I learned this a bit ago, and its one of the methods USMC snipers use on their sniper rifles.

• Materials
  To successfully paint your sniper rifle, you need the right materials. So here ya go:

• Spray Paint-For this job, you need the right paint. Look for any of the paint at military surplus stores that is specifically for camouflage. You need a light color for the base color. Try a light green or Olive Drab. Next, get some red-brown spray paint to add contrast and variety to the rifle. After that, get two darker shades of green.
  
• Quality Tape-Be sure to get strong, sealing tape, so that no paint will get on the valuable parts that shouldn't get painted.
  
• Leaves and Grass-You need a variety of plants and leaves that will look good on your rifle
  
• Burlap-This will be wrapped around the gun to break up it's outline. Choose OD Green or Brown.
  
• Cardboard or Newspaper-Be sure that this is longer and wider than your gun, because it will have the gun on it.
• 2-Starting Out
  Here is what you'll need to do to set up:
  Lay down the cardboard or paper, with the rifle set on top, or, if you have a bi-pod, take out the legs, and stand it up.
  Take out the tape, and tape off all of the areas of the gun you do NOT want to have spray paint on, or that shouldn't have spray paint on them, like the scope lens, magnification numbers, bolt, etc.

• 3-Base Paint
  Now, after shaking the OD or light color spray paint can fully, spray paint the entire gun that color. You may or may not want to let it dry for a day, but it's all up to you. Let it dry before you do the next steps, though.

• 4-Contrast and Variety
  This is probably the easiest step of all: simply shake the red-brown can of spray paint, and put strips of that color on the gun. These strips do not have to be evenly distributed, if preferred.

• 5-Touching Up
  This is the crucial part: after shaking the two cans of dark green, place some of the plants onto a part of the rifle. Now, spray paint OVER these plants, creating a plant-like pattern on your gun. Alternate between the two cans of spray paint and the plants, making the patterns spaced about 6-12 inches.

• 6-Burlap
  The last step in this is to wrap the gun up in burlap. Do this carefully, so as to not cover up any vital parts of the gun, such as the scope lenses, and the bolt, trigger, etc.