flag code
The Flag Code (Title 4) ch1, sec 1-10, about 15 pages, addresses and describes many issues concerning the the U.S. flag and the correct display. The Code does not list any penalties, so I suppose it serves as a source of guidelines. Title 18 (criminal and procedures) does list penalties. Given the commercialism and misdisplay and abuse of the flag these days, no one will come and charge you if you paint a rifle with Old Glory. There is nothing in Title 20 that specifically addresses painting a flag on a riflestock. In fact, doing so, in light of the current mentality of some of our countryman, seems down right patriotic indeed.
There are however sections that address the flag as display, decoration, or printed, embroiderd flags on items such as clothing, cups, plates, etc. By example, a star spangled bikini, hatband, ball cap, etc, is in violation of Title 20. So to wrapping paper, handkerchief, and any other dispposable item. The Code also addresses the flag used as decoration, and use as a tablecloth, for example is contrary to code as well. What is typically suggested for decoration is bunting, ie red, white and blue striping.
If I were going to create a patriotic themed rifle, I would go the "striping" route and forego the rendition of the flag. I'd keep the eagles too. I base this on the language that the "flag represents a living country and is itself considered a living thing." USC 4,ch1, sec 8. And while a rendition on a rifle seems totally appropriate, (certainly not in the same league as a tablecloth or handkerchief) , it dances around the edges of "decoration",.... to me, anyhow. So I just would't go there. That's me, But its the OP's rifle.....duly noted. Just some food for thought.