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did the spike damage the barrel ?

Posted: Wed Feb 13, 2008 8:37 pm
by mtngun
from the old forum:

After chopping the 30-06 barrel in a previous test, the cut-off pieces were measured, looking for evidence that the barrel had been damaged by shooting all those mega spike loads.

I used a Tesa tri-mic to measure the groove diameter of the cut sections as well as the new muzzle. The tri-mic is accurate to four digits. I took several measurements at each section. The smallest measurement was 0.3081" and the largest was 0.3087", with the differences being random. I could not measure or feel an expanded section. This barrel has not been damaged by the spikes. Bear in mind that Remington barrels are reputed to be heat treated and have a 130,000 psi yield strength. I don't have any way to verify the hardness except to say that it was pretty tough to cut with a hacksaw, enough to strip several teeth on the top-of-the-line Starret bi-metal blade. Some custom barrels are annealed to relieve residual stress so they may not be as hard as a Remington barrel. I would be very curious to have the Rockwell hardness of my barrel tested and compared to the barrels that Sisk blew up -- I believe two of the Sisk barrels were custom and the third was a Winchester SS barrel. If anyone has access to a Rockwell tester I would be glad to mail them one of my cut-off pieces to test.

Re: did the spike damage the barrel

Posted: Thu Feb 14, 2008 5:17 pm
by mtngun
Here's an update on the Remington barrel steel. A friend who has access to a college lab was nice enough to test one of my cut-off barrel pieces. The Rockwell hardness was 22C and the Scleroscope hardness was 39. According to the hardness conversion chart in my Mark's ME handbook, 22C = 230 BHN and 39 Scleroscope = 260 BHN -- yes, there does seem to be a discrepency, but that's the best I can do at the moment.

Assuming the steel is heat treated and tempered 4140, which is what gun barrels are normally made of, we can look up the properties for 4140 that correspond to our measured hardness. According to my ASM Metals Handbook, Edition 9, Vol 8, page 423, a BHN 235 corresponds to 705°C temper, 100 ksi yield, and 117 ksi ultimate. BHN 277 corresponds to 650°C temper, 114 ksi yield, and 130 ksi ultimate. Hence, we can say with reasonable certainty that the Remington steel is somewhere between 100 to 114 ksi yield.

Since some of my spikes redlined pretty hard at 105+ ksi, it's fair to ask, if the spikes are real, why didn't they bulge the barrel? (I haven't done the calculations for the various stresses that would result from having 105 ksi inside the barrel)

I'm toying with the idea of turning down the muzzle diameter of the next 22" barrel, in several steps. If our spikes are real, the turned-down muzzle should at least show some deformation, or maybe even blow off like Sisk's muzzles, and then I could use the formula for thick-wall stress to roughly estimate the internal pressure. It may be several months before I can schedule that experiment, because I want to do some accuracy tests on the next barrels before I destroy them.

Re: did the spike damage the barrel

Posted: Thu Feb 14, 2008 5:19 pm
by mtngun
I set up a spreadsheet to calculate the stress near the muzzle. Here is the how the tangential stress looks at various points across the barrel cross section, assuming 105 ksi pressure, 0.308" ID, 0.656" OD. There are other stresses present but the tangential stress is the largest stress.

D (inches) tangential stress (ksi)
0.3080 164.4
0.3428 138.4
0.3776 119.3
0.4124 104.8
0.4472 93.6
0.4820 84.7
0.5168 77.5
0.5516 71.7
0.5864 66.8
0.6212 62.8
0.6560 59.4

It does look like the tangential stress is enough to turn the inner portion of the barrel into putty. However, the outer portion of the barrel is not stressed enough to yield. Does the outer portion of the barrel hold it all together and keep the barrel from deforming? I honestly don\'t know, but I find it hard to believe that the barrel did not at least bulge a little bit, if the spike is real.

The spreadsheet says that the outer surface of the barrel reaches a tangential stress of 100 ksi when the inner pressure is around 180 ksi, so the barrel should fail at 180 ksi even by the most conservative estimate, and realistically, it will probably fail at a lower pressure. I don't how to predict the exact pressure required to make the barrel bulge or blow up, except to say that it is probably less than 180 ksi.

Re: did the spike damage the barrel ?

Posted: Thu Feb 14, 2008 5:25 pm
by mtngun
I'm still trying to understand why my 30-06 muzzle didn't get blown up by those 105+ksi spikes, if they are real. :?

Hatcher's Notebook provides some clues. He mentions that he "used pressures up to 130,000 pounds without any apparent ill effects on the barrels." [note: he probably means 130,000 CUP, which might be more in PSI, but I'll use PSI for my calculations]. He tried turning the barrel down to 1/8" wall thickness at the chamber, and firing it with regular and high pressure cartridges. As the results were not visible, [he] turned the barrel down so that it was only 1/16" thick over the chamber. It held three regular service cartridges perfectly. [He] then put a 75,000 pound shot through which blew a piece out of the side......"

I don't have a Springfield handy, so I'll use my M700 barrel dimensions instead and estimate the stresses created in Hatcher's experiments at the chamber area where my strain gages are mounted. The stress numbers are for the tangential stress only, and I was too lazy to make a seperate calculation for the brass case so I treated the brass as if it were 4140 steel. This shortcut will result in my calculated stress being only slightly smaller than the actual stress.

58 ksi load, standard barrel = 75 ksi stress inside, 17 ksi outside
130 ksi load, standard barrel = 167 ksi stress inside, 37 ksi outside

58 ksi load, 1/8" wall thickness = 113 ksi stress inside, 55 ksi outside
75 ksi load, 1/8" wall thickness = 147 ksi stress inside, 72 ksi outside

58 ksi load, 1/16" wall thickness = 162 ksi stress inside, 104 ksi outside
75 ksi load, 1/16" wall thickness = 209 ksi stress inside, 134 ksi outside, and it finally blew up

If Hatcher's barrel steel had a 100 - 115 ksi yield strength like modern 4140 barrels, then we can say that his barrel did not fail until the stress on the outside surface of the barrel exceeded the yield strength. If that is a general rule for predicting barrel failure, then my muzzle may not come unglued until the spike hits 180 ksi.