IMO, one of the worst fillers would be Plastic Wood.
Over time, it doesn't really adhere to the base wood and it doesn't take stains of any kind.
One which I have used is to collect a pile of sanding dust from the stock (I do this while I'm sanding even though at the time I may not have a use for it) and mix it with some white wood glue. The ratio I use makes for a very "wet" putty.
This doesn't accept stains well but it will absorb some. With this in mind, use the darkest stain you have, applied with a narrow brush and prior to applying a finishing oil be prepared to use one of those furniture "touch up" stains sold at the hardware stores.
I haven't used a epoxy type filler although some have had some success with it if it has the special stains added to it before applying it to the gaps.
Just remember that it will stick to anything that is not waxed so it can easily epoxy a metal part that you may want to someday remove in place.
Also, it will not absorb any stains of any kind.
If there is a long gap between something like the barrel and the stock, consider making a very thin piece of wood, the same type as the stock, and gluing it in place with a good wood glue.
After the glue has set, the thickness of this piece can be sanded or scraped to fit the metal part.
This has the advantage of looking like wood and accepting stains like wood.
Properly done it can be totally invisible.