Put the family crest in its entirety on a wrist plate inlay.
Even a deeper metal "carving" would be historically correct and much more appealing in a rifle.
The only issue is, that the rifle should be pre-revolution in style. A 1790 american longrifle with a coat of arms does not appear correct.
Worth looking for european rifles where there was a plethora of nobility of all levels having rifles/fowlers built for them.