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Old January 25, 2012, 02:16 AM   #1
PaulN
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If Bolt Lug Force Is The Concern Then How Relevant Is Chamber Pressure ?

I've been trying to understand how directly chamber pressure translates to load on a bolt-action's lugs.

Back 50 years ago P.O. Ackley had an article in one of his loading manuals which said that a cartridge's brass case can hold-back a significant proportion of the force generated when propellent is ignited (and the chamber wasn't oiled - that made a difference).

I've got a Spanish Army FR7 carbine which has an 1893 Mauser action married to a chamber/barrel that take a .308 dimensioned cartridge. A desire for safely indicates that a low-to-medium pressure loading is required but it isn't clear how low the maximum pressure needs to be.

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Old January 25, 2012, 03:18 AM   #2
Scorch
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Bolt thrust is equivalent to the pressure times the area of the case head. For example:
Assume you have a loaded 30-06 that operates at 62,000 psi measured with a strain gauge.
30-06 case head diameter is .470".
30-06 case head area is .1734944 square inches.
Bolt thrust would be the chamber pressure X the case head area, i.e. 10,756 pounds of pressure.

In reality, it is a bit more complex than that. You have to take into account the duration of the pressure impulse, the degree of taper in the case, etc, etc, etc, to determine the bolt load, but this will give you a quick working number. Shear is the force you are concerned about. The type of metal, thickness of the bolt lugs and the receiver ring will give you the info needed for the critical strength calculations. If you have, say, a Spanish Modelo 1893 Mauser, designed for 52,000 psi cartridges, produced from carburized mild steel, shear yield strength may be fairly low, bolt thrust from 9,021 pounds of pressure will be safe for a long time and 10,756 pounds might weaken it very quickly. If you are using a Weatherby Mark V rifle, produced from 41XX alloy steel, pressure excursions into the neighborhood of 70,000 psi are expected, the shear forces were calculate before the action was made, and the rifle is expected to fare very well for many thousands of rounds.

The brass cartridge case is just a seal or a liner for the pressure vessel itself, and adds very little to the strength of the action assembly.
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Old January 25, 2012, 11:01 AM   #3
Slamfire
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Quote:
Back 50 years ago P.O. Ackley had an article in one of his loading manuals which said that a cartridge's brass case can hold-back a significant proportion of the force generated when propellent is ignited (and the chamber wasn't oiled - that made a difference).
P.O Ackley was quite a bit of the salesman with a little shyster mixed in.

Remember at the time ole PO was selling his Ackley improved (AI) cartridges. He was blowing out the shoulders, straightening the case, to increase powder capacity and raising pressures. It is obvious that ole PO was taking flak from folks who were claiming that his high pressure cartridges were overstressing the action.

P.O Ackley cartridges are very interesting and P.O’s test of a straight sided cartridge holding pressure without a breech block has been duplicated. The tester swabbed the chamber out with alcohol swabs between shots. The Ackley cartridge held. However the other cartridges, such as the 30-30, 35 Remington, blew out of the breech at 1900 fps. A 150 grain cartridge case flying at 1900 fps will go through both sides of most people's skulls. No designer would weaken his mechanism assuming high case friction, you just cannot control that.

This would be an interesting side topic, but based on the cartridge taper advice found in my referenced design books, straight cases are not an ideal form. They don’t steer well and they don’t retract well. If the mechanism does not cycle well, if case sticking is a constant problem, if feed geometry is too fickle, the mechanism will be a failure.

Ackley was taking heat because he was getting high velocities from his improved cartridges, cartridges which were being used in actions not designed for the things. P.O. wanted to show that his speedy cartridges did not increase bolt thrust, and infact as a result of his experiments, claimed they actually reduced bolt thrust. Which was bogus as heck as these improved cartridges actually ran at higher pressures. Higher pressures increases bolt thrust, it does not lessen it.

Read carefully Boatright’s papers one of which he shows how a 308 case, in a clean chamber, can lock in and hold pressures by itself up to 25K psia.

Go to Jim Boatright’s web page.

http://www.thewellguidedbullet.com/

Look for yielding of the brass case in these studies

http://www.thewellguidedbullet.com/m...al_studies.htm


However once pressures go above 25K psia, Boatwright shows the brass case stretches and if not supported, the case head will blow off.

Regardless of taper, and all that mystical mojo, cases are made out of brass and will stretch. There may be bolt load reduction due to friction and stretching but it is inconsistent and not to be relied on in any way.

Apparently the standard 30-30 is strong enough not to lose its case head without the AI taper.

http://gunsmithtalk.wordpress.com/20...-thrust-tests/

If you notice, P.O. Ackley never conducted his test with a 30-06 or a similar high pressure cartridge. I am certain those would have blown right out the back of his lug less rifle. Ackley and others did not conduct sensitivity tests, varying chamber finish, (chrome for example), powders, primers, or much of anything else. I totally disagree with the implicit conclusion that Ackley and others have drawn, which is if Ackley improved cartridge reduces bolt thrust, a user can just pour the coal into the cartridge and let fly. Ole P.O. was interested in promoting his cartridges, found a “one off” and left a very misleading legacy in terms of case friction, load, and chamber roughness.


Action designers ignore case friction in sizing bolt lugs and locking recesses. Actions are designed assuming that the case provides zero resistance. At least in modern actions.

The Lee Enfield is an example of a legacy action designed in the blackpowder era that requires case friction because the action is so weak and flexible. The British NRA have warned not to shoot the things in the rain! I got the idea they were worried about bolt breakage. Major E G B Reyonlds reported in the American Rifleman that the point of impact change between dry cases and wet cases is 5 MOA. That is one stretchy action!

