Hi,
It is far easier to smoothly engrave a part if it is off the gun so that it can be spun in a vise. As others wrote, engrave as many parts as possible off the gun. You can also engrave those parts off the gun that will be nailed in place by fitting them and filing flush with the wood (if that is what you want). Then carefully remove them, glue to a block of wood and engrave them. Then replace them on the gun, drill the pin holes and counter sink them. Tap in the pin and carefully file flush if desired. If you are careful, you will only damage a small amount of the engraving when pinning the inlay, which can be re-engraved on the gun.
dave