Just to complete the SMG story I discovered that the New Zealand company was R.V. Crockett Ltd. They produced some guns from the parts they purchased from Tapmatic Corporation in the USA. The NZ guns seem to be the MKII MAGNUM versions which can use the later more powerful Grey SUB-AMMO and Brown SUB-AMMO with their stronger breeches and longer barrels.
You could make up your own SUB-AMMO if you purchased the SMG plastic sleeves and washers to use with .22 Ramset cartridges.
SUB-AMMO (Instructions)
SUB-AMMO SLEEVES are for use with .22 wadded blanks, Nos. 3 through 6, of the type used in powder actuated tools (manufactured by Ramset Speed Fasteners Company, Remington Arms, etc.)
LOAD-YOUR-OWN SUB-AMMO (only)
The blanks are loaded into the cup of the S.M.G. washer and then into the plastic Sub-Ammo sleeves.
For a permanent, waterproof seal apply a liberal coating of clear epoxy or plastic adhesive to the cartridge just prior to insertion. This bonds the cartridge to the plastic sleeve and prevents the possibility of leakage under pressure on deep dives.
SUB-AMMO GREEN for small fish, reef or cave shooting; SUB-AMMO YELLOW for medium fish at moderate range; SUB-AMMO RED for medium fish up to maximum range. Do not use with fibreglass spear. SUB-AMMO PURPLE for large fish at greatest ranges. Do not use with fibreglass spear.
To protect spear head do not use near rocks.