YES,
Special delivery routinely fly's, and is routinely X ray'd, the x-ray will show "a pistol" it will be refused flight & because it's a "gun" it WILL be sent to Belfast for inspection.
While not required by the rules, if I post an air gun I include inside the box a signed letter stating it is a UK legal sub 12/6fpe air rifle/pistol empty & unloaded with both seller & buyer details.
Strangely Post Office workers do not know every single posting rule for Royal Mail, and as their rules state, it is the senders responsibility to ensure all rules are met,
Air guns can be sent 1st class or RMSD by ROYAL MAIL, if they are too big for ROYAL MAIL they can be sent with PARCELFORCE,
If sent with PARCELFORCE they must be sent PF48, no other PARCELFORCE service is allowed.
Not exactly difficult rules to follow.