Back 50 years ago P.O. Ackley had an article in one of his loading manuals which said that a cartridge's brass case can hold-back a significant proportion of the force generated when propellent is ignited (and the chamber wasn't oiled - that made a difference).
P.O Ackley was quite a bit of the salesman with a little shyster mixed in.
Remember at the time ole PO was selling his Ackley improved (AI) cartridges. He was blowing out the shoulders, straightening the case, to increase powder capacity and raising pressures. It is obvious that ole PO was taking flak from folks who were claiming that his high pressure cartridges were overstressing the action.
P.O Ackley cartridges are very interesting and P.O’s test of a straight sided cartridge holding pressure without a breech block has been duplicated. The tester swabbed the chamber out with alcohol swabs between shots. The Ackley cartridge held. However the other cartridges, such as the 30-30, 35 Remington, blew out of the breech at 1900 fps. A 150 grain cartridge case flying at 1900 fps will go through both sides of most people's skulls. No designer would weaken his mechanism assuming high case friction, you just cannot control that.
This would be an interesting side topic, but based on the cartridge taper advice found in my referenced design books, straight cases are not an ideal form. They don’t steer well and they don’t retract well. If the mechanism does not cycle well, if case sticking is a constant problem, if feed geometry is too fickle, the mechanism will be a failure.
Ackley was taking heat because he was getting high velocities from his improved cartridges, cartridges which were being used in actions not designed for the things. P.O. wanted to show that his speedy cartridges did not increase bolt thrust, and infact as a result of his experiments, claimed they actually reduced bolt thrust. Which was bogus as heck as these improved cartridges actually ran at higher pressures. Higher pressures increases bolt thrust, it does not lessen it.
Read carefully Boatright’s papers one of which he shows how a 308 case, in a clean chamber, can lock in and hold pressures by itself up to 25K psia.
Go to Jim Boatright’s web page.
Look for yielding of the brass case in these studies
However once pressures go above 25K psia, Boatwright shows the brass case stretches and if not supported, the case head will blow off.
Regardless of taper, and all that mystical mojo, cases are made out of brass and will stretch. There may be bolt load reduction due to friction and stretching but it is inconsistent and not to be relied on in any way.
Apparently the standard 30-30 is strong enough not to lose its case head without the AI taper.
If you notice, P.O. Ackley never conducted his test with a 30-06 or a similar high pressure cartridge. I am certain those would have blown right out the back of his lug less rifle. Ackley and others did not conduct sensitivity tests, varying chamber finish, (chrome for example), powders, primers, or much of anything else. I totally disagree with the implicit conclusion that Ackley and others have drawn, which is if Ackley improved cartridge reduces bolt thrust, a user can just pour the coal into the cartridge and let fly. Ole P.O. was interested in promoting his cartridges, found a “one off” and left a very misleading legacy in terms of case friction, load, and chamber roughness.
Action designers ignore case friction in sizing bolt lugs and locking recesses. Actions are designed assuming that the case provides zero resistance. At least in modern actions.
The Lee Enfield is an example of a legacy action designed in the blackpowder era that requires case friction because the action is so weak and flexible. The British NRA have warned not to shoot the things in the rain! I got the idea they were worried about bolt breakage. Major E G B Reyonlds reported in the American Rifleman that the point of impact change between dry cases and wet cases is 5 MOA. That is one stretchy action!
Straight walled blackpowder cases need friction in the chamber. BPCR shooters report wet cases being stretched up to the lands and grooves. I have not seen any analysis but something about the low pressure curve of blackpowder, and maybe the bullet crimp, will pull a slick case up the barrel.