Looking for sectioned view of 357 Rem Mag case

[BattleRifleG3]

I'm doing some pressure/friction/backthrust calculations, and there's one critical piece of info I'm looking for - A sectioned drawing of a 357 Rem Mag case.

I know the basic outline, but I was hoping that somewhere out there was something with measurements.  The key element is the wall thickness throughout the length of the case.  I know not all brass is the same, what I'm looking for is the most typical example, or more appropriately the average.

In order to find these measurements myself, I'd have to section a case, measure at various points, and spline the line between them.  I've seen 3-d models of a 357 case online, including internal details, but no measurements, and no models to download.

It would be great if this was out there somewhere.  If not, I guess I have an appointment with a case and a micrometer.

[stimpylu32]

This should help .

http://stevespages.com/table3.html

Go down to the cases section and click on case dimrnations , top right cornor .

stimpy

[skb2706]

If you just have to know, section one with a die grinder and mike it with a ball mike. The whole process will take five minutes.

[BattleRifleG3]

That link has an amazing amount of info, thanks!

What I'm looking for in this case is the thickness of the brass from front to rear.  I may have to section the case myself, but if it's been done already I'd be happy to use it.

[skb2706]

Way back when I sought out that kind of info. I found that any two brands of brass vary considerably.

[Lone Star]

If all you want is to calculate backthrust, you do not need a sectioned case.  All you need is the maximum cross-sectional area of the case mouth.  In revolvers, maximum pressure usually occurs when the bullet has left the case and is in the forcing cone.  So the area on which the pressure works is the maximum exposed area, which is the case mouth.  Straight cases like the .357 are thinnest at the mouth and taper in building thickness towards the base.  But you should not need to know any dimensions other than the expanded case mouth area - unless you are calculating case expansion based on brass hardness/thickness.  Then you need to know the hardness - you need a real case and a Rockwell tester for that.


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[BattleRifleG3]

That formula for backthrust would apply in the case of case head separation.  That would be necessary for determining the load on the action in a worst case scenario.

Typical backthrust is lower, due to frictional resistance.  I have seen analyses of rifle rounds showing that the brass friction cuts down about half the backthrust force.

[Lone Star]

The formula apparently does not take into account the reality of shooting a firearm.  This is easily shown when firing a bottlenecked cartridge in a chamber giving excess headspace.  A perfect example is the factory .35 Remington, with short cartridges and long chambers the norm (these in headspace measurements).  Fire the cartridge and the case walls grip the chamber and do not allow the bulk of the case to move rearward at all.  But starting at the point where the case walls are too thick to expand, the case is forced back against the breechface.  This is how the typical head separation occurs.  The friction of the case walls - which expanded against the chamber - stopped all the force.   It was the case failure in plastic mode that allowed the thrust to reach the breechface.  Once the thrust exceeds the strength of the brass case, the brass fails and the thrust goes the only place it can - hard against the breech.  The same thing is seen with straightwalled cases, but less often.

It is not easy to calculate "stiction" which is what you really want, not friction.  Case/chamber stiction is a factor of pressure, case thickness and hardness, chamber finish and any lubricant (dry or wet) present.  Any cartridge that exhibits case head separation shows the fallacy of thinking that the case stops pressure from being applied to the breech.   (Cases whicdh exhibit primer protrusion indicate very little pressure/thrust reached the breech.)    This is where Ackley went wrong with his Improved cases.  Most normal cartridges exhibit plenty of stiction to the chamber, his cartridges attained more velocity (slight in most cases) by having more capacity and being loaded to higher pressures.  The brass case lets thrust get to the breech not because of its shape, but because of its plastic strength.


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[PaulS]

The only way you can get the information that you are looking for is to mount a strain gage on your breach face. The friction on the wall of the chamber changes with conditions from shot to shot. The cartridge thrust area is calculated with the bore of the camber (the largest diameter where the cartridge comes in contact with it.) The inside diameter or wall thickness of the case has no bearing on the thrust. I would think that the calculation is too complex to show here or to do in any practical sense on a personal computer. I may be wrong, but my mind starts whirling with all the variables that change each time you shoot your gun. Things like modulus of elasticity, coefficient of friction, pressure rise rates, maximum and average pressures under constant change and finding some way to even approximate them under real world conditions of use just boggle my mind. It would be easier to just install a breech face strain gage and take readings as they occur. At least that way your measurements would always be within 2% of actual experience.

[BattleRifleG3]

Pressure as a function of time would be another value needed for sure.

Reason I need something more precise than a worst-case-scenario is that I'm looking to design a straight blowback semi-auto rifle.  In order to know the force pushing the bolt back, I would need to know how much frictional (or stictional) force is applied in the other direction.

With that force (as a function of time) applied against the mass of the bolt and the recoil springs, I would then calculate how far out of battery the round is at a given time.  If the round is so far out that the case is unsupported enough to stretch or blow, the bolt would need to be heavier and/or the springs stiffer.