Straight walled blackpowder cases need friction in the chamber. BPCR shooters report wet cases being stretched up to the lands and grooves. I have not seen any analysis but something about the low pressure curve of blackpowder, and maybe the bullet crimp, will pull a slick case up the barrel.
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Old January 25, 2012, 11:17 AM   #4
Slamfire
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Quote:
I've got a Spanish Army FR7 carbine which has an 1893 Mauser action married to a chamber/barrel that take a .308 dimensioned cartridge. A desire for safely indicates that a low-to-medium pressure loading is required but it isn't clear how low the maximum pressure needs to be
An 1898 report of 1894/96 Mausers and their ammunition captured in cuba gave some interesting numbers

The report states that the Mauser Cartridges captured in Cuba had an initial velocity of 2200 fps with its 173 grain bullet.

I looked up in my 48th edition Lyman handbook for 7mm Mauser data. With a 175 grain Jacketed, loads that develop around 2169, 2165 fps with various powders, the pressures are 33 Kpsia, 32.5 Kpsia.

I have shot thousands of rounds of 308 Win, 200 yards standing and 200 yard sitting rapid fire, with a 168 Match and 39 grains IMR 4895. (AA2495 is a copy). The lower target was fired prone slow fire in a 100 yard reduced match. I believe this load or the same charge with a 150 grain bullet will not over stress a Spanish action, assuming it is metallurgically good.

If the receiver is soft, you will develop headspace and that could be dangerous.

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Old January 25, 2012, 03:24 PM   #5
thallub
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i had a rude awakening when my Ackley recommended .280 Ackley Improved load was found to generate in excess of 72,000 psi.
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Old January 25, 2012, 09:52 PM   #6
lonniemike
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Paul, you're in uncharted waters. I think Slam is giving some good insight. Strangely, from the Spanish you'll get few facts. Many people will have many opinions. Plenty of people do shoot 308 in the 93's. Plenty more say don't. Whether it is okay or not is something the individual must decide. I get into enough headaches when I talk about shooting my Swedes at Norma/RWS/European velocities(2800 vs 2600fps) instead of the Swede military round speeds.
I've no dog in this fight, but I'd like to see anyone show/demonstrate/prove the 93 model was designed to take a steady diet of 52,000 psi. The 93 was designed to shoot the 7X57 with a mid 40's KCUP pressures not unlike the Swede 94 and 96. Those pressures would be less than 50,000 psi by any measuring system, strain or piezo. In the early 1950's or so the Spanish did convert to 7.62CETME from 7X57. Dimensionally, it is nearly identical to 7.62NATO but at much less pressure. I'd find it hard to believe that the Spanish would stamp 7.62NATO(they did join NATO but it was in the 80's) or 308Win on their CETME rifles. However, I can easily believe that some importers would do that. AlltheBest and my flame suit is on. Go
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Old January 26, 2012, 01:31 PM   #7
F. Guffey
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“If Bolt Lug Force Is The Concern Then How Relevant Is Chamber Pressure ?”

My opinion, I believe there are those that take themselves more serious than I (do) take them seriously. It is cute when a reloader says I want to feel the case seat when I close the bolt, I want to feel the resistance to bolt closing, I want to crush the case slightly???? There are reasons for resistance to bolt closing, a few of those reasons can increase pressure enough to crush the case head and remove all the writing on the case head when fired. I am a big fan of “time is a factor”.

I have a press(es), I have dies, lots of dies and I have shell holders, because all shell holders are not alike I have lots of shell holders as in different brands, all my shell holders have a deck height of .125.

I chamber ammo that fits, I do not chamber ammo that puts a preload on the chamber, closing the bolt is an incline thing, raising the ram on my press is an incline thing, when I size a case I size the case, I have nothing to gain by putting the press in a 9 line bind after the case is sized, 9 line bind, that is the part where the die is screwed in an additional 1/4, 1/2 or 3/4 turn after the die contacts the shell holder, unless the case being sized is tougher than the ability of the of the press to size the case. I am sure the action is tougher than the press, the press when abused stretches, tweaks, spreads, jumps sideways? or goes into some kind of a strain (that can be measured), so? I choose not to put a preload on my receivers and when closing the bolt others live in a dirt, grit and grime free environment, I don’t, so, I choose not to lap my lugs every time the bolt is closed. I did not say it does not sound cool when someone says they like that feel of crushing the case when closing the bolt, I said I choose not to put the receiver in a pre-load mode/bind.

And I use the feeler gage, the companion tool to the press, I am not the fan of allowing the case head a running start before it hits the bolt face, I am the fan of cutting down on all that case travel.

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Old January 26, 2012, 08:15 PM   #8
Nevmavrick
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If you're using the M93 as a hunting/field rifle, there is NO reason to run the pressures up. The small-ring Mausers make fine little rifles, a little lighter than the M98.
An M93(94, 96, etc) with a 20"-22" barrel will kick a 150gr bullet at 2500+fps from a .308 and, if put in the right spot, will drop any animal for which the .308 was intended, at any distance that an animal should be shot.
Ask me sometime what I think of my Mexican Mauser/.250 Savage Improved.
If you want to make a .308 equal a .300 Weatherby, use a Mark V action!
The taper of a standard chamber versus an Improved DOESN'T relate to chamber pressure, directly, and there are other factors that would dictate whether one would choose one over the other.
There is no reason to load down to Black Powder velocities, but seeing if you can blow primers with every shot is not necessary, either.
The .308 is a minimum-taper case anyway.
As a hunting gun, you don't want the ammo to fit tight, anyway. It should chamber with the minimum of resistance, but not have excessive headspace.
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