<< bonanza
According to my reloading manual, the case rapidly expands to fill the chamber, grips the walls of the chamber so tightly that the head will stretch if the head space is not correct giving that bright ring around the base. How can the case produce back trust if it is griping the chamber walls? >>

Bonanza, you have provided the answer to your own question. The case stretches. For a rifle, the first correction in calculating the back thrust is to subtract the force required to stretch the case.

<< new guy
Maybe the question isn't which is stronger...
But rather: "Are both Strong Enough?" >>

Yes, this is really the question that matters. The answer is an unequivocal yes. Provided the action, whichever type it may be has been designed for the pressure at hand. You can make a bolt action out of pot metal, but I wouldn’t want to use it for anything beyond a cap gun.

DUGABOY1

The technology involved in the drop barrel or break open action is not 18th century technology, but rather 19th Century technology. And the bolt action is also 19th century technology. In both cases improvements have been made since then using 20th century technology, most notably the improvements in metallurgy.

Judging the strength of an action type can be deceptive. A Winchester Model 94 looks strong, it has a massive breech block locked with two good sized sliding bars. But it is weak because the sliding bars are at the rear of the breech block. On firing the breech block compresses while the frame stretches. With too much pressure this allows the cartridge case to elongate too much and the brass fails. Note the brass fails first, not the action itself. We end up with a SAAMI maximum average pressure of only 42000 psi. The Winchester Model 95 is much stronger, not so much because it’s bigger as because the locking bars have been moved forward eliminating much of the compression and stretch.

The weak link in today’s technology is the brass cartridge case. The case restricts the maximum average working pressure to 65000 psi. This allows room for a proof load with 30% more pressure than the working load. When pressures go beyond this region the brass begins to flow.

The action is stressed by the back thrust. Using modern metallurgy either action can be built for a 65000 psi working pressure. (See new guy’s comment.) If you had a hypothetical cartridge case that would withstand a gadzillion psi, then I suppose as you increased both the action strength and the pressure you would reach a point where one or the other action types would prove inherently stronger. But that’s a moot point because we only have to handle 65000 psi as an average working pressure.

The third fastener, debates not withstanding, does do something to stiffen the action. The debate should not be, “Do they do something?” but, “Are they really necessary?” It’s a simple experiment to prove they do something, you can find a good example of such an experiment in Greener’s book, although he is guilty of slanting it to make his third fastener look better than every body else’s. But then Greener was never noted for being shy about blowing his own horn.

Now as DUGABOY1 indicates, there is nothing to gain by trying to hot rod a double rifle load. The older rifles were designed to work at the pressures they worked at, regardless of the action characteristics. There are some of the old bolt actions you don’t hot rod either if you have any sense. In addition, old rifle or new, a hot rod load is likely to affect the regulation and can also be sticky on extraction, not good in a mixup. On the other hand, I wouldn’t expect to download the 375 H&H to match the 375 Flanged, again you might have an adverse affect on the regulation. No harm in trying though, since we would be lowering the pressure instead of raising it.

The double rifle cartridges ran low pressure for two reasons, reliable extraction, and because there is simply no need for high pressures in a cartridge designed for a double or single shot rifle.

So the bolt action has an advantage in extraction, but not in pressure. It’s easier to design for gas handling in the event of a failed case or primer in the bolt action. The “advantage” of from two to four cartridges in the magazine is largely an illusion. When things get hairy there will not be time for a third shot, any “third” shot will come from your backup man.

As for the extractor, yes rimless type extractors work. But I have to go with DUGABOY1 on this, the simpler the better when reliability counts. And I also go along with him on the non-auto safety and double triggers.

All this said, the question of a rifle or gun being “off the face” is a good question and deserves some real answers. There are different types of drop barrel actions, some rely on the hinge pin taking almost the entire back thrust. On the other hand the Purdey type action has one or both of two the two lumps taking back thrust. If so fitted there is also a third fastener. All this assumes proper fitting, which is one of the factors that make our pets cost so much more than a typical bolt action rifle. Given sufficient wear, any rifle or gun will go off the face, but I suspect the majority of cases are due to either a cheap gun or abuse of one form or another. How about slamming the action shut?