Usable black powder can be made without sulfur according to Ulrich Brescher's measuments http://www.musketeer...der/recipe.html It is less explosive in a bomb or mortat but a long barreled musket can still get almost as much energy from it, and has the advantage of being less smoky and less corrosive on your weapon, but the black powder armies never realized this because they always assumed the most explosive powder was the best for firearms. His website is also a good source of information on black powder in general, its history and how it was made.
In a gas operated weapon the black poder would probably clog up the gas port forcing you to cycle it manually, but a recoil operated machine gun could probably work If you reduced the the strength of the recoil spring. loaded into the cartridges of modern weapons black power would considerably reduce their range and accuracy. Wikipedia lists the muzzle velocity of a black powder .303 rifle as 2040 feet per second bested by some modern magnum revolver pistols. The muzzle velocity of the early smokeless cordite filled .303's is listed as 2441 feet per second however that is the same velocity as a AK47 which shoots a similar size bullet from a much smaller cartridge demonstrating the improvements smokeless powder chemistry in those intervening 53 years.
In a post apocalyptic scenario a couple alternatives to flint locks or percussion caps to consider are 1 pneumatic ignition. A spring loaded piston would forcing the air though the fuse hole due to compression would easily reach the ignition temperature of the powder. Diesel engines work the same way. It also make a handy alternative to matches: http://www.primitive...st_century.html
2: Piezoelectric ignition found on BBQ's depending on how extensive this end of electricity is, quarts it naturally occurring.
Some early breech loaders such as the Dresler Needle Gun used paper cartridges. The leakiness of the breech was apparently solved by adding leather washers to the bolt. You could swap the needle for a pneumatic piston and then you dont need the mercury fulmate.