This is a bit off the wall but...
I have a mate in Tamworth that has a vehicle signage business, he specialises in vinyl wrapping. (and other sticker bits n bobs) This wrapping is like you would see the graphics on london Taxis that have pictures/images on them all over the vehicle. I am sure he could print a photographic (real)grass/moss/undergrowth pattern on a large sheet of self ahesive vinyl, that you could cut and apply to your weapon, the great thing is it could be easily removed with a hair dryer and is really resilient to the elements and any rubbing wear.
You would have to cut a paper pattern to fit to your gun components, trace it onto the printed sheet, cut it out and apply it but I am betting with a little care it would look supurb !
It could even be removed without damage to the weapon allowing you to maintain its resale value. Plus if damaged, just replace the section !
Is this a mad idea or just perfect?.
I suppose if there were popular makes/types of weapon he could make a pre-cut pattern set but obviously the cost of this would increase to that of a DIY sheet !
If you think its of interest I will give him a call with the proposed idea. Then he can talk about cost etc.
Steve