lubricating cases to fireform

[M-Tecs]

I can't prove this but I believe a case moves to the bolt before it seals to the chamber. The case is relatively light and as soon as the pressure rises enough to push it away from the bullet it scoots to the bolt. I don't know how much pressure it takes to release the case from the bullet but it can't be much, maybe a few hundred psi. As soon as the pressure is high enough to separate the case from the bullet the case will move the .001/.002 to the bolt. I believe the release pressure is much less than what it takes to seal/stick the case to the chamber. I have seen blackened cases when shooting relatively low pressure loads. Apparently these blackened cases are caused by blowback because the cases haven't sealed completely to the chamber. This makes me think it takes a substantial amount of pressure before the case sticks to the chamber giving the case plenty of time to move to the bolt and then push the front of the case forward. Also straight wall cases don't seem to stretch like bottle neck cases. I believe this is because there is little to push forward. If cases stuck to the chamber and then stretched to the rear, straight wall cases would stretch as badly as bottleneck cases. And no, I don't believe oil on a case has any affect on bolt pressure

We know for a fact that the case is still in the forward position when the neck and shoulder expands and adheres to the chamber walls. How do we know this? Simple --- chambers with excessive headspace have case head separation issues. Create a false shoulder or use the bullet to jam into the rifling and those issues go away in the same chamber.

Straight walled cases will sperate from excess headspace also. Straight wall cases have less of an issue mainly because most straight walled cases operate at lower pressures. A buddy had a Siamese Mauser in 45/70 that separated cases at higher pressures. No issues at Trapdoor pressures. I set the barrel back and that resolved the issue. The 458 Win. Mag and the 458 Lott are noted for having head separation issues.

I have a couple S&W 625 revolvers in 45 ACP revolvers that I shoot a lot without moon clips. One headspaces well without the clips but one definitely has headspace issues without a clip. At 45 ACP pressures it doesn't create case issues. Kick the pressure up to 65K (of course that can't be done in a 625) and I am willing to bet stretch and or separation will become an issue.

Most people believe that it is only the firing pin impact that pushed the case forward. That is only partly correct. The primer upon detonation pushes out and needs the case setback to reseat it. A primed case can be used as a crude headspace indicator. Not recommend for revolvers' since it may lockup the cylinder. Low pressure loads in bottle neck cases will set the should back due to primer pressure and due to the low pressure the shoulder does not get blown forward.

I have shot light 30/30 cast loads that didn't reset the primer. In that case the only bolt loading would have been from primer pressure.

[rbuck351]

I see your point. If jamming a bullet to rifling in a chamber with excess head space prevents case head separation, the case must be sticking forward and blowing back to the bolt. So would an extractor that held the case firmly against the bolt also prevent separation in a chamber with excess headspace? Curious.

[M-Tecs]

I see your point. If jamming a bullet to rifling in a chamber with excess head space prevents case head separation, the case must be sticking forward and blowing back to the bolt. So would an extractor that held the case firmly against the bolt also prevent separation in a chamber with excess headspace? Curious.

If the extractor truly held it against the bolt it should work. Different method of doing the same thing but the "the o-ring method" in the 303 Brit works. http://www.milsurps.com/showthread.php?t=24699&page=4

[M-Tecs]

Interesting read hear.

https://www.24hourcampfire.com/ubbth...Bolt-Thrust%22

A number of years back, Dan Lilja wrote an article for Precision Shooting on Bolt Lug Strength and included the simplified formula for "Bolt Thrust" which was the same as what's posted on this thread. (internal cross sectional area of case*pressure)
What I found interesting was that it assumed zero friction between the casewall and the chamber which he acknowleged as being significant, but for the purpose of firearms design is ignored. In fact the apparent convention in firearm design is to design in a safety factor of 2 with full backthrust (zero case/chamber adhesion)...I like that Dr Oehler <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />.

I'm surprised no one has invoked PO Ackley's writings in his book "Handbook For Shooters and Reloaders/Vol 1". In his chapter entitled "Pressure" (pp.134-149) he discusses case design's effect on backthrust and describes his various experiments that support his assertion that case design can reduce backthrust.

It interesting reading for me and I would guess for most on this thread as well.

Here's some thumbnail descriptions of his experiments;
Savage 99 in 250/3000 loaded to the point where action would lock up. Rechambered to "Ackley" config (only body taper changed). No extraction problems until primer pockets swell. (a different weak link becomes significant).

Winchester 94 chambered in 30-30 Ackley. Barrel unscrewed two turns. Dry round fired, brass maintained proper cartridge headspace in spite of increased chamber headspace. Primer backed out exactly the amount of the increased headspace. Oiled round fired; Case separated.

Same Winchester 94 with Ackley Chamber. Fired with Locking lug completely removed. Bolt held closed only held forward with finger lever. Fired cases are normal except for excessive primer protrusion.

Here are some quotes off the chapter;
"...This method gives a fair ideo of the actual chamber pressure exerted upon the walls fo the chamber in all directions, but bears little relationship to the amount of this pressure actually transmitted to the bolt or breech block in the form of thrust. The English method actually measures actual thrust against the bolt...which in turn is converted to breech pressure."