[PaulS]

The 9mm is the highest pressure round that I have ever heard of in a straight blow-back action. Most often the delayed blow-back system is used even in rounds half that of the 357 magnum.  I don't think what you are trying to do is possible, but I would love to be proved wrong. I can tell you that you are over thinking the math... For direct blow-back action you need to know only the mass and velocity of the bullet. From that you make the mass of the bolt high enough to lower the velocity to usable limits.
say you have a 158 grain bullet at a muzzle velocity of 1400 fps. Your bullet weighs 1/3 of an ounce so if you use a bolt that is 2 ounces you have six times its mass - it will travel at 233.3 fps. Now all you have to do is find a spring that will stop the bolt as it reaches the end of its travel. (or slightly before). The friction will serve to delay the action until the pressure drops a bit.
I know it can be done, I did it for a 9mm about twenty years ago. I had to play a little with the springs (I used two) but it worked.
whether you can get it to work with a 357 or not is the question.
Paul

[BattleRifleG3]

Have to say that case friction would be far more significant in a dynamic system (ie a straight blowback) than a locked bolt.  You can make a straight blowback system for any cartridge, even a 308.  But the bolt would be far too heavy and the spring too stiff to cycle by hand.  The question is how heavy would the bolt have to be for a 357 Rem Mag.

I once tried to calculate these based on conservation of momentum, but the resulting numbers were unfeasible for something that I knew already existed, ie a 45 ACP straight blowback.  The MAC-10, M3 Grease Gun, and even Hi-Point 45 ACP pistols use straight blowback.  This would be a rifle with an enclosed receiver and a heavier bolt than could ever be feasibly used on a pistol.  I'm thinking 1.5-2x the weight of a 45 ACP bolt.

[PaulS]

I wish you good luck then because the internal dimensions of brass varies with manufacturer and lot. Some use more metal at the base of the round and others less. Some use more taper on the walls and others use less. There is no "standard internal dimension" for any cartridge. The more room your have in there the better you are for powder room but the less case strength you have. The 357 has considerably more range of variance than a 45 ACP. You say that you can make a straight blow-back system for any cartridge? The information that I have read says it is limited by the amount of pressure a round produces. When pressure drops enough that it begins to move the case back with the bolt it must be low enough that it doesn't blow the case open. Having a bolt or breach block that heavy for a 308 would keep the arm from cycling. I think you will run into the same issue with the 357 magnum.

[S.S.]

Careful on this site http://stevespages.com/table3.html,
My Anti-Virus will not let me open it at all.. Must be spreading
something nasty around..

[BattleRifleG3]

I wish you good luck then because the internal dimensions of brass varies with manufacturer and lot. Some use more metal at the base of the round and others less. Some use more taper on the walls and others use less. There is no "standard internal dimension" for any cartridge. The more room your have in there the better you are for powder room but the less case strength you have. The 357 has considerably more range of variance than a 45 ACP. You say that you can make a straight blow-back system for any cartridge? The information that I have read says it is limited by the amount of pressure a round produces. When pressure drops enough that it begins to move the case back with the bolt it must be low enough that it doesn't blow the case open. Having a bolt or breach block that heavy for a 308 would keep the arm from cycling. I think you will run into the same issue with the 357 magnum.

I understand completely, and you may be right that the bolt would be too heavy for a 357.  But I wouldn't assume it would be too heavy just yet.  I've read that a straight blowback 308 bolt would need to be somewhere on the scale of 20lb.  If a 9mm bolt is something like 1 pound, it obviously isn't directly proportional to energy, momentum, or any other commonly identified factor.  I'm going on the expectation that a 357 is far closer to a 9mm than a 308.  But comparative calculations will tell more.

[PaulS]

I believe it has more to do with pressure than any other single factor. The mass of the bolt has to be overcome for the action to begin cycling. If this happens before the pressure drops below the yield strength of the brass it will simply blow the head off the case and leave the rest of the case in the chamber. In a 9mm the pressure starts out lower than the 357 and drops off very fast. The higher pressure of the 357 would deform the brass as it began to move the bolt back. If the bolt was heavy enough to delay the movement longer then the pressure would be gone before the action could function. Maybe you can find a balance in there but others who have tried have found it easier to design either a delayed blow-back system or a gas operated system. As I have said before, good luck.

[BattleRifleG3]

The mass of the bolt is always overcome, it just accelerates and moves at a different rate.  Your description of finding the just the right balance (weight) is exactly what it would take, as well as the burn rates being very significant.  The question is whether or not the required bolt will be too heavy.  What makes this different from other designs is being a rifle, not a handgun.  The only 357 Rem mag semi-autos I know of are the Coonan semi-auto 1911 based pistol, the Desert Eagle, and modifications of the M1 Carbine.  All of these were based on existing designs.  357 semi-autos haven't really been explored enough to close the book, in my opinion.  Research and analysis will tell, and if they give a green light, experimentation will give a final verdict.