A resin kit with a working trigger? As we all know, Boba only fired his Webley once or twice at Luke in ESB and then confronting Luke, in ROTJ, he launched himself from the sail barge to attack, only to be immediately disarmed, his blaster sliced in half by Luke's Saber, not one shot in the whole film from the Webley (well he did shoot a little, except it was his laser drill on his gauntlet, but like most stormtroopers, a very bad shot?, he then gets piked by Han, blind and backwards and behind him... and dies in the giant Vagin...oops, I mean the Sarlaac Pit.
The amount of effort to create such working feature, might as well buy a real Webley if you can find one. There is a kit coming from the UK for a Webley, I believe they are about to make the moulds. I can suggest they make a custom for you...I imagine there would be an increase in price.
Simon and Pete are the guys I talk too, very friendly and they might be able to pull off a request like this. Frank is the guy that makes the stuff, but we have not spoken. They sell stuff on ebay under frank-n-son ID and they have a 100% rating.