Quite honestly not even that much hey.
Get a couple of cans of Aervoe or I personally use Rustoleum because it is what I can get.
De-grease the gun. Tape up the optics of your rifle and possibly temporarily fill the muzzle.
Make sure the rifle is warm (put it in the sun for half an hour) and it is an above 20*Celsius day (I do realise this means you could be waiting forever, you can do it at above 15* but curing time is longer). Shake the **** out of the cans to make sure it is mixed properly.
Spray a base coat with gentle wafts of spray. Mist it. Then obscure the gun with something like a camo-net or such to temporarily mask of sections. Mist it with your next colour. Move the temporary masking and repeat. Total elapsed time is around 40min. Gun will be dry and ready to shoot in about 2 hours. Leave to dry in sun, or not

Total paint curing time means you should probably leave it for 24 hours. You can matt clear coat it if you want, but this is not neccesary.
As the seasons change or your tastes change you can do it another colour.
If you break the rifle down into smaller parts you can cure the paint in the oven. 70*Celcius and the paint will be hard as nails.
If you scratch it it will simply add to the character and if you need to you can touch it up.
Oh, and if you shoot LOTS and get you barrel REALLY hot, the paint may change colour. But I'm talking probably polymer melting hot.
Total cost what, $20? £15?
KBK