I'm not sure why anyone would calculate energy based on the total weight of projectiles rather than individual ones. The 1969 secondary legislation is very clear when it says "capable of discharging a missile so that the missile has... kinetic energy in excess... of 12 ft. lb" (my italics etc).

Dust shot can do little damage, which after all is the unstated objective of the law (notwithstanding preventing US competition to UK airgun manufacturers, which was the real reason...).



STATUTORY INSTRUMENTS
1969 No. 47

ARMS AND AMMUNITION
The Firearms (Dangerous Air Weapons) Rules 1969

Made
13th January 1969
Coming into Operation
1st May 1969
In pursuance of sections 1(3) and 53 of the Firearms Act 1968, I hereby make the following Rules:—

1.—(1) These Rules may be cited as the Firearms (Dangerous Air Weapons) Rules 1969.(2) These Rules shall not extend to Scotland.(3) The Interpretation Act 1889 applies for the interpretation of these Rules as it applies for the interpretation of an Act of Parliament.(4) These Rules shall come into operation on 1st May 1969.2.—(1) Subject to paragraph (2) below, Rule 3 of these Rules applies to an air weapon (that is to say, an air rifle, air gun or air pistol) capable of discharging a missile so that the missile has, on being discharged from the muzzle of the weapon, kinetic energy in excess, in the case of an air pistol, of 6 ft. lb. or, in the case of an air weapon other than an air pistol, of 12 ft. lb.(2) Rule 3 of these Rules does not apply to a weapon designed for use only when submerged in water.3. An air weapon to which this Rule applies is hereby declared to be specially dangerous.
James CallaghanOne of Her Majesty's Principal Secretaries of StateHome Office
Whitehall
13th January 1969