Not an easy task. Your best bet is to clean the tube really well w/ sandpaper and then a solvent. Don't let any thing touch the barrel(includding your grubby little oily hands

). Then use an etching primer followed by really expensive automotive paint.
A neat trick for cheating is to cover the barrel with a thin layer of roven woven fiberglass cloth and epoxy (a 2oz or 4oz cloth is plenty). Let it dry coat w/ primer then any type of paint on top. This will make the gun heavier, which is something that I personallylike. I'm sure you won't glass it so on to trick number three...
Tape it. Omer sells mimetic tape as does numerous hunting stores for rifles. Before I glassed my barrel I used 2" wide electrically tape as per instructions from DB Sultan o'.... 2 lengths wil cover your 1" od barrel, but don't put any on top of the rail, and since it has an extruded (instead of plastic track) you can try any tape you want and simply remove any residual goop w/ acetone (just don't get any on the plastic muzzle or handle, it will melt them).
I just saw your in OC, Maryland. I used to hunt the breakwaters in Lewes, De all the time. How long have you been in the area? Stop by Pirate scuba yet/ the're very helpful. If your using that Rob Allen in MD it's probably a 75cm-90cm (unless they make a 55

).