The term "overbore" is not a technical term used in official ballistics, however much is discussed on this term and what it means.
Basically overbore is a term that describes the efficiency of a particular cartridge. 25%-30% is about the limit of powder efficiency as the limit of bore capacity. The remaining energy is lost to heat, friction, expanding the brass, etc.
25% is generally considered the lower end limit for average efficiency. Anything above that is an "efficient" cartridge design.
Overbore then, can be considered any cartridge/powder capacity that produces less than 25% efficiency.
No official measurement of overbore exists, but some simple math can help in defining overbore. We are looking for a way to define the amount of powder used for any given caliber and the resulting velocity so that we find the point where increase in powder begins diminishing returns in velocity. The point where you start burning lots more powder just to get a small increase in velocity.
The entire exercise is a relationship of bore diameter and case capacity in grains of water ( a standard used to measure case capacity. This does NOT mean that is the maximum load of powder as powder has air spaces between the kernels, always consult published load data).
An easy, consistent formula to use is Bore diameter x bore diameter x 1000 = maximum case capacity (in grains).
So, for your question of the 22 Cheetah, let's look at what our formula finds:
.224 x.224 x 1000 = 50.1 grains
So, any 22 caliber round that uses more than 50.1 grains of powder is technically "overbore".
Hodgdon reloading data for the 22 Cheetah MkII shows the following loads for 50 grain HDY SP bullets at maximum loads:
47 grains H380 for 4057 fps
51 grains H4350 for 4069 fps
Clearly the 47 grain load is within the 50.1 grain overbore limit and this efficiency level is shown when compared to the 51 grain load of H4350 (overbore) which only increases velocity by 12 fps. Obviously, using 4 more grains of powder to eek out an extra 12 fps increase in velocity is not efficient and wastes powder. It's also interesting to note that 51 grains of powder is only slightly over the 50.1 grain limit, yet it already clearly shows diminishing returns in velocity.
Please note too that different powders will return different efficiency numbers and load tables can also show efficiency for a single powder type if the min and max loads cross the overbore capacity limits.
In your case it would seem the 22 Cheetah MkII is right at the verge of being overbore, but most published loads for it keep it just inside the overbore limit. Technically speaking then, it is not an overbore round.
