First of all are you talking about small volume of a large case ? If so then, In fact you have it backwards regarding the fast powder providing inputus to the projectile and than stalls as it enters the rifling..

This is the case with reduced loads of SLOW powders and large capacity cases.
The pressure again rises but the projectile is lodged in the rifling and a condition similar to a plugged bore occurs. In a strong bolt action or Ruger NO.1 this condition won't do much except flatten primers or maybe pierce them but a double?
Holy shit!

The Wheatherby line of cartridges are loaded to the max case cpacity and operate at upwards of 62,000 psi. the free bore allows to bullet to move unimpeded by the friction of the rifling
thus keeping the pressure curve more gradual.
So, This is why many handloading manuals provide reduced loads whith rather fast burning powders .

Regarding the over/under, The fact that the lower barrel is essentially surounded in the action bar makes it far stronger than the sxs action and yes the top barrel may impart higher bending moments and so those in the know opt for, In the interest of safety ,a 3rd or 4 fasteners.. correct!

Your obvious knowledge of physics are still not able to explain anything away here.
Citing these principals looks good on paper to some people but it's mostly elementary.

Im just making a case for the 3rd fastener..

When I dig through my books and find the notes on the slow powder DR blow up phenomenon that I speak of I will certainly post it.
Tom