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Known Loads too hot now..why
- Thread starter clhman
- Start date
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Yes, if you're saying the cases won't let go in wet weather, I agree. But case stretch isn't the evidence being identified and described by BuffaloBob, clhman, and 300remum. They're describing additional case head thrust against the bolt face to the extent they experience case head flow into the bolt face, and the telltale associated difficult bolt lift.
So... - retail purchasers of factory ammo would still complain of overpressured loads if they experienced difficult bolt lift due to brass case head flow into the bolt face, along with plunger hole and extractor slot marks on the case heads with their factory ammo. Retail users would interpret this as overpressured factory loads, even though they aren't. Just normal pressured factory loads fired with water-wetted exteriors.
The factory probably loads them to pressures just enough less than us wildcatters are doing such that - IF the case heads hit the bolt face with a slightly heavier thrust with wetted cartridges - that the case heads are still left largely free of case head flow into the bolt face indentations, which would also prevent any difficult bolt lifts. That's my best guess. Post back if I misunderstood you...
I think you make a good point. New brass is also has more spring back than work
hardened cases. I do think case stretch is what is being described though. The hard bolt thrust is the first part, the case stretch makes the bolt hard to
lift with the accompanying bolt swipe. Good thread, it's
got everyone thinking.
hardened cases. I do think case stretch is what is being described though. The hard bolt thrust is the first part, the case stretch makes the bolt hard to
lift with the accompanying bolt swipe. Good thread, it's
got everyone thinking.
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I realize I'm a new member and don't want to tread on anybodies toes, but if you go back and read Phorwath post where he talks about the case head separation he mentions .008 headspace. What I'm thinking (wondering) is how this relates and if it could be the cause of the ejector mark. Would there be a difference in what a case head looked like if you fired a lubricated round with a slight crush fit vs a lubricated case with .008 or so headspace. I'm assuming a max load or close to it in both cases.
Here's an interesting read about chamber finish that has some relation to this topic.
Rifle Chamber Finish & Friction Effects on Bolt Load and Case Head Thinning Calculations done with LS-DYNA
Rifle Chamber Finish & Friction Effects on Bolt Load and Case Head Thinning Calculations done with LS-DYNA
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bigngreen
Well-Known Member
Holy cow man, when I saw the separated case I thought it had gone all bad for you! In the BPCR shooting you will see the cases separate if you don't catch on to the early signs of moisture and you get to pound out a case. In BPCR shooting your actively trying to keep the bore and fowling moist by breathing down or patching the barrel but if you get sloppy you can get moisture back into the chamber and cause problems like pressure signs or separation.
I imagine you wouldn't even notice without the load being hot, it would be interesting to know what the pressure difference is, that would be the most conclusive test.
It will be interesting to see what clhman has happen!
I imagine you wouldn't even notice without the load being hot, it would be interesting to know what the pressure difference is, that would be the most conclusive test.
It will be interesting to see what clhman has happen!
The good thing about my experience is that i was thinking i was
over pressured so i decided to drop to 92 gr retumbo instead of 94 gr.
I've been trying to get this load to do better than 1"-1.5"
first try at 92 gr resulted in a .712" 5 shot group LOL !!!
Will try again soon to make sure it's not a fluke
over pressured so i decided to drop to 92 gr retumbo instead of 94 gr.
I've been trying to get this load to do better than 1"-1.5"
first try at 92 gr resulted in a .712" 5 shot group LOL !!!
Will try again soon to make sure it's not a fluke
If the case exterior is slickened such that the coefficient of friction between the brass and the chamber wall is reduced, I would expect less case stretch with equal pressured loads compared to bone dry case walls and dry chamber (with the greater coefficient of friction) because the sidewalls of the brass case will bind less to the chamber walls and the entire cartridge case (sidewalls included) will slide/shift back with less case sidewall elongation/stretch (tensile force), as the case head is driven firmly against the bolt face. I think we have pretty much all agreed that the internal case pressures will remain the same, whether the exterior of the brass casing is wet or dry.
Now - a water-wetted cartridge/chamber surface (with the reduced coefficient of friction) enables the cartridge case sidewalls to slide and shift back with less friction and resistance during the moment of case head setback against the bolt face (at the moment of maximum cartridge pressures). This will result in the case head setting into the bolt face with some minimal additional thrust, compared to dry casing sidewalls and dry chamber walls. If the dry sidewall of the case is held tightly to the dry chamber wall, then the lbs of tension force acting to stretch and elongate the case sidewalls at the location where case head separations occur (which is where the sidewall of the cartridge case becomes thickened, rigid and unable to expand enough to tightly grip the chamber wall) is a tensile force acting to restrain the case head thrust. The lbs of case head force is reduced by the lbs of tensile force generated at the thickened web location where case head separations occur. I'm repeating myself, but this is the location along the sidewall of the case where the sidewalls are too thick and stiff to be blown out into firm contact with the chamber. This is the location where the tensile force occurs if the sidewalls are able to grip the chamber tightly enough from this location forward to the shoulder of the casing. Under repeated firings and stretchings, this is the webbed location where the case head will eventually separate from the forward portion of the casing. The location is as shown in my four photos where that case head almost completely separated last weekend.
