Steve,
The rings depend on [1] the scope, [2] your gun, and [3] your neck !
Should you not be interested in a full explanation

then please cut straight to the bottom line, below.
- scope tube diameter: most cheaper scopes (like the one in my pic on the Hatsan) have a 1 inch / 22.5mm tube diameter. (Different from 30mm tube for many side focus scopes.) Such scopes often come standard with low or medium height 1 inch dovetail (11mm) rings in the package. Sometimes you may also choose, and that's good because of the other factors in play:
- type of scope rail on your gun: often dovetail (11mm width) for long air guns and picatinny/weaver (20mm) on bullpups. The Flash pup has both dovetail AND picatinny/weaver (20mm). But he Kral PB comes with a weaver rail only.
- height: a 50mm objective scope often needs high rings to clear the barrel. High rings may also be required by your gun to clear the magazine, even for smaller scopes. And the kral PB, because of the design of the butt, needs high rings + a riser unless your neck is made of rubber.
BOTTOM LINE: my 4usd pair of risers for the Kral PB are weaver all around, so my rings needed to be high, 1 inch diameter, and weaver ! I had to order them separately ...
Success!