Put Yourself in Tactical Positions That Maximize Your Guns' Effectiveness
Each gun has a range that its most effective at. Guns might also require you to crouch or go prone to aim effectively. If all you have are long range weapons, you should avoid towns like the plague. If all you have or short range weapons, stick to house to house combat as long as possible.
Obviously, its best to have one good long range weapon, and one good short or medium range weapon, but early on.. you might not have that choice.