By golly if reloaders don't know the answer, noone does. For decades armorers told us "don't oil your cartridges, it increases your chamber pressure." I always took their word for it. I figgered they had no reason to fib; it all pays the same, and I tried to keep them as dry and clean as conditions allowed. I read similar things in a Marlin manual. But I would like to know... how big an issue is this? And what about rain water or other fluids? Sometimes, conditions in the field are not ideal.

Would enjoy your thoughts here.

My take on it:

When a cratridge is fired, the brass expands starting near the front of the cartridge and working its way back. Whent the brass expands, it grips onto the tiny imperfections of the chamber which pretty much holds the cartridge into place. The brass will stretch and flow until the base hits the boltface. The action of the brass briefly gripping the inside of the chamber takes alot of pressure off the bolt lugs and bolt face. If the cartridge is lubed, it won't stick to the inside of the chamber as nicely and will slam with increased force into the boltface- giving the appearance of and the same effect and wear and tear on a rifle as an over pressure load.

Also I could imagine a series of lubed or wet cartridges pushing water further and further into the chamber where the moisture or oil gets in front of the bullet. Since liquids aren't compressable, this could in away, act to shorten the freebore of the rifle's chamber. The Free bore allows the bullet to start moving down the barrel and out of the case before the burning powder reaches peak pressure. If you already have a near maximum load and the bullet gets held in the case a few milliseconds longer, the pressure will increase beyond the maximum normal pressure range before the bullet is able to start moving.

That's my best theory on what happens.

Oil in the chamber prevents the brass cartridge case from clinging to the chamber wall during peak pressure, thus increasing the back thrust on the breech.

question, many many moons ago we left some sizing lube on some cartridges, shooting them in a bar and they would not eject, we thought that the lube was sticking to to the chamber?, we wiped down the cartridges and they shot fine, but i always thought they were sticking, would it have been that they were moving faster than the action could recover from or what, we have rectified the situation though we all now shoot bolts or single shots, just wondering, oh yea we also make sure we remove any sizing lube[:-]

Very informative comments. We have some real champs on this board. Thanks