The GC-45's general design followed several decades of work by Bull with fin-stabilized artillery shells, starting at the Canadian Armament Research and Development Establishment (CARDE) and later at Project HARP. In these efforts accuracy was not a huge concern, the objective was muzzle velocity, and the test articles were finned darts representing missiles. Yet with the removal of the rifling and the soft-metal driving band on the shell itself, the shell could be designed purely for ballistics, as opposed to having the external constraint of the driving band. A system combining some sort of rifling for accuracy without a driving band would result in a longer-range weapon. However, such a design was never achieved.
After years of research at his Quebec firing range, Bull eventually settled on a solution. The resulting Extended Range, Full Bore (ERFB) ammunition was key to SRC's designs: a "pointy" looking shell with much lower drag at supersonic speeds. For longer range applications he added a base bleed system (invented in Sweden) that could be screwed onto the standard shell, as well as an even longer-ranged system with a rocket booster.
The gun designed to fire it had a 23,000 cm3 (1,400 cu in) chamber, a 45-calibre rifled barrel with 1/20 right hand twist fitted with a conventional muzzle brake. Its breech was a conventional screw with interrupted thread.
Key performance data, from the Firing Table are:
- ERFB-BB shell, weight 48.0 kg (105.9 lb), M11 Zone 10 muzzle velocity 897 m/s (2,940 ft/s), QE 898 mils, time of flight 112 s, range 39.6 km (24.6 mi; 130,000 ft). Probable error in range 212 m (696 ft), in line 36 m (118 ft).
- ERFB shell, weight 45.5 kg (100.4 lb), M11 Zone 10 muzzle velocity 897 m/s (2,940 ft/s), QE 881 mils, time of flight 99 s, range 29.9 km (18.6 mi; 98,000 ft). Probable error in range 189 metres (620 ft), in line 42 metres (138 ft).
- HE M107 shell, weight 43 kg (95 lb), M119 Zone 8 muzzle velocity 675 m/s (2,210 ft/s), QE 764 mils, time of flight 65 s, range 17.8 km (11.1 mi; 58,000 ft). Probable error in range 59 m (194 ft), in line 12 m (39 ft).
The dispersion of the EFRB shell is more than three times that of the FH-70 field howitzer at its maximum range of only 5 km less, and is twice as great as FH-70s at 20 km (66,000 ft; 12 mi). Its maximum range with the M107 projectile is the same as any 39 calibre 155-mm gun and its dispersion about the same. (The "dispersion" figure means that 50% of shells will fall up to the stated distance either side of the mean point of impact, but 100% will fall within 4 times the probable error either side.) Dispersion of this magnitude significantly reduces the tactical value of the equipment.
In 1977, Bull's work put him in touch with (what is today) the Denel SOC Ltd company of South Africa. Denel designed a new mobile mounting that was able to handle the increased recoil. It used a sole-plate to lift the carriage to take the four wheels off the ground. The chassis had the option of being powered by a small diesel engine acting as an auxiliary power unit, driving hydraulics that could set up the gun in two minutes, and move it short distances. This feature had been previously been included in the 1960s design FH-70 carriage by Vickers. Bull, meanwhile, started production of $30 million worth of rounds, shipping them via Spain to avoid the international arms embargo against South Africa.
At first, the U.S. chose to overlook Bull's actions and, according to him, the Central Intelligence Agency actively mediated the deal between Space Research and the South Africans. However, when the Carter administration joined the international efforts to sanction South Africa's apartheid government, Bull was arrested by U.S. Customs agents in 1980. The investigation did not go far, and active work on the case was ended by direct intervention of the White House. Bull pled guilty and was sentenced to a year in prison, serving six months. Having expected some sort of "slap on the wrist", he was embittered and made statements to different newspapers that he would never set foot in North America again. He left Canada and moved to Brussels where he continued his work.