Bob,
Every year for the past 5 or 6 I have been doing a demo at the Gunmakers'Fair showing how to address issues like this.
You can move both the wood and the metal although metal like being moved much more than wood.
Fist you have to draw-file the top surface so you can clearly see where the gaps are. From you photo I can't see enough.
The best option is to push-over the steel to meet the wood. Mark the areas that have to move with a Sharpie. Pull the plug from the barrel, lay it on your anvil and start peining the metal where it is marked. I sue a ball-pein hammer with a head about the size of my thumb. Lots of little taps all along the edge that you want to move over. It doesn't take a lot. Pein a bit, try fit, pein a bit more, try fit, slowly causing the metal to flow and fill the gap. This will close a 1/16th inch gap in a few minutes. Sometimes if a gap is only on one side, for example, you will have to pein both sides to keep thing symmetric and that will mean you have to widen the inlet in spots. To do this make a little scraper and use it to scrap away the wood where you need some extra width. Also keep in mind that if you pein one side of a long part like a tang that you will cause the whole tang to bend slightly away from the side where you are peining. You might have to corrct some bend.
You can also move the wood. To do that you must first wet the wood and once well wet apply a hot iron to turn the water to team and, using the hot iron in a manner similar to a punch tap the iron while its steaming the wood to shove the wood over. Wood does NOT like this kind of treatment and wood treated this way will take stain differently, usually more intensely, which will show up when the gun is finished. It might be more noticeable in plain wood than in highly figured wood.
So you can fix inletting gaps without resorting to abominations like saw-dust mixed with glue or stained epoxies. All it takes is a little time. Go slow and you can achieve great results.
Good Luck!
JMC