With nitrocellulose propellant manufacture being a continuous process industry and production ramped up enormously during wars, when hostilities cease, the authorities end up with large unwanted quantities in the ammunition production pipelines and still coming out of explosives factories before their lines are shut down. This was a huge problem after WW1 in particular and thousands of tons of the stuff sat in warheouses around the world. It also applies to things like stores of guncotton and naval gun propellants which go out of date and have to be destroyed, not nearly so much nowadays as in the days of big gun navies.
Until not that long ago, large quantities of such heavier stuff went by rail to Shoeburyness in Essex for taking out to sea and dumping - I doubt if that's allowed now.
After WW1 and to a lesser extent the last war, arsenal staff had to find ways of disposing of rifle, cannon and artillery propellants. Nowadays, much can be recycled into making new powder, this being in the slurry and still method developed to produce ball powders by Winchester Western in the 1930s, one of its great pluses. Before that in the 1920s American ordnance staff had to 'lose' huge amounts and it was found through experiment that the only practical way was by burning it. Experiments were carried out to find safe maximum weights for the fires. Ultimately huge amounts were disposed of by creating hundreds of heaps, each several pounds weight, a set distance apart on isolated empty ground and workmen manually firing each individually with a taper on a long pole. I remember seeing a photograph of one such burning ground on the site of a US government ammunition factory. So .... burning a few ounces outside of the house hardly carries major risks if basic safety precautions are applied.
I have read, but don't know if it's accurate, that many modern smokeless powders don't break down easily or quickly when spread on the garden and they are certainly environmentally unfriendly. I did try to get rid of some old ball powder this way some years back spreading it very thinly over part of our back lawn. It did the grass no good at all, and my other half was extremely critical, ordering me never to repeat 'that stupid stunt'.