Unraveling the mystic of firearms design

[Hannibal]

So is it accurate to say that due to the expansion of the case against the chamber wall the cartridge is essentially held in place during firing?

I realize this question makes a few assumptions that can not be ignored. One being that the chamber walls are sufficiently scuffed to hold the case and not allow it to slip. Another being that the chamber wall and case sides are free of lubes and oils. And a third being that the powder and charge used are sufficient to rapidly expand the case to the chamber wall to hold it in place.

If the above is correct then the bolt face and action are constructed in such a manner that if one of the conditions is absent or incorrect the pressure will be contained by the bolt face and action rather than injuring the shooter?

I've no intention of experimenting with limits nor do I suggest or encourage anyone else to. This is simply a theoretical discussion to aid my personal knowledge.

[JSnover]

What cartridge and barrel contour? Were they only 0.0625" thick in the chamber section? Did they use normal barrel steel or something stronger?

A 30-06 barrel would not burst even if only 0.0625" thick at the muzzle but closer to the chamber it would. I am sure Hatcher knew and understood that before he even started the testing.

Tim

Those were 1903 Springfields. Forgive my failing memory and my inability to upload the photo right now, here's a quote from page 202 of "Hatcher's Notebook." As it turned out the barrel did fail but not with standard pressure cartridges.

"I turned the barrel down so that it was only 1/16 inch thick over the chamber. It held three regular service cartridges perfectly. I then put a 75,000 pound shot through which blew a piece out of the side, as can be seen in the photograph."

[dtknowles]

Those were 1903 Springfields. Forgive my failing memory and my inability to upload the photo right now, here's a quote from page 202 of "Hatcher's Notebook." As it turned out the barrel did fail but not with standard pressure cartridges.

"I turned the barrel down so that it was only 1/16 inch thick over the chamber. It held three regular service cartridges perfectly. I then put a 75,000 pound shot through which blew a piece out of the side, as can be seen in the photograph."

I did some digging into this. I was wrong to think that the chamber would not hold with only a sixteenth inch of steel. The 1903 barrels were 4150 steel and if it was tempered properly, it would have a minimum yts of 180ksi and a uts of 190ksi. Using thin-walled cylinder stress calculations (ignoring the strength contribution of the brass) and using spec max 58ksi chamber pressure, the thinned barrel chamber was not certain to hold together as the stress would be 208ksi. With the help of the brass, ammo not at max spec pressure and the steel being better than minimum strength, it is not unreasonable that it did not fail with standard ammo. It was probably on the hairy edge. Hatcher probably could do the same calculations I did and might even have had actual tensile strength data for barrels like the one being tested. The increased pressure of the 75ksi load breaking it was almost a certainty and probably would have failed with less of an increase in pressure but maybe only producing a crack not sending a piece flying.

Factors of safety of 2 or 4 (2 or 4 times as strong as needed) against burst are common design criteria depending on the application. The analysis is done with worst case pressures and temperatures and minimum material properties with additional factors for stress concentrations like dovetails, tapped holes, etc.

Tim

[Hannibal]

So is it accurate to say that due to the expansion of the case against the chamber wall the cartridge is essentially held in place during firing?

I realize this question makes a few assumptions that can not be ignored. One being that the chamber walls are sufficiently scuffed to hold the case and not allow it to slip. Another being that the chamber wall and case sides are free of lubes and oils. And a third being that the powder and charge used are sufficient to rapidly expand the case to the chamber wall to hold it in place.

If the above is correct then the bolt face and action are constructed in such a manner that if one of the conditions is absent or incorrect the pressure will be contained by the bolt face and action rather than injuring the shooter?

I've no intention of experimenting with limits nor do I suggest or encourage anyone else to. This is simply a theoretical discussion to aid my personal knowledge.

Anybody care to comment on this?

[dtknowles]

So is it accurate to say that due to the expansion of the case against the chamber wall the cartridge is essentially held in place during firing?

I realize this question makes a few assumptions that can not be ignored. One being that the chamber walls are sufficiently scuffed to hold the case and not allow it to slip. Another being that the chamber wall and case sides are free of lubes and oils. And a third being that the powder and charge used are sufficient to rapidly expand the case to the chamber wall to hold it in place.

