Groovy Mike - oil does not increase the pressure of the load by burning. Every oiled ctg. I've used was still 'oiled' when I removed it from the chamber - at pressures over 60,000psi. Oil on the case walls increases the pressure against the breech block as an oiled case cannot grip the chamber walls in a normal manner.

In this case & other words meaning the same thing, the pressure of the load is decreased against the chamber walls and increased at the base.

To get dieseling, there must be sufficient combustable oil in the bore itself, to add to the conflagration of the powder. This happens more in pellet rifles, rather than in modern high powered firearms.

Normally, when the firing pin hits the primer, the case is shoved forward in the chamber until it stops and the powder starts burning, the rapidly expanding gasses expanding the case in all directions - pushign the primer back against the breech face or bolt face & the case releases the bullet into the bore. The case expands in proportion to the thickness of the case at various locations and the temper of the case walls base too. Due to the strength of the ctg. base, it does not expand behind the 'web(the thinning of the base into the case wall itself).

This expanded case, which has stopped against the front of the chamer & primer hard bck against the bolt or breech face, grips the chamber walls as much as it's pressed friction will allow until the pressure drops after the release of the bullet from the muzzle. Soem cases grip harder than others. The more taper, the less gripping ability the case has. The straighter (less taper) the case, the more gripping ability the case has - all else being equal as to finish, temper of the case, etc.

What also happens during the increase of pressure in normally tapered rounds developing pressure in excess of about 45,000PSI, the brass at the web loses it's ability to resist the pressure (passes it's elasticity level) and stretches, allow the case head to stop at the bolt or breech fase, reseating the primer whatever amount it protruded and sometimes excessively flattening it as well. That is why predicting pressure by reading primers alone is meaningless - barring blown primers, that is. It is also why some rounds and rifles show protruded primers when firing facotry ammo - sometimes extremely protruded primers - the pressure was below about 45,000PSI and perhaps the rifle has a headspace problem if protrusion is excessive. Excessive is regarded to be in excess of .006" in the States and .007" in Europe.

Now, getting back to the oil - a viscous oil could cause a swollen or ruptured chamber due to being incompressible as far as your rifle is concerned. Too much oil could reduce the expansion of the ctg. thereby decreasing the actual capacity enough that pressures were elevated slightly, but that would be against the breech, not the chamber walls. Personally, I would not allow anyone to place an oiled ctg. in any of my chambers. do use a very fine oil, applied with a lube pad when fireforming some wildcats, but not all.

I wonder how many or if any firearms being proofed - swelled the chamber and therefore "failed proof"?

How many of those were due to an overly-oiled ctg. being fired in them? These are merely questions - food for thought. There was a caution penned by a reputable gunsmith on a Wildcat forum I frequent about using a thicker oil than WD40 when fireforming. Perhaps the almsot straight sided cases are the only ones susceptable to this - I don't know.