It wouldn't be difficult, but it would be expensive.
Let's say materials cost 50 quid.
But you'd need to pay for the time, and the fact the fella/lass who makes it has a lathe, grinder, polisher, pillar drill, workshop area ect.
It would actually be quite simple, but not financially viable.
Personally I'd cast the reciever/hammer/levers ect out of a variation of Zamak and make the barrel from mild steel, with a long 8mm wide slot in the base of the barrel. The actual pin (is it on the hammer or a transfer?) ideally made from polymer.
The last thing you want to make is something that can actually go bang.