If the above is correct then the bolt face and action are constructed in such a manner that if one of the conditions is absent or incorrect the pressure will be contained by the bolt face and action rather than injuring the shooter?

I've no intention of experimenting with limits nor do I suggest or encourage anyone else to. This is simply a theoretical discussion to aid my personal knowledge.

If the cartridge case is held in place (pushed forward by the firing pin strike) when fired you will likely see the primer backed out of the pocket a little bit. This usually only happens if the pressure is low.

More normal is for at least the head of the cartridge case to push back against the breech/bolt face. This can happen two ways. Either the whole case moves back or the forward part of the cartridge case grips the sides of the chamber and the case stretches. Sometimes it is a combination of the two with the case moving back some and stretching some.
How much does the case stretch?
This is where headspace come in. It is best if the cartridge case does not stretch past its elastic limit, does not yield. Tight headspace and a stiff action minimize the stretch. Loose headspace and/or a springy action will have more stretch. More stretch leads to case head separation when the stretch is more than the elastic limit. When really bad the case can separate on its first firing but the damage can be cumulative and only fail after multiple firings.

I doubt there is a magic load where the cartridge case would remain in the chamber without a breech or bolt backing it up and certainly the primer would blow out. Sometimes the force needed to hold the case in the chamber is tiny like loads in blowback actions that fail to cycle the action.
Tim

[405grain]

As an engineer I will comment on this. There is no mystery to firearms design. There might have been prior to the advent of the industrial age, but not any more. There are several principles that are used to determine the design of a firearm. These are material strengths and properties, mechanical design, and the mathematics to predict mechanical behavior. All of these things are well established and can be easily looked up. As a standard practice, firearms (as well as many other critical devises) should be designed to withstand at least 150% of their functional loading. The yield strength of the weakest component of the assembly should be considered the point of failure. The cartridge case does expand to form a gasket against the chamber walls, but it should never be included in the strength prediction of any given firearm as if there were a failure of the breeching mechanism the case head would instantly give way.

The formulas for determining hoop strength are applicable for determining the margin of safety for chamber wall and barrel thickness. The formulas for determining load over surface area are used to determine if the action will approach yield strength using a given material. Practicality of design and simplicity of operation will be at the ingenuity of the designer. If you are personally interested in machining and building your action, two sources are probably the best for a beginner as they are good learning resources. The first is any of the designs from Frank DeHaas's book Mr Singleshots Book of Rifle Designs. pdf's of this book are available online. The Chicopee centerfire action is by far the most built action from this book. The second design would be the Darcy single shot. PDF's for this action can be found online. A more updated version of the Darcy falling block was posted in a thread in the "special projects" area of this website several years ago. Both the Darcy and the DeHaas falling blocks use a round breech block, which simplifies the machining process. Either of these actions can be built by a reasonably mechanical person with some effort and some power tools.

[elmacgyver0]

I am not intelligent enough to add any relevant information to this thread, but I will say, a friend of mine when we were kids used to fire .22 LR cartridges out of a .22 magnum pump rifle.
They seemed to work good enough to hit ground squirrels, our form of prairie dog shooting when we were kids back on the farm.
The ejected cases were interesting to look at, they were swelled towards the front of the case.
I suspect he would have preferred to use magnums, but economics dictated .22 long rifles.
Why the pump .22 magnum? Well one of his sisters won it in a raffle.

[Hannibal]

As an engineer I will comment on this. There is no mystery to firearms design. There might have been prior to the advent of the industrial age, but not any more. There are several principles that are used to determine the design of a firearm. These are material strengths and properties, mechanical design, and the mathematics to predict mechanical behavior. All of these things are well established and can be easily looked up. As a standard practice, firearms (as well as many other critical devises) should be designed to withstand at least 150% of their functional loading. The yield strength of the weakest component of the assembly should be considered the point of failure. The cartridge case does expand to form a gasket against the chamber walls, but it should never be included in the strength prediction of any given firearm as if there were a failure of the breeching mechanism the case head would instantly give way.

