"That acute angle actually makes the action want to stay closed
when cartridge back thrust occurs, even if there was no
locking mechanism!"

I tend to agree.

After all, if every time you fired the gun it forced the action open
all Double Guns would come loose a lot quicker than they do.

I agree about dry guns / pivots - it's amazing how many people
do not grease / lube the pivots and or locking lugs of Double Guns
they use.

Personally, you can produce as many force vector analysis's as you
like, I really couldn't give a s2#@t what forces are acting where on the gun,
as long as it works when I pull the trigger, shoots where I point it
and stops me from being trampled.

While you spend your time reading force vector analysis's and
working out backthrust and it's affect on Double Guns,
I'm going to spend my time getting ready to go hunting
with my Double Rifle.

500 Nitro