"...By studying these pictures it can easily be seen that the brass cartridge case WILL withstand at least SOME pressure, that the minimum body taper takes the "load" off the bolt or locking system, that oily chambers increase the bolt thrust...."

Finally, I found that Harold Vaughn had done experiments with strain gages to document bolt thrust in his search to eliminate receiver error( "Rifle Accuracy Facts", pp55-58). Similar to Ackley, he found that Oiled cases produced significantly greater backthrust than Dry cases (7500 lbs. versus 800 lbs in his .270 chambered Remington bolt), primers backed out proportionately with excess headspace ala Ackley.

Vaughn says Zero headspace produced maximum bolt thrust verus excessive headspace. In his text he lumps it with the oiled versus dry case numbers and I couldn't tell if he isolated the effect (though I suspect he would have to have made the statement).

[M-Tecs]

Some actual bolt thrust tests here.

https://gundigest.com/gear-ammo/ammu...th-bolt-thrust

[yeahbub]

There is some excellent analysis in this thread. The internal details are very interesting.

In my experience, lubricating a case allows it to slide to contact the bolt face while forming to the chamber rather than stretching and thinning at the expense of side wall thickness to get there. It increases case life, improves the number of cases which survive fire-forming, and reduces case stretch per firing to nearly zero. Fire-forming with a thin film of lube and neck sizing thereafter will maximize case life. In gas operated semi's, neck sizing works okay for a few firings, but eventually the shoulder will have to be set back a few thousandths for reliable lock-up on a round so sized. In bolt actions with their greater camming force, this can be delayed until there is noticeable resistance on closing the bolt on chambering. Note that this is shoulder movement due to action stretch and bolt compression on firing which is proportional to chamber pressure and at normal pressures is unavoidable. I routinely lubricate cases precisely to reduce case stretch and maximize case life. Some fellow had a Rossi '92 in .44-40 which stretched cases .008 per firing (sloppy headspace). The repeated case head separations were an expensive and serious show-stopper until he started lubricating the cases. Stretch per firing was reduced to .001 or less.

Thank you, 35remington! Your explanation is precisely correct. Bolt thrust cannot be "magnified" to the rear or any other direction, and will be the same to the rear as on all other surfaces, pressure dynamics being what they are. It always mystified me when someone with a stoutly built bolt action would express fears of catastrophic bolt failure should the lubricated case exert a few more pounds of pressure to the bolt than with a dry case. As you note, why would an action be designed to withstand 100-120kpsi, but the bolt left barely able to withstand normal chamber pressure? When an action lets go due to over-pressure, it's rarely the bolt which fails. Does a lubricated case increase bolt thrust? Yes, by the amount of force it takes to stretch a case to seat against the bolt face, which isn't much. I recall reading a report on two M96 Swedish Mausers tested to destruction by H.P. White Labs. One let go at 98,000 psi and the other at 106,000 psi, thus proving their strength. If the mean operating chamber pressure is 45kpsi, they'll survive a lubricated case just fine.

[M-Tecs]

Bolt thrust cannot be "magnified" to the rear or any other direction, and will be the same to the rear as on all other surfaces, pressure dynamics being what they are. It always mystified me when someone with a stoutly built bolt action would express fears of catastrophic bolt failure should the lubricated case exert a few more pounds of pressure to the bolt than with a dry case.

No one is claiming that lubed cases magnifies bolt thrust. The claim is that lubed cases transfers more of the force to the bolt face. That claim is correct. One poster did make the claim that lubed cases "increase bolt pressure to dangerous levels and are a big nono". This is less of an issue with modern firearm's since the bolt lugs and receiver lug face have a significant safety margins but it has real validity for older firearms like the Krag Jorgensen rifle, some of the older 98 Mausers, original Remington Rollling blocks and many others marginal actions still in use.

The concern is not catastrophic failure but increased stress leading to things like lug setback or frame stretching. I see lug setback mostly in 98's that have the receiver lug face trued and reduced the case hardening thickness. Going more modern a marginal design for some calibers is the original TC Contender. Great guns but they been chambered in cartridges that will stretch the frame. That was the reason for the increased amount of metal on the Gen 2's. Going even more modern the AR15 bolts that are opened for larger case heads are known for breaking one or two lugs. In these firearms increasing bolt thrust is not a good thing.

Looking a one of the more common firearms like the Remington 700 increasing the bolt thrust on a 223 from lubing is a non-issue. The bolt lug in 700's are the same size in a 223 as the high pressure mags. One bolt lug size for 3 different case head size in 700's In most firearms its an non-issue. In marginal designs bolt thrust is a real issue.

Claiming that case design, chamber finish or case lubing has no affect on bolt thrust is flat out incorrect. What is true that in MOST firearms it is not an issue but it MAY be an issue in marginal designs.

[13Echo]

When the firing pin falls the case can move forward. How much depends on headspace. Initial pressure from the burning powder will expand the thin case against the chamber wall. The thicker wall at the base of the case resists expansion a bit more. With the forward case gripping the chamber wall the rear is forced back against the bolt face. With excess headspace this will result in a stretch ring a bit in front of the web in the base. This will result in separation of the case head if resized and fired again. Excessive resizing between loads can have the same effect.