The formulas for determining hoop strength are applicable for determining the margin of safety for chamber wall and barrel thickness. The formulas for determining load over surface area are used to determine if the action will approach yield strength using a given material. Practicality of design and simplicity of operation will be at the ingenuity of the designer. If you are personally interested in machining and building your action, two sources are probably the best for a beginner as they are good learning resources. The first is any of the designs from Frank DeHaas's book Mr Singleshots Book of Rifle Designs. pdf's of this book are available online. The Chicopee centerfire action is by far the most built action from this book. The second design would be the Darcy single shot. PDF's for this action can be found online. A more updated version of the Darcy falling block was posted in a thread in the "special projects" area of this website several years ago. Both the Darcy and the DeHaas falling blocks use a round breech block, which simplifies the machining process. Either of these actions can be built by a reasonably mechanical person with some effort and some power tools.

I have both of those books and have built a Chicopee and plan to start a Darcy action soon. My questions are for my own knowledge. I like to understand things completely so it appears that I've a good deal of equations to look into.

These things have indeed been known for a long time. I would say that it's not a straight-forward proposition to research these things for one's self however.

Most people are content to know that a machine will operate as they want it to provided they've been taught how to operate it. How and why it works are usually of little if any concern.

Anyway, that's why I'm asking questions. Hard to understand the details if you aren't sure what principles apply.

[Hannibal]

If the cartridge case is held in place (pushed forward by the firing pin strike) when fired you will likely see the primer backed out of the pocket a little bit. This usually only happens if the pressure is low.

More normal is for at least the head of the cartridge case to push back against the breech/bolt face. This can happen two ways. Either the whole case moves back or the forward part of the cartridge case grips the sides of the chamber and the case stretches. Sometimes it is a combination of the two with the case moving back some and stretching some.
How much does the case stretch?
This is where headspace come in. It is best if the cartridge case does not stretch past its elastic limit, does not yield. Tight headspace and a stiff action minimize the stretch. Loose headspace and/or a springy action will have more stretch. More stretch leads to case head separation when the stretch is more than the elastic limit. When really bad the case can separate on its first firing but the damage can be cumulative and only fail after multiple firings.

I doubt there is a magic load where the cartridge case would remain in the chamber without a breech or bolt backing it up and certainly the primer would blow out. Sometimes the force needed to hold the case in the chamber is tiny like loads in blowback actions that fail to cycle the action.
Tim

Agreed. I'm not suggesting that a cartridge can contain all the pressure from firing with no support on the base. What I am thinking of is just how much support is required under ideal circumstances vs the amount of support required when a mistake is made.

[john.k]

Cast boolit shooters probably do push the limits without knowing .......most guns shooting cast will get the chamber coated with bullet lube ........then I shoot some full pressure loads for a 1000yd shoot ........these rounds will have near maximum backthrust for the case size.

[fixit]

A comment was made about how much pressure would be needed to blow an unobstructed barrel....there is an interesting series on Kentucky ballistics called, I believe, "When guns go boom". Kinda fun seeing what he goes through to get some of the guns to let go!

[Recycled bullet]

When center-fire cartridges are loaded and fired in revolvers with out gun powder the primers stand proud and tie up the cylinder and prevent rotation. This rattle room or difference in dimensional min-max is the headspace.
When regular cartridges are fired the bolt thrust pushes the cartridge to the rear and seats the primers flush against the recoil shield and firing pin bushing.

Have you ever experienced the joy of ear plug or hot glue bullets?

[fixit]

This was black powder, so the rules are a little different, but Sam Fadala ran a series of experiments to test several theories concerning ringing and blowing up muzzleloaders, and for expediency sake, he used copper pipe embedded into coffee cans of melted lead to mimic a muzzle loading barrel. One of his comments in the process of these experiments was a sense of amazement at how much pressure the pipe would take before letting go. I don't have the article nearby but, the pipe, ignited by fuse through a touch hole, would withstand several shots that would be considered overloaded in a standard production rifle. All of this highlights the amount of over engineering that goes into design to make for a safe product!

[dtknowles]

..... All of this highlights the amount of over engineering that goes into design to make for a safe product!

What you are calling "over engineering" is what engineers call safety margin.

I am not involved in the firearms design industry so I don't know what their rules are but I don't think every gun is proof tested anymore.

In my industry untested factors of safety are necessarily high, at least a factor of two if we don't proof test an item and that is when we used tested material and processes and just don't proof test the finished product.

Tim

[GONRA]

GONRA sez - lottsa good advice am sure!
Just be CAREFUL when "Famous Gun Writers" comment on semiauto actions ..... OUCH!