Okay. The wood is split and the stove is pumping out the heat. We heat with wood, exclusively so I get to get warm twice.
Now, back to the back thrust.
When the powder ignites and pressure starts to build, the bullet moves forward as the neck expands. In most cases, the neck probably expands until it contacts the neck of the chamber (but not always). This starts the seal (case as a gasket) as pressure builds, more of the case contacts the chamber wall, thinner portions first, until the entire surface of the case is contacting. Once contact is made and pressure builds further, the case will adhere to the chamber walls sufficiently that it cannot be moved rearward. The claim is that a case with less body taper (like the 308 Winchester) will grip the chamber with more force than will a case with more taper (like the 303 British, plenty-o-taper). Guess what? This is undoubtedly true. There are formulae and computer models which prove it (sort of) but here's the thing; once the case is gripping the walls with sufficient force that the head will serparate (if poorly supported), any more grip is meaningless. We know that a Lee Enfield, especially if it's loaded heavy, will separate cases just ahead of the web. If that well-tapered 303 case was not gripping the chamber wall, it would just slide back and the head would not come off. In a Lee Enfield fitted with a 7.62 barrel, exactly the same thing happens. Plainly, both cases are gripping the chamber wall with sufficient force to hold the brass enough that the head will separate. This is the first part of the explanation, now for the second.
The Lee Enfield separates case heads so well because, under high pressure, the bolt compresses and deflects while the receiver stretches. It is this attribute, by the way, which made the Lee Enfield such a fine battle rifle. You can force the bolt closed on a deformed case where there would be no chance of doing the same thing in a Mauser, for instance, and a Remington 700 would end up with the bolt handle on the ground.
On the other hand, that Remington 700 bolt, when the rifle is fired, deflects... hardly at all. Probably less than .001" under normal circumstances. So, any resistance to bolt thrust beyond .001" of deflection, doe not even come into play. I once did an experiment where I made up some chambered stubs (in 30/40 Krag) and split them on one side. I made three pieces. One was chambered to half depth (measuring down to the web of the case). One was chambered to 2/3 depth. The last one was chambered so that about .2", ahead of the web, was exposed. This is about where a case usually separates if it is going to. I made up a rod with a tee handle. The rod was about two inches longer than the chambered piece. I then stuck a case into the first stub and clamped it in the vise. I set up an indicator with the magnetic base mounted on the stub and the plunger against the case head. I then pushed on my tee handle to see if I could get the case head to deflect measurably. I could. In fact, it wasn't even all that hard to get the needle to move .002". Then to the depper chamber. I could still get the needle to move but I had to push harder. This made sense. There was less length of brass which could stretch and the exposed brass was thicker. The deepest chamber took a lot more effort but I was still able to move the needle .002". How much force? Well, I'm kind of old and won't be seen competing in any strongman competitions so I expect I might have been pushing a couple of hundred pounds. The brass would stretch this much then spring back. Now, given that the pressure contained by the brass is likely to be up around 15,000 lbs of thrust, a couple of hundred is a drop in the bucket. Add to this the fact that we have already establish that a tapered case will hold the chamber walls enough to allow the head to come right off, if unsupported, you can see why I don't believe case shape has any bearing on bol thrust. More after I eat supper.
