Conclusions
A casual glance at the "Average Performance"
graph suggests one fact that is obvious to experienced varminters.
While the 22-250 gives a usefully flatter trajectory than does the
223, the 22-250 also heats the barrel so much faster than the 223
that heating can create a significant problem when shooting conditions
are good. Conversely, the striking advantage shown by the 20-caliber
(5mm) numbers, compared to either of these popular 22s is likely something
of a surprise to many readers.
A tiny part of this differential stems from the fact that the 5mms have a slightly thicker barrel wall (0.010" at any given location), which results in a tiny decrease in deformational heating resulting from any given stress loading – however, this difference amounts to only a couple of percentage points. The big heating difference stems from the fact that these particular 5mms both do a comparatively better job of trapping unburned propellant in the case.
Consider the 223 versus the 204. In this set, the 204 has a usefully sharper and wider case shoulder – such a design will trap a far greater percentage of propellant that was not ignited by the primer within the case (primer flash only ignites propellant within about one-half inch of flash hole – refer to this author's primer plume penetration study, elsewhere on this site). The predictable result is that the 204 will show less barrel heating because less total mass will accelerate into the bore. For the same reason, even when loaded with the same weight bullet, the 204 will generate far less "felt" recoil and sight picture disturbance.