You might consider emailing Wayne Watson (
Welcome to Wayne P Watson's 18th Century Guns), as he built the gun for the movie. He's a great guy and loves to talk gun building (at least he did the last time I talked to him almost 15 years ago...). I'd talk to him about the process of building a functional muzzleloader to replicate it, and then use what you need for your project. Don't know how he feels about non-functional guns.
If nothing else, there is a fantastic shot of DDL on his page with the gun, you could get your proper length by doing some measurment conversions based on his height and the fact the gun is sitting on his foot. Hope this helps.