One method not mentioned by Dfariswheel in his otherwise excellent overview of gun painting is dip, hydro-print or hydro-graphic printing. This is a process where colored inks on a water soluble film are placed in water. The film dissolves leaving behind the ink on the water surface. The ink will adhere to (most) anything that is dipped into the water.
Here is a custom example.
The downside is cost. A complete bolt action rifle... stock, action, barrel, rings and scope will run about $250-$300. But... this includes disassembly, bead blasting of all metal parts, primer application, hydro-printing and multi-stage clear coat application after the print is applied.