Always the but. A dummy launcher used for traning gun dogs, and an ideal line thrower, is classed as non firarn because it is a spigot launcher. So if smoke dischargers are Pyrotechnics, where does this bit of section 5 come into it?
'Any weapon of whatever description designed or adapted for the discharge of any noxious liquid, gas or other thing' Chemical smoke is definitly noxious.
Vets use blowpipes to anthetis animals. For this a Section 5 is required, as the dart discharged contains a 'Noxious liquid'.
ACPO have a nasty habit of interapting things to suit themselves. They do not make legislation, that is Parliment's job, and the Courts interpret. ((That's why 'Reasonable' is the word that makes the lawyers a fortune! )