I had a guy bring a Savage Model 99 in .243 Win to my shop. He had a case head separation and it blew the buttstock off the rifle. He had some problems with extracting empty brass and he was oiling his ctg cases. It increased headspace due to increased bolt thrust. The field gauge would rattle in the chamber. He wanted me to re-barrel it and I refused. You could see the indentation in the locking surfaces in the rear of the receiver.
I would be curious to know what loads the 243 shooter used. I have no confidence that his loads were anything but over pressure reloads. Whatever bolt thrust reduction dry cases provide, it is inconsequential. Cases are always operating in yield: they are stretching, cartridge cases are not structural members, they must be supported or they will rupture.
This idea that lubricated cases are dangerous start with Hatcher and the 1921 tin can ammunition coverup. It was further extended by P.O Ackley and his claims that case taper reduces bolt thrust, therefore his overpressure Ackley Improved cases were “safe”
These claims are and were 100% bogus. High pressures are a result of too much powder in the case. A sloppy reloader adding a couple of grains over max can raise pressures well over 80 Kpsia, well over 100 Kpsia, to the point that the brass flows as a liquid.
There were many fielded machine gun designs that used lubricated cases, given normal cartridge pressures they did not, and will not, increase headspace.
Section 11 Cleaning for the Model 42B Ljungman is quite explicit, lube your cartridges, when the chamber gets dirty, clean it and lubricate the chamber and the cartridges.
The Schwarzlose machine gun was another, look at the wonderful pictures at this Swedish site, and of course, you can see the oiler.
The Japanese Nambu used an oiler:
The Italian Breda 30 used an oiler
Oilers were designed out of mechanisms after WW2. The Germans captured a Russian machine gun that had a fluted chamber, copied it in their assault rifles, and that ended the need for oilers.
I don’t know why FN did not use chamber flutes, but on their 5.7 cartridges, they are using Teflon. I have read on other forums that injuries have occurred when shooters reloaded their cartridges and rubbed off the Teflon. It is likely the cartridges ruptured on extraction, but I really don’t know what happened.
FN's 5.7×28mm cartridge cases are covered with a special polymer coating for easier extraction with the PS90 carbine due to the high chamber pressures and lack of case tapering. In addition, this coating ensures proper feeding and function in the magazines.
As for those who claim case friction is necessary or the bolt will be overloaded, what do you do with these two piece polymer cases? Do notice the brass case head that provides the case seal. How much load can that polymer/brass joint actually carry?
It turns out not much. The problem with that ammunition was that the front of the case stayed in the chamber messing up function with the next round.
So this is polymer case 2011, Oh there is a metal case head in there someplace, but the sidewalls are plastic.
There are two factors which lubricated cases could cause problems. The first are bottlenecked cases with too much headspace. A case with excessive headspace is going to peen the bolt face, lubricating the case will increase action peening. The second is for people who load over pressure cartridges. There is a tiny amount of load reduction on the bolt face with dry cases, remove that, and the load is 100% on the bolt face.
Model 1893 Spanish 7x57 rifle. brought in because the owner wanted to know what caliber it was. I told him it was a 7x57 Mauser. He said the guy where he bought it said it was a .308 Winchester. I tried to check the headspace and the gauge would not work. There was a ctg case in the chamber. I used a headless case remover and discovered a headless .308 case in the rifle. He said he fired the .308 in it. He had to beat the bolt closed to chamber the round. He also said it kicked like hell. I checked the headspace and it was in tolerance. I told him to use 7x57 ammo next time. I also checked the locking lug recesses for an signs of setback and there was no sign of it. So much for Spanish rifles being soft.
It is remarkable that this Spanish rifle did not blow. I am certain the shooter would have been in sad shape had the case head ruptured. Whether or not M1893's are "soft" or not, and there are plenty of examples of "soft" M1893's, these early actions frag when they blow and they have virtually nothing that will protect the shooter from gas/brass particle release.