You automatically pick up guns you dont have, and hold LB for ones you dont.
Example:
I have sawed off shotty, i walk over sawed off shotty, i get ammo added to inventory.
I dont have any shotty, i walk over sawed off shotty, i get shotty and ammo.
I have sawed off shotty, i walk over combat shotty, i can hold LB to replace sawed off with combat shotty.
You can only carry one type of hand weapon, one type of pistol, one type of shotty, one type of SMG, one type of assult rifle, one type of sniper and one type of grenade.