Not too much help, but this is what I found about the tapered bores:

The idea of tapering a bore to improve accuracy and velocity is not a new concept. The German Army designed its famous 88mm cannon with the tapered bores in order to gain the super-high velocities that made them such infamous tank killers. In the United States, the old Springfield Armory also conducted substantial R&D on taper-bored weapons with positive performance results. In fact, the research published since the 1940's has consistently identified a tapered bore as the ideal bore configuration for both accuracy and power.