does the spike go away when you shorten the barrel?

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mtngun
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does the spike go away when you shorten the barrel?

Postby mtngun » Wed Feb 13, 2008 8:34 pm

Hacksaw time. :mrgreen:

I shortened the 22" barrel on my 30-06 in increments. At each step I fired one jacketed validation load and two 180 gr. cast loads. The cast loads are known to have consistent mega-spikes.

Unfortunately, I overwrote the trace for the 22" barrel. Ooops. However, there is plenty of data for the 22" barrel from previous tests, showing the cast load spikes almost always redline the PT at 105,000+ psi.

The graphs are self explanatory. Note that the cast loads do not trigger consistently, making it difficult to pinpoint the exact time when the spike begins.
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I would have like to continued cutting but with my luck a BATF agent would have shown up at the range just as I turned my rifle into a pistol.:|

As you can see despite the erratic triggering, the spike did not go away, but it did move closer to the breech. :? No disrespect to Charlie Sisk, whose work I admire, but this contradicts his experience. Charlie's spikes went away after he blew the muzzle off.

If the spike is due to the powder "catching up" to bullet's base, how can we explain the spike moving as the barrel is shortened?

Another theory goes like this: the air being pushed out the barrel ahead of the bullet forms a supersonic shock wave (verified by shadowgraphs). When the shock wave hits the muzzle, part of it reflects backwards and starts traveling backwards (pure speculation). The spike is caused by the reflected shock wave colliding with the bullet. Actually, that's not a bad theory, because reflected shock waves are known to be very destructive, but if the spike is caused by a reflected shock wave, then why do Winchester powders greatly reduce the spike? After all, no matter what powder is used, the air in front of the bullet is still going to act pretty much the same, right?

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Re: does the spike go away when you shorten the barrel?

Postby mtngun » Thu Feb 14, 2008 5:32 pm

This data was given to me by someone who wishes to remain anonymous. Don't ask, don't tell. The important thing is, we have some good data.

The usual ammo -- 30-06, 165 gr. jacketed validation load and 180 gr. cast spikey load.

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Note the primary pressure is abnormally high with the cast load -- it should be 50 - 60 ksi. Maybe as the spike moves toward the breech, it pushes the primary pressure up??? Ya, it sounds crazy, but do you have a better theory? Pressures remain normal for the jacketed load.
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At 12" a third spike appeared. :?:
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At 10.5" the third spike vanished.
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With a 22" barrel, the spike began around 1.1 ms. As the barrel was shortened, the spike moved backward, seemingly maintaining a certain distance from the muzzle. I have some theories but let's just keep collecting data and sort it out later.

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Re: does the spike go away when you shorten the barrel?

Postby mtngun » Thu Feb 14, 2008 5:33 pm

Changing the barrel length changes the vibration patterns and critical frequencies, yet the secondary spike never went away.

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Re: does the spike go away when you shorten the barrel?

Postby Nitroman » Sat Nov 15, 2014 5:09 pm

This is a wonderful experiment; seeing a "spike" and trying to determine if it is a real pressure excursion or an artifact in the equipment.

The projectile takes time to move down the barrel. According to the trace, the time and distance from the chamber this occurs should remain static, i.e., if it occurs about 2" from the end of a 26" barrel (at 24"), with a certain load (pressure and time), if you were to shorten the barrel to say 22", about 2" before the "spike" in the 26" barrel, it would not be there. So you know it is not due to pressure or distance from the muzzle. It is an artifact.

That you continue to see it as you shorten the barrel only reinforces that it is artificial, something in the equipment, but in no way affects the precision or accuracy of the equipment as I am convinced this is occurring after the bullet has left the muzzle.

I have PressureTrace and use it extensively on my rifles. It is THE best diagnostic tool there is as far as I am concerned. Coupled with a good chronograph like the Oehler 35p, you are set to go.

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Re: does the spike go away when you shorten the barrel?

Postby mtngun » Sat Nov 15, 2014 9:00 pm

I never determined to my satisfaction what was causing the spike, though there was some circumstantial evidence that unburnt powder was igniting when it exited the barrel.

Even if the powder is reigniting upon exit -- and we know that some powders do -- it's not obvious why that would cause a pressure reading at the chamber?

I heard that another experimenter had narrowed down the cause, however I never saw his report.

There are more experiments I would like to do but they are on hold until I get halfway caught up with work and chores. :/


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