Pablo, here is how I paint German Rifles- give it a base coat of Valijo sunny skin tone, once that is dry, (not very long), go over it with raw umber or burnt sienna oil paint. wait about 30 minutes and the with a clean brush very lightly wipe down the length of the rifle, this nwill give you the wood grain. You will have to wipe it down with the brush several times but make sure each time you do it make sure the brush is clean. The two pics below show one rifle done in burnt sienna and 4 done in raw umber. IMHO most rifles are a dark walnut color so the raw umber is your best bet.
Hope this helps you out
Don