Although I know many cringe at the idea of a modern product on their finished longrifle, Microbed is an option that I have used for just such a purpose and worked well, assuming you don't mind if the inlay is likely going to stay put forever. I find it much easier to use for these situations than "Accraglass" as long as you are using a relatively dark stain. Microbed is already stained dark - you mix the two tubes like epoxy - mix up a small amount and I then let it set up till it is about the consistency of play-dough, then push a tiny amount into the gaps - pack it in, then immediately wipe off all excess from the metal, and lightly sand it the area with the wood around it to mix dust from the wood into the surface of the Microbed. Stain right along with the rest of the wood when finishing. Works well with a dark stain, it makes a less visible fix.
Guy