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Pressures, .50 caliber rifles, strength of materials, PART 2

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Pressures, .50 caliber rifles, strength of materials, PART 2

Old 03-12-2006, 11:20 PM
Nontypical Buck
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Default Pressures, .50 caliber rifles, strength of materials, PART 2


As promised, this thread is dedicated to showing how pressures in the breech translate into stress in the barrel. Again lets consider the 25-06. A glance at the barrel and it is obviously wider at the breech than at the muzzle. This of course isbecause pressures are higher near the breech than at the muzzle.

When the powder is ignited, pressure in the breech, impresses force on the inner surface of the barrel, which must be born by the elastic properties of the barrel. The barrel expands straining the metal within it until sufficient stress is born by the barrel to completely and totally offset the forces on the barrel's inner surface. This is an action - equal reaction of the Newton type.

We know that the force related to thestressin the metalof the barrel is equal and opposite to the force impressed by the charge on the barrels inner surface. So if we know the dimensions of the barrel and the pressure, we can easily calculate stresses which are in barrel.

First we calculate the force on the inner surface of the barrel for a given length L.

Force= Pressure*PI*ID*L

The stress in the barrel is calculated:

Stress= Force/Area(single wall section)

The area of a singel wall section = (OD -ID)*L/2

Since Stress = Force/Area(single wall section) then by substitution

Stress = (Pressure*PI*ID*L)/((OD-ID)*L/2) which reduces to:

Stress = 2 Pressure*PI/(OD/ID -1)

My 25-06 is a minumum 7/8" at least8' past the lands. So I am going to use this as a measure of the outside diameter of the barrel. The inside diameter is .257. So at peak pressure(51,000 psi) the stress in my 25-06 barrel is 133,258 psi.

Now lets calculate the stress in the barrel ofa 25,000 psi load in my 50 cal muzzleloader. Just past the breech plug, in the area where the powder charge is the diameter of my 50 cal inline is 1". The inside diameter is .510 (I used groove diameter on both rifles). So at peak pressure (25,000 psi) the stress in my 50 cal ML barrel is 163,490 psi.

Please, please, take a moment and reflect on that. Considering that with a Muzzleloader there is always the potential to double load, short seat, and so on. Wouldn't you think it would be a whole lot better to have some room to play with regarding stresses? The 25-06 with its special smokeless steel is literally stressed less than the ML barrel with a magnum load.

A big bore doesn't carry pressure in terms of the stresses in the barrel as well without going really heavy and thick on the barrel. Even a 50-140 Sharps smokeless load is never to exceed 27500 psi. A Sharps is much heavier with a much greater wall thickness, especially at the breech. It is no wonder Hogdon Powder company PROHIBITS the use of more than two pellets in any 50 cal muzzleloader. This is not to say, as T7 so eloquently put it, everything is OK as long as nothing goes wrong. But for any of you out there that are under the impression thatML barrels have a "BUNCH OF SAFETY FACTOR BUILT IN", think again. Without knowing the actual minimum strengths of the steels used, I can notbegin to calculate how much safety factor is there. But it should be clear, if something does go wrong, the stress in the barrel can climb to twice the stress of a high powered rifle barrel.

The good news is this, if you reduce your charges so that the maximum pressure is say, 12500 psi, then you cut the stress in half to 81745 psi which is manageable and a fraction of smokeless stresses.

Now U.C. has suggested that I have some kind of selfish alterior motiveand Wolfhound has been trying to trip these threads up. If I have offended anyone with the way I have responded to them, I apologize. I am completely sincere when I say that I feel this is good information, information that we can all use to help us be more safe. Even should one choose to use magnum loads, at least he does it with the knowledge that there is little safety factor built into his muzzleloader. He must never short seat, never double load, and always break-in a new rifle on lighter loads first.

I sincerely hope that all this rhetoric can be dismissed with, so we can get to the bottom of all these safety questions. If we are going to say a Spanish barrel is "soft", lets know what its strengths actually are. If we are going to say that White isstronger than a Knight or a BPI, then lets know why andby how much. Too many have just spouted off seeminglyundocumented andpotentially libelous things. I have always found it curious that they arecommonly followed by recommendations of where to spend one'smoney and how to"add multiply and divide" and, of course, I am not talking about me.

Happy Hunting, Phil

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Old 03-13-2006, 02:45 PM
Nontypical Buck
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Default RE: Pressures, .50 caliber rifles, strength of materials, PART 2

OK. Been searching for material properties of steel suitable for rifle barrels, particularly my .50 cal. So far only found two which are capable of even withstanding stresses of my .50 cal at 25,000 psi. One of them, the strongest, is a chrome-moly alloy (4140)which has a yield strength of 195,000 psiif oil quenched. Tensile strength is 230,000 psi.

The other, a precipitation hardened Stainless steel(Inconel), was strong enough with withstand breaking, but not strong enough to operate within the elastic properties of the alloy. I do wonder whether my ML could withstand 25,000 psi for any substantial lentgh of time. I mean, maybe a partial milisecond of this pressure isn't sufficient to burst it but maybe prolonged exposure at the same pressure would.

I have yet to find a Stainless alloy with greater that 168,000 psi yield strength. Yield strength is the stress at which the material begins to deform plasticly. Tensile strength is the stress required to rupture the material.

Any comments or suggestions for other alloys?

Happy Hunting, Phil
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Old 03-13-2006, 04:24 PM
Nontypical Buck
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Default RE: Pressures, .50 caliber rifles, strength of materials, PART 2

OK, I foundthe problem with the equation:

The equation should be

Stress= PI * Pressure/2(OD/ID -1)

Thisquarters the stresses calculated in both barrels. So the stress of the 25-06 is33314 PSI, the stress in the .50 cal ML is 40,872.

Now all of a sudden there are many alloys capable of handling the .50 cal load at 25,000 psi. Alloys included, now are 1020 Mild steel with yield strength of50,800 psi.

Not much safety factor there, though.

4140 is a "smokeless" steel.If the 25-06is made ofoil quenched 4140 its design safety factorwould be6 to 1. Anyone know what alloy their ML is made of?

Happy Hunting, Phil
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