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HogWild Offline OP
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Mr. Howell,
<br>
<br> Under "Custom Rifles & Wildcats" we briefly discussed the 260 Imp, 40deg shoulder. The point was brought up that Steve Timm claims "improving" a cartridge to a 40 deg. shoulder and/or minimum body taper reduces bolt thrust. This is one of the points Timm brings out in the July 2001 issue of The Varmint Hunter in an article on the 260 Imp. That article did not include an explanation for the reduction of bolt thrust.
<br>
<br> Would you care to explain Timm's reasoning? Do you agree the bolt thrust is reduced with the Imp case?
<br>
<br>
<br>Thanks for your time.
<br>
<br>HogWild


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Any claim that the 40� shoulder reduces bolt thrust uses imagined "facts" and "logic" that I believe to be unsound. Any claim that reducing the body taper reduces bolt thrust depends on reasoning that I believe to be erroneous.
<br>
<br>By the time the powder gas reaches its peak pressure (assuming at least 50,000 lb/sq in.), the case has long since gripped the chamber wall with its softer neck, shoulder, and forward body, and the increasing pressure beyond that much lower level has already stretched the case enough to press the head of the case HARD against the face of the breech. If I understand these relative pressures -- and the corresponding force against the breech -- correctly, the pressure (thus the force) exerted against the breech is the same with any body taper (given, remember, the same peak pressure).
<br>
<br>Since this theory was introduced many years ago, many have parroted it -- but I've never seen any data that overrode the facts and relationships that argue against it. I'll test it carefully once I determine how best to design the tests properly.


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Ken, the so called logic of using less taper and a 40 deg. sholder to reduce bolt thrust seems rather unsound. it seems the same logic could be put forth to support a case for more bolt thrust. This logic is far too simple to explain all the reactions that occour in the miliseconds when a round goes off. I believe it is difficult for most people to understand the speed at which things happen when a round is fired and the chain of events that occour. All of which happen in a particular order. Simplifing this to a time frame we can relate to may cause misconceptions of the actual facts.
<br>
<br>Bullwnkl.


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Ken, as an afterthought to the above ramblings, would not the theory used in the function of blow back actions disprove the bolt thrust theory to some degree?
<br>
<br>Bullwnkl.


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Mr. Howell,
<br>
<br> Thank you for taking the time to respond to my post. You share my opinion on the subject. Here are my thoughts:
<br>
<br> The pressure generated when a case is fired acts in every direction, ie, radially, on the base of the bullet, and against the base of the case and primer. If the pressure exceeds the yield strength of the brass (which is generally the case) the brass will "flow" or "stretch" in every direction till contact is made with the chamber wall (radial expansion) and the bolt face (rearward expansion). At this point the brass is not "containing" or "holding" the pressure as it does not have the strength to do so. Only the bolt face and chamber wall (not the brass) have sufficient strength to contain the force generated by the pressure.
<br>
<br> Therefore, the force, or thrust, applied to the bolt is strictly a function of the pressure and surface area of the bolt face (the area to which the pressure is applied). The shape of the case is not a factor in the equation. If, during expansion, the case happens to make contact with the chamber wall before making contact with the bolt face, the case simply "stretches" rearward till contact is made with the face of the bolt. The full pressure is then acting against the bolt face.
<br>
<br>HogWild


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First let me agree that the shoulder/ taper of the case has no effect on the bolt thrust. One detail needs to be discussed; In a dry chamber the friction between the pressurized and expanded cartridge head will hold the case forward (if that is where it winds up) until around 40,000 psi or so. The head is not stressed beyond it's elastic limit and contracts after the pressure drops. This observation is supported by several facts:
<br> In a chamber with headspace the primer firing (not the firing pin) will drive the case fully forward. When the cartridge fires the neck/shoulder will stay there. If the pressure is low the primer will push back against the bolt head and remain protruded.
<br> If you increase the pressure, or oil the case the case will lengthen to fill the chamber.
<br> Now comes the part that I think is right but haven't proven to myself yet. It would seem that if the case has enough pressure to break the static friction and move back it will exert the same force oiled or not. I have been playing with heavy loads in a 35 Remington that has a little headspace, and can span this pressure range. For an indicator I have been using a pressure sensitive film.
<br> Much has to be done to reduce the variables, but I do believe that much of what we read, including Ackley shooting the 94 Winchester without the locking lugs, is horse apples.
<br>Good Luck!

