Durs Egg is not a lock type - he was a well-known maker of London who settled there from Switzerland. You can easily google his name and see how well-respected his products are.
His son was Joseph Egg who partnered with Henry Tatham in the early
1800s - Tatham & Egg of Charing Cross, London.
Yours is, as pointed out, what is commonly called a screw-barrel pistol, or as you just as correctly call a cannon-barrel pistol.
D. EGG of London - though the inscription looks added or enhanced the pistol is old certainly - DrLaw alludes to this: "The engraving on this gun, not being the best of quality" but it isn't 17th century (1600s) it is (if right) 18th century (1700s) - the pistol is probably 18th c.
The Frasier Museum is a waste of time - you'll find more online than good infor from them.