The last time I played through Crysis I tried to see if I could keep the SCAR through the whole game. It can be done (I did it) up to where the alien ship opens at which point, you loose all your weapons. I was able to keep the SCAR by using the dart to neutralize every NPC in the game and finish them off with the silenced pistol (which is what I did).
It was fun and stretched the game time out considerably. =]