TEN years of suicide data in the wake of John Howard's decision to ban and buy back half a million semi-automatic rifles and shotguns has produced a stunning conclusion.

A paper forthcoming in the American Law and Economics Review finds the buyback cut firearm suicides 74 per cent, saving 200 lives a year.

A former Australian Treasury economist, Christine Neill, now with Canada's Wilfrid Laurier University, says she found the result so surprising she tried to redo her calculations in the expectation the effect would be smaller.

''I fully expected to find no effect at all,'' she told The Age from Waterloo, Ontario. ''That we found such a big effect and that it meshed with a range of other data was just shocking, completely unexpected.''

John Howard's agreement with the states to ban and buy back more than 600,000 weapons in the wake of the 1996 Port Arthur massacre cut Australia's stock of firearms 20 per cent and approximately halved the number of households with access to guns.