun onwards its just too easy. The "open world" also takes about 3 minutes, if that to get from one side all the way to the other, so open world, generally yes, tiny world, also yes.
The main bases for the biggest missions are very nicely put together, although mostly VERY short, it feels like a few normal levels of say HL2, with long runny bits inbetween getting to those levels to extend the game to me, nothing more nothing less. The atmosphere is nice but theres little to do.
I have stalker but couldnt get 'hooked' on the game, the guns just never seemed to hit or do damage!
Thought it was a fn2000best gun on the game was the VSS Vintores though.