So for the engineers on the Forum: With a slickened surface between the exterior of the casing sidewalls and chamber surface, the coefficient of friction between these two surfaces is reduced. Reducing this friction results in the entire casing shifting back firmly against the bolt face and if the sidewalls of the casing are unable to bond tightly to the chamber wall, then sidewall elongation/stretch, and the associated tensile force that would act in the direction opposite of the thrust force driving the case head into the bolt face, would be reduced. The simplified mathematical expression (assuming no case sidewall stretching tensile force) is:
[lbs case head thrust] = [internal case pressure X internal case head cross sectional area].
IF the case sidewalls tightly grip the chamber walls from the location of case head separations forward, sufficient to prevent the shifting or slipping of the case sidewalls along the chamber surface, then the case side wall will enter a condition of greater tension or tensile force acting in the opposite direction of case head thrust toward the bolt face. A sidewall tensile force can be created in the relatively rigid area of the case sidewall (where case head separations occur) because the side walls of the casing forward of this location are driven tightly enough against the chamber wall to prevent case sidewall slippage. The mathematical expression of case head thrust in lbs of force is now expressed as:
[lbs case head thrust] = [(internal case pressure X internal case head cross-sectional area) - (tensile force associated with case sidewall stretch)].
This tension force in the case sidewall is increased with a dry chamber and case exterior because the cartridge sidewalls forward of the location of case head separations is so tightly bound to the chamber that a larger counteracting tensile force is able to develop as the case head is driven backwards into the bolt face.
In all honesty and humility, I didn't expect this tensile force to decrease substantially enough from the dry cartridge/chamber condition to the wet cartridge/chamber condition to increase case head thrust into the bolt face with the additional force required to initiate brass case head flow into any voids in the bolt face. My experiment today opens my mind to the possibility that - IF case pressures are already being redlined to the hilt - then the act of firing a wet cartridge (or a dry cartridge in a wet chamber) could allow sufficient case slippage in the chamber to reduce tensile forces in the cartridge sidewall. And these tensile forces act on the case head in the opposite direction of the pressure-induced case head setback and thrust. So this reduced conteracting tensile force COULD result in enough additional increased case head thrust (lbs of force) to cause brass flow into the bolt face, and the associated stiff bolt lift.
That's my engineer's explanation for the forces at play at the time of ignition, and how case head thrust against the bolt face could be increased (restrained to a lesser degree) by a wet cartridge case exterior or chamber, compared to a dry cartridge and chamber.
In all my years I have never experienced enough increased case head thrust (as would have been noted from brass flow and stiff bolt lift) from any inclement weather or oiled chambered to take notice. If I did experience increased case head thrust with my single soaped cartridge firing today, I probably only noticed because that was the whole point of firing that load over the chronographs, and I paid careful attention to bolt lift and case head markings.
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Great post, thanks.
Tee Hee Hee...
Yeah I probably had you set up for the sting more-so than anyone ...
Have to admit I amused myself.
dodgefan,
I haven't visited your link or the previous one at Sniper's Hide yet. It sounds like it should be directly applicable to this subject matter. I'll check it out in a bit and likely comment further after reading. It will be an interesting read now that I've committed myself to my own interpretation and explanation of the phenomenon. I'm going LRH Forum keyboard crazy at the moment.
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dodgefan,
Thanks for locating and providing us with that link. The article is directly applicable to the subject matter at hand. I'll pat myself on the back as my engineering intuition led to the same theoretical cause & effect findings as resulted from Varmint Al's computer modeling.
I took a graduate level Finite Element Method engineering course at Michigan State Univerity my last year of attendence. It's a high-tech method of mathematical modeling with tremendous value for applied engineering design. It had already acheived "state of the art" recognition as an engineering design tool with the auto and farm machinery manufacturers. It's so mathematically intensive and analytically exhaustive that it can't really be emplyed without computers. And our computers back in 1978 were slow clunkers compared to today. It's a specialized enough method of mathematical analysis that one wouldn't remain proficient unless working with it on a frequent and sustained basis. I've visited Varmint Al's website before. He's describes his background and he was employed in a field of work that resulted in extensive exposure to the use of Finite Element and Finite Differences analysis and modeling methods, as well as supportive computer software. He's probably come up with a pretty realistic projection of case head thrust versus case to chamber wall coefficients of expansion.
Varmint Al modeled about a 700 lb difference (~15% increase) in case head thrust between a rough chamber and a polished chamber. Most of us would probably experience a somewhat smaller percentage increase in case head thrust moving from a dry cartridge/chamber to a wet cartridge/chamber surface. That's more or less what I expected and that's why I committed myself to the position that a wet cartridge/chamber was a nonsensical explanation for the brass flow and stiff bolt lifts as reported by clhman, Buffalobob, and 300remum.
Varmint Al's modeling of case head thrust versus differing friction coefficients more or less confirms the position that if a wet case/chamber produces sufficiently increased case head thrust, so as to result in brass flow and stiff bolt lift, that our loads are probably already exceeding maximum pressure loads as published in the available reloading manuals. Get the internal cartridge pressures high enough to approach brass flow conditions with a dry case/chamber, and the added case head thrust allowed by the wet case/chamber could provide just enough case head to bolt face contact pressure to result in brass flow into the voids of the bolt face.
Enough!
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clhman
Active Member
Phorwoth, very thorough indeed! Tomorrow I'll be at the range to try my loads so we can put this to rest. The only thing I'll do is clean my gun, everything else will be identical. I'll chrono them and report results. Thanks for all the work you put into it.
