Uncharted 2 trophies help

I have a bit of trouble with the trophies for m32 hammer and gau 19...
i am not able to get it because of the very few times it comes in the game.
i have finished the game on crushing and bought these guns off the store... how do i use these guns in the single player campaign again?
or are the kills in multiplayer counted for the trophies ? anyone's got platinum on this?

alright got it... just go to the store and say give weapon :) easy :)

Do it on the chapter you have to pull the RPG guy out of the truck, then chloe throws a nade. Get all the way to the part where the second truck comes out, but dont kill the gunner. They respawn a few times and you get it easy.

Yes i did exactly that... Got the plat :) Thank you
My first plat by the way :)