Not that easy to correct. If you increase the acceptable diameter, then you start excluding some grenade launcher from the ban, which was the point to begin with. If you try to exclude shotguns, then you open a whole new can of worm, which is to define what a shotgun is (nothing even close to that in the FA act). You could end up with 88mm howitzers that are also shotguns.
The root of their problem is that a 20mm grenade launcher is technically pretty much the same thing as a 10G shotgun.
As for the energy problem, it goes way beyond shotguns. There is a possibility that everything that's 308Win or larger is prohibited at this point. It is possible to generate 10kj of energy in a case that's the size of a 308Win, so in theory, the calibre is "capable of discharging a projectile with a muzzle energy greater than 10,000 joules". In practice I don't think so, but:
1-Anything that's 338LM or larger is capable of that in practice, I have no doubt about it;
2-With advancement in smokeless powder technology, the minimum calibre capable of discharging a projectile with a muzzle energy greater than 10,000 joules will shrink. An increase in energy/volume comes with a decrease in the case capacity required to discharge a projectile faster. There will come a time when very small cartridge will be capable of discharging a projectile with a muzzle energy greater than 10,000 joules, so the amount of prohibited rifles will grow. Basically, some day a scientist comes up with a new smokeless powder, and your rifle is now prohibited.
Keep in mind that nowhere in the FA, the criminal code or the OIC that any concept of "regular" or "standard" cartridge is expressed. The word "capable" very much includes anything that's atypical, not matter how weird or unsafe the cartridge can be.