Can you please provide the source for your definitions? You sell them as SS definitions but they differ compared to the Springsharp2 guide I am aware of.
The decision to use either type of gun designation has also no effect on the sim of a ship in SSv2.1.
One should also note that BL and QF are british terms first place, meant to seperate guns of different breech mechanisms and different ammo. Neither definition refers to elevation or limits thereof, like you do, because elevation is not a matter of the gun but of the mount used.
From wiki: "BL, in its general sense, stood for breech loading, and contrasted with Muzzle Loading. [...]" and also wiki: "A breech-loading weapon is a firearm (a rifle, a gun, etc.) in which the cartridge or shell is inserted or loaded into a chamber integral to the rear portion of a barrel. [...] The main challenge for developers of breech-loading weapons was sealing the breech. This was eventually solved for smaller weapons by the development of the self-contained metallic cartridge. For weapons too large to use cartridges, the problem was solved by the development of the interrupted screw." and from NavWeaps: "BL - British designation meaning Breech Loading cannon. [...] By the early part of the twentieth century, BL had become to be used only for bag ammunition guns."
From wiki: "QF came from "Quick-firing". The designation was put into use in late 19th century in two different meanings. In naval terms it was first used to describe small guns firing fixed ammunition i.e. a complete round formed from a metal (brass) cartridge case containing the propellant and projectile in one unit thus enabling higher firing rates. An early example was the QF 6 pounder Hotchkiss. In later pieces, the charge was sometimes separated from the shell to reduce the individual weight of loading, but the charge was still loaded in a brass case, rather than a cloth or silk bag typical of "BL" guns." and from NavWeaps: "QF - British designation meaning Quick Firing cannon. In the late 19th and early 20th centuries, this term was used to define any gun that could be fired several times per minute whether it used bag or cartridge ammunition. By the 1920s this designation was given only to guns that used metallic cartridge cases."
According to these definitions both BL and QF guns can be used for either single peace ammo or multi piece ammo. Both can use cartridges to seal the breech. In fact, there is no difference for guns of 6" caliber or less.
[Note that I have edited my entry on the Mountain class. Their main guns are now mounted in dual-purpose mounts.]