"The rotary bolt opens due to pressure on the slopping surface camming it around, sort of like squirting a watermelon seed."
The slope in locking surfaces is around 6 to 7 degrees. I find it hard to believe that the forces tending to throw a double gun open are so great that they can force such shallow angled surfaces to act as cams.
If those forces were so great doubles would not be able to fire when held by two layers of scotch tape (per Gough Thomas), or no locking bolt in place (Greener).
More likely is the design that locates mass in the "wrong" place so that the inertia during recoil gets the parts moving. Note how Hoof reports fixing his opening problem by replacing weakened springs. This echoes my experience with this problem.
Slow motion videos on Youtube show that the opening levers and hammers (of hammerguns) oscillate under recoil. And let us not forget that inertia in Benelli autos is strong enough to compress a stout recoil springs that cannot be hand compressed.
