Depends on the pattern you want. One of the easiest is the mesh or snakeskin look.
Lay down a dark basecoat, then put some mesh down on the rifle. Spray over the mesh with a lighter color(s). Keep in mind if the mesh isn't directly touching the rifle, the edges or pattern will be blurred instead of distinct. A mix of both looks good.
I've known guys to use leaves for stencils as well.
Try this first on some other medium. Once prefected, hit your rifle up and most importantly, POST PICS!!!