I follow this tutorial. Requires photoshop but there are some other free programs that will work as well.
How to take decent gun photos in your bathtub . . . - AR15.COM
Took the words out of my mouth. It's nearly impossible to get enough light indoors. Rifles look cool laying in the grass and on rocks anywaysTake them outside. Sunlight is the best backdrop. Using the flash generally kills the looks and detail.