I guess you about have to paint the rifle and stock at the same time?
That's how I do it.
Here's an example. I just put tape wherever I don't want paint. Some say clean with alcohol first, it's a good idea if your rifle is really oily. I don't usually bother with that though.
Base coat dark brown. Note the tape.
Tan accents. Diagonal passes sprayed through dry grass,
Added some sage green accents, kind of overlapping the tan,
Pulled the tape,
In natural environment,