You can make a wooden plug just slightly larger than the gun, I assume its gripping around the barrel/foresight area. Draw around the gun on a suitable piece of wood & jig/band saw it out !

Now soak the holster in cold water for a few hours & then shake off the ecess water & fit the plug. Leave to dry naturally, do not use heat, like a radiator or anything. You should now have a slightly stretched holster. Oil it once it is fully dried to keep it supple.

Chris W.