Valve mod - pcp pistol

You could try  rust bluing for your barrel its a fairly simple process.

## Equipment Needed:

* Rusting Solution: A commercially available, pre-mixed rust bluing solution (typically a very mild mix of nitric and hydrochloric acids with iron filings).
* Degreasing Agent: Acetone or a dedicated industrial degreaser.
* Boiling Tank: A clean steel or stainless steel tank long enough to hold the barrel.
* Water: Distilled or demineralized water (tap water contains minerals that will ruin the finish).
* Carding Brush/Wheel: A specialized ultra-fine (0.002" to 0.003" wire) carding brush or degreased #0000 steel wool.


## The Process:

   1. Perfect Prep and Degreasing
   Polishing the steel to your desired finish (usually 320 to 400 grit). Put on nitrile gloves. Clean every square millimetre of the metal with acetone. From this point on, never touch the metal with bare skin, as skin oils will cause splotches. [7]
   2. Applying the Solution
   Dampen a clean cotton swab or sponge with the rust bluing solution. Squeeze out the excess so it is barely damp. Apply a very thin, even coat along the barrel in long, single strokes. Do not let the solution puddle or run. [8]
   3. The Rusting Phase (Damp Cabinet)
   Place the barrel in a warm, slightly humid environment (a bathroom after a shower works well) for 1 to 3 hours. A fine, uniform coating of velvet-like red rust ($Fe_2O_3$) will form on the surface. Do not let it rust for too long, or it will pit the metal. [9]
   4. The Conversion Boil
   Bring a tank of distilled water to a rolling boil. Submerge the rusted barrel for 10 to 15 minutes. The boiling water changes the chemical structure, converting the red iron oxide ($Fe_2O_3$) into black iron oxide ($Fe_3O_4$). When you pull the barrel out, the red rust will have turned a dull, matte black. [10]
   5. Carding the Loose Oxide
   Let the barrel air dry (the heat of the metal will dry it instantly). Use your degreased #0000 steel wool or carding brush to gently rub the barrel. This removes the loose, powdery outer layer of oxide, revealing a faint, smooth, dark grey-blue layer underneath.
   6. Repeating the Cycle
   A single pass will look thin and uneven. Repeat steps 2 through 5 completely. Each cycle deepens the color and builds the layer of protection. Most barrels require 5 to 8 full cycles to achieve a deep, rich, uniform factory black.
   7. The Final Cure
   Once the desired depth of color is reached after the final carding, coat the warm barrel heavily in a water-displacing gun oil or water-displacing preservative. Let it sit for 24 to 48 hours to cure, cure the metal, and seal out future moisture.