The guns are sold as primarily as shotguns but Crosman advertised and sold both shot and solid ball for them. whilst shot cartridges are almost unobtainium, .375 ball can be easily obtained from Muzzle loaders suppliers. Another point is the word 'Capable' . if the shot energy is tested As a 'DISCHARGE FROM THE MUZZLE' then the shot will not have time to spread and will be treated as a solid missile. Would you take your chance at getting hit by a charge of small shot at 6" range?
There is no escaping the fact that the muzzle energy exceeds the limit, the missile is almost irrelevant as the gun is not designed for or ever advertised as suitable for use with non lethal ammunition .
EDIT. The word 'CAPABLE' in the legislation is the reason it is almost impossible to get a S1 air rifle downgraded to non FAC. Once it is deemed capable . That is final as far as the authorities are concerned.