One thing nearly all the gunmakers did before WWII was, to one degree or another, the whole factory was the "custom shop".
Meaning, some, if not most would build you anything you wanted, and paid the premium for, (within physical limiitations) even when it was outside the standard offerings of features.
Today, if you want a combination of features the factory doesn't offer, they will still make it for you, if you order a few hundred (or more) guns at once.
One thing I've learned, like the Bond movie title "Never say Never".....