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After reading my post a clarification is in order. I do not doubt that Ackley fired the 94 winchester without a locking bolt. What I do question is his claim that it was made possible because of the improved chamber. Note he didn't fire an unaltered chamber to show the difference, and his pressure was in the range that a dry chamber would hold either case forward. To furthur prove that the forward part of the case will not hold thrust, only the case head need be lubed to allow it to come back. Or in another way of saying it; The case neck stays forward, oiled or not.

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This has been alluded to, but pressure is not one-dimensional. It has both force and time dimensions.
<br>
<br>Similarly, bolt thrust has a time and force dimension. A cartridge fired with a very fast burning powder at a peak pressure of 60,000 psi will result in an entirely different amount of stress than one fired with h1000, at 60,000 PSI. The area under the curve (above the 40,000 PSI yield strength of brass) is simply larger.
<br>
<br>Further, I would not be surprised if there is a "time delay" introduced by the cartridge case head. It takes time for the brass to yield, move, stop and push. How much? Only a well designed experiment, with strain gauges located in at least two places will show if it is measurable, and significant. FWIW, Dutch.


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Thank you Ken, my point exactly on bolt thrust. The body taper and shoulder angle has no baring on bolt thrust. Flinch


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Using "common sense" or "obvious logic," I picked the wrong answer on a physics quiz in 1956 (not my only wrong answer, but one mistake that I haven't forgotten). The question was about the internal pressure on the bottoms of two conical containers of water -- identical in all dimensions but with one resting on its broad base and the other resting on its narrow base -- each the upside-down twin of the other, both the same height, with the same quantity of water inside.
<br>
<br>Now that I know the physics involved, I have no trouble "seeing" that the internal pressure exerted on the narrow bottom of the one container is exactly the same as the internal pressure exerted on the wide bottom of the other container. But Boy! I couldn't "see" it when Dr Jeppesen said they were the same!
<br>
<br>Many proven facts and principles of physics aren't obvious or compatible with "common sense" -- at first look.


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I actually did really well in physics. I just picked the answers that seemed to be the least obvious (grin). Flinch


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Ken, that same question is still around, or at least it was in 1990...seems that the laws of physics do not change no matter how many opinions are tossed at them. [Linked Image]
<br>
<br>Bullwnkl.


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HogWild Offline OP
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Dutch,
<br>
<br> I think I agree with you on the time component. Note when fireforming Ackley Imp. brass that if there isn't sufficient pressure generated, the case will not be sharply defined by the chamber walls. I must assume the pressure was above the yield strength of brass (as most of the forming took place) but the pressure wasn't above the yield strength long enough for the brass to completely form. The higher the pressure, the shorter time required to comletely form the brass. The simple solution is increasing the powder charge which increases the pressure. The brass completely forms (with good definition) in one firing.
<br>
<br>HogWild


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Hogwild, I don't know. I've been re-reading Mr. Bromel's introduction to interior ballistics, and I can't seem to "make all the pieces fit".
<br>
<br>I happen to have a falling block that is quite sensitive to bolt thrust. Using Quickload to estimate pressures, I run into a noticeably harder dropping block at about 45,000 psi with Varget or slower powders, but I can get to 55,000 PSI with 3031 or H322, as long as I use a light bullet.
<br>
<br>So, in light of that, I conclude that bolt thrust has a time component, somewhere. It may just be that the steel in the action needs time to move. Whatever the second component is, peak pressure is only part of the answer. I would suspect that Ackley's model '94 experiment would come out differently with slow burning powders, but I'm not the experimenter that's going to find out in this case..... FWIW, Dutch.


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