The legal limit is 6ft/lb for a pistol - the hw45 is pretty close to this - but to be honest I don`t see the need for a powerful airpistol - their plinkers not hunters.
Sure i`ve heard of people using them for rats - but even rats deserve to be knocked over cleanly.
The only multi-shots I know of are the gas cartridge type, the ones i`ve used produce only about half the legal limit.