If you click on my (MUSKETMAN) profile, you will see myself holding a .69 caliber musket that has brass tacks on it.
FIRST: MAKE SURE THIS IS WHAT YOU REALLY WANT...
You have several types and sizes you can choose from, I used brass-round head tacks for the musket in the picture.
You can get them from any hardware store, they also sell chrome tacks too, but they're tacky. "UGGH, bad pun."
Brass tacks were a big trade item with the indians, patterns are whatever is pleasing to your eye.
I pre-drilled the tack holes first with a fine drill bit, just to keep from splitting the wood or bending a tack and marring the stock.
Also, once the tack is started and almost hammered down to the stock, put a small scrap of wood over the tack and finish hammering it down. This will keep the hammer's head from hitting your stock and disfiguring it.
Once you do this, there is not turning back...
You will have holes in the stock and the outline of the tack's head if the tack is ever removed.
You could also use small brass nails for a finer "tack" outline.
Draw in the design first using a crayon or washable marker, do NOT tack as you go...
You might be off by a tack and then it would look bad.
Keep it simple and symetrical, the tacks I had along the barrel channel were the same on both sides.
Good luck and work slow.
Measure twice and tack once.