“It was suggested to me by a friend that I try using some motor oil to lightly oil the steel cases prior to firing, to add "slipperyness" to them. Has anyone else heard of this idea, and if so is there any problem with using plain motor oil or should something else be used (i.e gun oil)?”
What you are experiencing is an example of excessive breech friction. Without a doubt the steel on steel contact between the case and the chamber has created so much friction that the mechanism is unable to extract the cartridge.
It is hard to say why. Steel cases different in hardness and temper, and dimensions. With a hard material like steel, your mechanism can’t swage the cases down as easily as a brass case. So maybe you have an interference fit leading to excessive breech friction.
There are good reasons why brass is the preferred cartridge material. Brass will will contract more on firing than steel, while I have not looked this up, I believe that frictional forces between brass and steel are less than steel on steel, and brass is harder to corrode. It is also a lot easier to manufacture brass cases than steel. Making steel cases is difficult, and if the temper or the material is a little off, you get extraction problems.
Adding a lubricant into the system will reduce the friction between the case and the chamber walls. This reduction may be enough to allow your rifle to function. Try putting an oily patch in a bag, add cases, and shake. Motor oil would be fine. You do not need much oil, ( I really doubt you need a film thickness greater than .001”) for this to work. The surface only needs a thin, definitely not dripping, coating of oil.
When a cartridge gun is fired, the chamber pressure forces the sides of the cartridge against the walls of the chamber, sealing the gases in. If the cartridge is lubed, the cartridge just slips to the rear, putting all the pressure of the firing on the bolt face.
This is not a good thing to do to a bolt face. The gases may be able to escape and come rearward (toward YOUR face!), or the bolt may seal the chamber, but at the cost of enduring more wear and tear from the high pressure.
As a matter of fact, many machine gun designs had integral cartridge oilers, aiding in feeding and extraction of rounds in combat conditions. Google the Hotchkiss and Breda, among others.
They were also designed to handle the extra bolt thrust of oiled ammunition.
Bottom line: Good for machine guns, bad for Yugo SKS and other rifles not designed for that application.
Gentlemen: Lubricated cases, whether oiled, waxed, lacquered, or covered in water, will seal the bore. I was surprised to find out that a case can have a lot of lubricant and the weapon will still function. I have used RCBS water soluble case lube, Johnson paste wax, and Stick Wax. No gas blow back, no excessive pressure indications. If you had seen the gobs and film thicknesses of Stick Wax I had on CAVIM 308 ammo, with perfect functioning, I doubt you would believe that gas blow back is likely. I do not recommend using Stick Wax, the stuff is very difficult to clean off the rifle, cases, and the human. The stuff went everywhere! But it was something I tried in a FN/FAL.
If you examine the literature on the design of semiautomatic weapons you will see that case friction is not taken into account for bolt loading. The bolt is designed to take the full load of combustion. The only dispute is whether the load is based on the outside of the case head, or the inside of the casehead.
What is totally true is that a lubricated case will transmit more load to the bolt than a dry case in a dry chamber. There is unfortunately a lot of friction between case walls and the chamber walls.
I say unfortunate because breech friction is bad. Breech friction means the designer has to use extra energy to peel that case out of the chamber. That means more gas venting, heavier operating rods, heavier extractors, etc. Life would be so much easier if cases were frictionless. All you would have to do is use your little pinky to open the mechanism and the case would fall out.
The reason you don’t see oiler and lubers anymore is that they are just another thing to go wrong. Early designs needed these contraptions, later designers figured out how to get decent primary extraction without them.
If however you are using overpressure ammunition you already have a problem. Lubricating overpressure cases will encroach even more on the design margins of any mechanism. So, don’t load overpressure ammunition and never lubricate the stuff.
