An interesting request looking for an elegant solution.
I would approach this in a different way.
I wouldnt go the mould and resin route.
Taking a mould from the existing is too risky.
I would make a hard wood replica of the pistol grip as follows. Get
some white card, like a large stiff cornflakes packet. Lay the rifle
on its side on this.
take a fine felt tip pen held vertically and draw a profile of the
grip onto the card. Make sure the dimensions are accurate and
identical to the grip.
Repeat this on another piece of card with the grip on its edge. IE
turned through 90 deg. Cut out this profile and lay onto your wood
draw around to copy the profile onto the wood. then band saw this 1st
profile out to make a copy of the original from a suitable piece of
Repeat with the second profile. Now you have a rectangular copy in
the 2 dimensions matching the original. With rasps and spoke shaves
carve this copy to match the original, hopefully the owner will let
you have the rifle for say a morning whilst you do this. Use a vernier
to measure the original to make sure the replica is the same.
Obviously youcan polish it with sand paper etc and varnish it too, as
the customer is paying you to make this pattern its logically his
property, you could give it to him along with the silver mount.!!
If your wood is strong enough and hard enough, you might just be
able to use it to form the silver around. Let us know how you get on.