For this reason I like to make very sure there is some way of headspacing when fireforming. In my .257 AI I load a bullet long so it seats hard in the rifling. I also will very lightly oil the case to help make any stretching as uniform as possible.

[35remington]

If the action is spec’d for a given pressure level, not exceeding it will ensure lubricant or not is a not an issue. Truthfully it is simple as that.

Relying on a case to do this or that when the actual pressure is the determining factor is to ignore the real contribution to whether a gun is safely fired or not.

I will note, again, that even unattainable levels of case slippage in the chamber in the quoted Varmint Al article are discounted as being capable of causing any significant additional strain on the rifle.

Higher pressure than an action is designed for is bad no matter what the case does. Examples of guns wearing out or shooting loose because of too much cartridge in too weak a gun is a design and suitability problem and has little to do with lubricant.

Brass cases are not a gun strengthening device.

[Sig556r]

I've fireformed a lot of 7.62x39 brass to 6.5Grendel & .308Win to 6.5Creedmoor without additional lube other than the applied Lee water-based lube prior to sizing. I've used the lightest load to cycle ARs with no extraction issues.

[blackthorn]

While it may seem a bit of a stretch (no pun intended), my mind is drawn to an example: I do not believe one insult to an action is likely to result in catastrophic failure of the firearm, but consider that if drops of water fall on a rock face, often and long enough, the rock will be worn away. My opinion (and it is worth what you paid for it) is that many repeated insults to some actions may result in major damage (over time) to the locking mechanism, even to the point of failure.

[35remington]

Let us clarify concisely. If the action of a firearm is spec d for a given pressure level for the cartridge used, and the pressure level of that cartridge is within that limit, the bolt face will see only that pressure level even if the case and chamber is in a frictionless state.

Lubricant or zero friction will not increase the force on the bolt to levels the action cannot stand.

The force on the bolt in terms of pressure per unit area will merely be equivalent of that elsewhere in the chamber.

Actions are rated as to pressure they will accept based on the cartridge used. Nowhere is it stated or implied that an action takes more or less psi depending upon a lubed or bone dry chamber condition.

A weak action is not saved by using degreased chambers and cases when pressure is beyond its limits. Nor is it harmed by using an oiled case when pressure is within its limits. This is rather like suggesting a Stevens Favorite can safely and repeatedly fire a .32 H and R magnum when cases are dry but the 32 Rimfire will harm it when cases are oily.

[35remington]

Pressure within an action’s limits is not an “insult.” Pressure beyond an action’s limits certainly is. An oiled case does not stress an action beyond its limits as long as pressure is within its limits.

[EDG]

Much of the damage to .303 British brass occurs on the first shot when a factory round is fired even if the rifle has acceptable headspace. This is due to the chamber being about .010 larger in diameter than the case heads resulting in diametral stretching. Another cause is the shoulder of US brass is about .040 short of the shoulder in most Lee Enfields. Blowing the shoulder forward damages the brass some. If you want even more proof trim and fire .30-40 Krag brass in a .303 Lee Enfield. Due to the shorter Krag shoulder I have had the cases separate completely in the chamber on the first shot. Finally if you want to spend a few minutes doing the math you can calculate the spring of the Lee Enfield action under the force of the chamber pressure. I did that and I got .007" combined between the compression of the bolt and the stretching of the receiver. Even with beginning headspace of .000" you still have .007 movement of the bolt face. With the typical rifle having .006 clearance between a case head and the bolt face you are looking at .013" stretching of a cartridge case pressure ring on the first shot. Then add the .013 longitudinal stretching to the .010 dimetral (.005 radial) stretching. That is why you see some .303 cases that are almost separated with the first shot using factory ammo.

[EDG]

I think it is unreasonable to expect a cartridge case to act as a brake to protect the action. Any action that i marginal is defective by design, materials and heat treat.

Pressure within an action’s limits is not an “insult.” Pressure beyond an action’s limits certainly is. An oiled case does not stress an action beyond its limits as long as pressure is within its limits.

[MostlyLeverGuns]

I believe that the case neck and shoulder expand gripping the chamber, THEN the case head is pushed back to the boltface, stretching the case at its thin unsupported area in front of the case head. The dreaded incipient case head separation is caused by this. Excessive stretch may cause the case head failure immediately or it can happen after multiple firings with just a little stretch happening each time.
Light loads that DO not move the case back to the boltface avoid the stretching as do 'zero' headspace reloading techniques. Very smooth chambers reduce stretch as the case is not gripped tightly by the chamber. I do agree with Varmint Al's analysis. For moderate loading in brass new to a rifle I will lubricate the front 'half' of a case with case lubricant so the case fully expands forward AND to the rear without gripping the chamber. I use Redding Competition Shellholders to control headspace when full-length sizing, finding cases last much longer under 'full-pressure' loads in my Savage 99 300 Savage, 308, 243 and 358 rifles. Case failures in these rifles are usually the start of a 'ring' in front of the case head, a sign of case failure due to stretching. Brass is simply a gasket or bladder to seal the steel chamber.