Claude Blair in his book
Pistols of the World 1968, THE VIKING PRESS, NEW YORK, says on page 17,
"
The Turn-off Pistol This form first appeared in the 1640's and appears to have been used in England more than anywhere else. The barrel was made to unscrew just in front of the chamber so that the latter could be loaded directly. The advantage of the system was that the ball could be made to fit the bore more tightly than was possible with a true muzzle-loader so reducing the escape of gasses round it and increasing both power and accuracy. The system also made the use of rifling a more practical proposition, and many seventeenth-century turn-off pistols are rifled. They were known as
screwed turn-off pistols , the modern practice of calling any pistol with a barrel that unscrews a screw-barreled one being incorrect.
Turn-off pistols of the second half of the seventeenth century are usually stocked only to the breech. Many pistols of this type have a running ring round the barrel linked to a small rod projecting from the fore-end. This facilitated the process of loading by keeping the barrel permanently attached to the pistol. Another aid to loading that seems to have appeared first at the end of the seventeenth century was a small wrench for unscrewing the barrel: it normally engaged with a lug on the latter.
Sometimes an internal wrench shaped to fit into what at first sight looks like muzzle-rifling was used...
The turn-off horseman's pistol had gone out of general use by the end of the seventeenth century and the system was henceforth used almost exclusively on belt- and pocket-pistols...
Throughout the period when turn-off pistols were in use, ordinary muzzle-loading pistols of exactly similar design but with fixed barrels were also made. Both forms in the late seventeenth and early eighteenth centuries frequently had a raised molding at the muzzle, like that on some cannon, whence modern collectors sometimes refer to them as
cannon-barreled pistols ..."
I find this latter comment interesting because Pedersoli's "Queen Anne" pistol fits this very nicely. At least according to Claude Blair it is a PC firearm.