Home Forums Hunting & Shooting Ask The Gunwriters JB...Down the Barrel Pressure Spikes Questions

I was reading the June 2004 Handloader last evening and re-read your piece on pressure signs and actual pressures.

I noted you mentioned significant pressure spikes down the barrel, as the bullet slows and gasses catch up. This is of particular interest to me, because I shoot combo guns with very thin rifle barrels. Moderate stretching and flexing might fracture solder over time was my first reaction.

I would appreciate your thoughts regarding the pressure spikes; where they occur, pressures realized, different component effects, etc.

Is it possible that very thin rifle barrels might suffer damage by the spikes, being outwardly imperceptible and unnoticed? Could the spikes actually reduce velocity as gas escapes from the barrel being stretched for a brief moment?

As I recall, the problem was eventually traced to the instrumentation. The spikes weren't "real."

http://www.shootingsoftware.com/barrel.htm


Public debate over whether these secondary spikes are real was finally put to bed when Charley Sisk at Sisk Rifles blew the end off two barrels. We have also verified changes to the rate of acceleration just prior to, and after these events. Case Closed, it is real!

There are some things we know and some things we don't know about the situation. I apologize for not having more data, but I haven't gotten back to testing and writing since my accident a couple of years ago.

1. It's not a real pressure spike. There is not enough energy in a load of powder to pressurize 20" or so of barrel to the levels indicated. Also, bullets are not leaving the muzzle at extraordinary speeds, as would be required if the indicated pressure were real.

2. Hot dogs always split lengthwise on the grill. That is because the strain around the circumference of the hot dog is much larger than the strain along its length. For this reason, barrels that burst from excess pressure always split lengthwise. Charlie's three broken barrels all broke the end off the barrel, almost as though they had been sawed off. This is not an excess pressure failure.

3. I'm reasonably sure that Charlie's failures are related to the same root cause as "barrel rings" that are sometimes seen inside barrels, near the muzzle. If you set the barrel back a thread, you get a new ring, one thread farther down the barrel. I believe that the cause of the rings, and of Charlie's barrel failures, is related to the "shock collar" of condensed water vapor that sometimes forms on aircraft and rockets. My theory is this: The weight of the air in a barrel is around 2 grains. As the bullet initially rapidly accelerates, the air in the barrel ahead of the bullet is compressed because of its own inertia, raising temperature and pressure. When the bullet nears the muzzle, there is much less air left in the barrel, and the bullet is no longer accelerating so much, so pressure and temperature drop. When that happens, the very compressible water vapor in the barrel turns into very incompressible liquid water. The bullet traps some of that, and causes a ring, or causes the end of the barrel to blow off.

I thought that Charlie blew of threaded muzzle brakes.

Pretty sure they weren't threaded.

They were muzzle brakes, correct?

At least some of them had muzzle brakes to start with. The tip of the barrel popped off just ahead of the brake if I understand correctly.

Just got back from running around, and left a message on Charlie's phone. Will post more when he provides some facts. It was over 10 years ago so my only recollection (other than the brakes busting off) was that the "pressure spikes" did turn out to be an instrumentation problem--which they'd have to be.

I must agree with John. The apparent pressure spikes can be introduced by sloppy instrumentation.

Just bring up what's out there and being used as a sales tool I would guess. Lots of conflicting info out there. Thanks JB



Sir, Really like the 35P I picked up from you. Now if you could just add storage and Downloadable data it would be perfect.

Just did talk to Charlie. He's got some sort of flu so we didn't talk long, but he never did figure out why the muzzle brakes blew off--and neither did anybody else--but the best guess was gas pressure combining in some bad way with the brake design.

There was no proof of "secondary pressure spikes" being the culprit, or how such spikes could even occur, other than being an instrumental aberration.

John
Sorry for being short today. I got a dose of the TTPOA and then the NRA show crud.
I never really could absolutely determine what was going on. The only thing I could do every time was blow the end of the barrel off. In 338 Win mag, 21.7 inches was how much barrel you ended up with regardless of how much you started with, or whether it had a brake or not. I used Pressure Trace, which some say is junk, and others praise.
The way I would do this is start loading, go up a grain, shoot, repeat. Then WHAM, there went a few inches of barrel. I used the same peice of brass over and over. Seems that up to where the last few inches of barrel departed, the pressure was not high, meaning no sticking bolt, ejector marks, etc. Then with the newly shortened barrel, keep shooting that same load, and all was well. A little louder, but all else was was fine.
308, 375 H&H, and 338 Wim mag were the easiest to make this happen. I didnt matter factory or custom, or contour( which puzzled the [bleep] out of me), just load enough very slow powder and a light bullet and WHAM!!!, you just got a shorter barrel.
I do remember that about 40 feet was the normal distance inside my range that I would find the piece of barrel. Not burst like peeling a banana, just like it was cut off with a saw.
Charlie

This evidence is not conclusive, but I think it is interesting.

The PressureTrace is not designed to handle cases where the trace wants to swing below zero volts, nor should it have been (steel in compression instead of tension). The instrumentation amplifier in it is a three op-amp design. That type of device sometimes goes a little squirrely. One case is when you tell it to produce an output below zero and don't give it the supply voltages that would allow that. The "shock collar" phenomenon I posted about earlier might cause the barrel to "ring" in such a way as to cause a negative excursion on the output voltage.

So I built up my own pressure measuring system. This one was designed for the output to swing below zero. I even put a full four-resistor (two gauges, two precision resistors) with plus and minus power supplies on the barrel. The gauge amplifier fed a small computer based oscilloscope.

I got very nice traces, but try as I might, rain, shine, warm, cold, fast powder, slow powder, I could not get it to produce the spikes.

Does this prove that it's an instrumentation problem? No. But it does point the investigation in that direction.

In the meantime, the effect does not interfere with the measurement and is only an item of curiousity.

Mr. Sisk,
Thanks for your response, I found it informative as well as entertaining. Seems there may be some credence to the secondary pressure under certain conditions and sounds repeatable by your post. Thank you.

I just got off the phone after almost any hour with Charlie, and don't know if he'll feel up to posting again. But I will comment that the circumstances of this happening were VERY different than 99.9% of us would encounter, because we simply don't load ammo like Charlie was doing it during his experiments, with light bullets for the caliber, and slow-burning powders so compressed they often pushed the bullet out of the case neck after seating.

He eventually gave up on the experiments because all they proved was that if you load in extreme ways, extreme stuff can happen.

Perhaps that's why Hodgdon developed Extreme powders....so we don't have to get extreme in other ways.


(And, perhaps experiments with pre-Extreme H4831 in the 41 Mag revolver I did back in the 80s were ill-advised then? )

No one ever explained why strain gages on the breech would
measure pressure at the muzzle. Funnies can happen with strain gages. Barrel "whip" would put a bending moment that unless
extra gages were used would look like pressure. Tension on
the connections could impart enough strain on the gage .

My setup was dual trace, so I could watch down near the muzzle for a "twang" from rapid condensation and see the breech and muzzle curves in their true time relationship. What I found was nothing of interest. All I got was nice, smooth pressure curves with the gauge near the muzzle simply rising abruptly as the bullet passed by, and exponential pressure decay as the bullet exited.

Since the gauge is glued down, lead tension does not seem to affect it. However, I always do strain relief with a little dab of JB Weld. Otherwise, it is very easy to yank the lead and its solder pad off the gauge.

Which strain gauge did you select? (Ohms)