I would not paint a rifle if I was trying to camouflage it. I do own a few rifles with desert or woodland "camo" paint but I've never paid extra to add it. The paint might do some good at close range but there is no effective camouflage if you don't break up the outlines which say "human with gun" to an observer at a far greater distances than where paint patterns can be resolved at all.
A simple method I use is a piece of fairly fine netting mottled in natural colors found in the environment to drape over myself and the rifle to break up outlines. A 4x8' piece of netting is light, compact, cheap, comfortable and easy to put on and remove unlike a ghilli suit. From the inside it's practical to see out with a partial loss of resolution and brightness. Sure, it's still visible and if it's moving it can be detected. (like a ghilli suit). When still and sitting or lying down its difficult to tell from rocks even in direct sunlight. As important as the netting and it's color is to make use of the environment by mingling with shadows and similar size and colored objects. Use terrain and foliage when possible to not be in full sight of whatever you're trying to prevent detecting you.
Nearly all mammals and birds are very sensitive to movement in their field of view. Avoid unnecessary movement when game is within line of sight. Many species including humans can recognize a human form at distances well over a mile.
Camouflage is not about being unseen. It's about being unnoticed and unrecognized.
There's is little point in being unnoticed visually if you don't also control noise and odor.
