Firing lubricated ammunition increasing chamber pressure

[condor bravo]

I seem to recall from an American Rifleman article that firing lubricated or oiled ammunition, due to a hydrolic effect, can increase chamber pressure. Probably more of a concern with rifle ammunition. Does anyone have any insight on this?

[gwpercle]

It doesn't increase the chamber pressure per se. Normally, upon firing, the brass case expands and will for an instant grip the chamber wall. When the pressure lessens the brass releases it's grip, springs back and can be extracted from the chamber.
By lubricating a case the gripping effect of the brass is lessened, now the rearward case thrust is greater.
The chamber pressure is the same but the rearward thrust is increased.
In a hand gun, especially revolvers, the cases will move rearward and tie up the gun, especially if the case is tapered like the 22 Remington Jet fired in the S&W Model 53. That problem didn't help sales and they haven't been manufactured in years.
In rifles, the increase is in bolt thrust, which would have the same effect of increasing chamber pressure, but it doesn't increase the pressure , it increases the rearward thrust.
There is a whole lot more to the dynamics than my simple explanation but I was trying to keep it understandable. Hope this helps.
Gary

[Bart B.]

If the case is lubed, peak pressure will still be the same but its body won't grip the chamber walls as good. Back pressure from the case head to the bolt face will be more, but typically not enough to be dangerous. If the metal compression from peak pressure is enough to minimize case springback, extraction may be difficult.

Some folks do use a bit of lube on cases without any problems.

Sierra Bullets tests their stuff for accuracy as they come out of the final ogive pointing dies still covered with a lanolin based lubricant. Those greasy bullets don't cause any overpressure situations. But too much liquid or greasy stuff in the bore will cause overpressure sometimes "ringing" the bore (swelling it over size) so it looks like a snake with an egg in its belly.

[condor bravo]

Roger on tbat guys. Yes it's the.back thrust that increases and not the chamber pressure.

[Unclenick]

This topic was studied in some detail by Varmint Al. While it is theoretically possible to double bolt thrust in the static case, it turns out that to do much more than 20%-25% increase (a highly smooth and mirror polished chamber plus grease on the case) or so takes lubricant coefficient of friction that is typically too small to actually be achieved in practice. What actually happens with some lubrication is the slicker the case gets, the further forward in the chamber the front edge of the pressure ring moves, while the back end stays in place. By including a longer portion of the case in the stretch zone, the amount of thinning of the pressure ring is spread out and is dramatically reduced.

Board member Slamfire has reported getting up to or slightly over 20 reloads out of a .30-06 fired in the Garand by oiling them and without forming a pressure ring or getting incipient head separation. The downside is that with less axial stress on the brass, the head is less resistant to flowing radially outward. As a result, if you experiment with this with maximum loads in a bolt gun, you may find you have no pressure ring problem, but that your primer pockets get loose.

[Hummer70]

There is however a way to get the case heads to flow radially outward.

A number of years back I had to shoot a 200 yard rapid fire string on a alibi relay at Perry and it was in a driving rain and everything was soaking wet.

I shot and picked up my brass and looked at the primers and they were flattened as if they were high pressure. I saw Larry Moore on the Assembly Line and showed him the cases and he explained they found the same thing in rain tests at Aberdeen many years before and the water on the ammo/bolt face would not allow the head to "grip" the bolt face and thus the head just opened up which opened up the primer pockets thusly ruining my cases.

I do not run max pressure loads as they are generally less accurate so I knew I did not have a pressure problem.

I tried to reload them when I got home and he was right, the primer pockets had become a lot bigger and primers just barely wanted to stay in thus I chunked the brass. From then on I kept a box of the same load with brass I didnt mind losing in my stool for shooting in the rain.

Logically with commercial brass having softer heads the problem would likely surface much quicker but I run strictly LC or FA Match brass in 30.06 as it gives much longer life (until it gets wet haha)

[Jimro]

Fullbore competitors using the 7.62 No4 Enfield conversions noticed the same issue with shooting in the rain, but due to action design the additional bolt thrust can cause the Enfield action to stretch.

I wouldn't expect that to be a significant problem with dual front lugged actions, Garand or Mauser type.

Jimro

[Mike Irwin]

The big problem with lubricated ammunition is if grease is used. It can migrate to the shoulder and neck area (or if it's on the shoulder/neck to begin with).

This can create a situation where the neck can't expand to release the bullet, and that can result in chamber pressures that spike at MUCH higher than normal levels.

[rkittine]

I do not know much about the results of using cases that still have lube on them, but see everyones point. I have always been afraid of hydrolic lock after seeing barrels that were blown due to mud etc. in the bore. Sure this is nothing like this, but I wipe down all my cases after loading and through I swap the bore with gun oil after cleaning, I run a series of dry patches through my bores before a new round of shooting.

Bob

[condor bravo]

Very educational replies. Not that I intended to start lubing loaded rounds; something had jogged my memory about the Rifleman article. Not only back thrust but chamber pressure and loose primer pockets enter in the equation. (And I'll keep in mind not to fire any alibi strings in the rain.

[T. O'Heir]

This topic was studied in some detail by the U.S. Ordnance Department in the early 1920's. Covered in Hatcher's Notebook. More about them not wanting oiled ammo than anything else, but neither oil nor water compress well. That's why both can be used to move stuff.

[Mike Irwin]

There have been any number of machine guns and a few rifles that have used, or even required, lubricated ammunition to function properly.

These are normally semi-auto or fully auto firearms that have been designed without a slow, primary extraction phase. Lubrication is required so that the case heads aren't simply yanked off during extraction.

Most of the Japanese machine guns employed in World War II required lubricated cartridges.

Some had oil pumps on them that sprayed an oil mist into the chamber prior to a round being chambered, others had pads or brushes that lubricated the cartridge as it was being chambered, and I believe that in the case of an Italian machine gun (one of the Bredas, I believe) the rounds had oiler pads that were part of the magazines that lubricated the cartridges as they were loaded.

As can be imagined, this was something of a nightmare because the oil could quickly become fouled with carbon, sand, grit, etc., and cause stoppages or undue wear to the gun.

One of the American semi-auto designs (Bang? Pedersen?) that was in early competition with the Garand also required lubricated ammo, which knocked it out of the running early.

[F. Guffey]

I want nothing between my case and the chamber but air,. even thought the cases is embeddable I do not want dirt, grit or grime. And, I have no problem with a shiny chamber and a smooth-clean case. I am the fan of 100% contact.

Measuring case head diameter before and again after?

F. Guffey

[buck460XVR]

I think this is not a matter of lubricated ammunition, but a matter of an over lubricated barrel. Folks used to suggest always squirtin' some oil in the barrel before puttin' the gun away for the season. This was supposed to prevent rust and pitting in the barrels when a gun was only used during a specific season....i.e., deer rifles. Was more of an issue back before most homes were air conditioned and the use of dehumidifiers became the norm. This is why so many of Granpa's old guns have a black stain on the stock where it meets the action. That dirty oil seeped outta the barrel, thru the action and deeply into the stock. It is a proven fact that too much oil in the bore can cause problems with over pressure and is related to many of the bulged barrels that exist today.

[mete]

Study of the HK delayed blowback guns can illustrate this well. When fired the bolt immediately starts to move back at the very time when neck grip is highest ! That invites case stretching !
For any combination of pressures, case hardness, bolt design etc. those conditions work for or against you.Make sure you see the difference between chamber pressure and bolt thrust --load carefully.