doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 2.75" shells in 2.5" chambers

Thank you Miller. Good for the ego. :-)
Especially coming from you.

JC

JC;
I have no problem at all with recognizing & supporting "Facts" which put down old wives tales. Your reasoning on the max pressure occuring in the chamber was totally logical & supported by facts. One can also recall when Bell reported on blowing up the Parker Damascus @ a pressure of around 30K psi, it blew in the chamber area where it was "Thick", not down the bbl where it was "Thin". Again obivious that max pressure did not carry down the bbl to any extent, as does the illustration you just posted. I do recall this incident being reported here on the BBS.

Diagnosis? 1914 CHE. Unfortunately, no history available

The pressure in a shotgun barrel peaks within the first 1/4 - 3/4 inches. At 16" the pressure will be down in the 1000-1500 psi range and, say, 500-1000 psi at the muzzle. The base of a 12 gauge wad should be about 0.42 square inches. The thrust on the payload near the muzzle, then, would be 210-420 force pounds. Acceleration would be thrust minus drag. The payload will continue to accelerate until thrust equals drag. We know that thrust and drag are getting close as the loss of velocity from shortening a shotgun barrel is small.

If a shotgun barrel is over-pressured, the burst will occur at the first point where the hoop stress exceeds the ultimate strength of the barrel steel. Depending on how the barrel is bored and profiled, this could be anywhere along the entire length. Had the chamber walls been thicker in J-C's example, the burst could still have occured further forward as in revdocdrew's example.

In my opinion Drews example shows all the characteristics of an obstructional burst. This is, however one of the problems with trying to diagnose when no history is available.

Pete,
Currently there is a straingage system available that will plot pressure vs. time. I purchased one a couple yrs ago. Here's a sample plot of five shots overlayed.

ChuckH:

Do you still have the website you posted earlier this year where one could purchase a strain gauge?

I have a non-double question: a hunting buddy fired what looked to be a Winchester 1400 12 bore this past weekend and the first shot resulted in approx. 2 1/2" long thin crack in the top of the frame just right of top dead center(looking from the receiver to the tube) and allowed the barrel to jump forward about 1" showing most of the threads; obstruction? I was busy loading a muzzle-loader and only took a glance as he passed to fetch another gun. I asked if that was the only shot and if he had looked down the tube before shooting. It was his uncle's gun and it had been in storage for about 5 years since his uncle expired. Would a dirt-dobber's nest accomplish such an event?

Kind Regards,

Raimey
rse

Raimey,
Here you go. http://shootingsoftware.com/

That's Paul's '89 Remmy "salesman's cutaway". At least it is now.

jack

To get back to Burrard, Thomas, etc on recoil and pressure:

Miller, I'm not sure that Thomas' theory as to WHY the faster burning powder creates less recoil is correct. However, even you state that you don't think he was a liar, and that the IMI tests to which he refers did happen, and the results were as reported. Did he report "complete" results, including types of powder, etc? Not anywhere I can find. However, if you're going to fault Thomas for that, then you also need to fault your hero Burrard, who only posted the CONCLUSION that long shells (appropriately loaded), when fired in short chambers, do not increase pressure. Thomas, on the other hand, provided an actual chart comparing 10 shots fired in a 2 1/2" chamber and 10 fired in a 2 3/4" chamber, with pressure readings taken at 1" and 6" from the breech, as well as velocity readings. (In this respect, his information is more complete than Bell's, although it only involves one type of shell.)

Burrard did indeed conclude, as I quoted, that excessive constriction of a shell would result in increased pressure. (P. 153, "The Modern Shotgun".) Bell essentially confirmed that in his tests, although in most cases the pressure increase was not very great.

And your recollection of the first "burst" produced in Bell's test is not exactly on target. What happened is that the chamber did indeed peel back where it's thickest, but the source of the weakness was not the solid chamber wall itself. Rather, it peeled back from the hole between the chambers that houses the ejectors. An obvious weak spot, if you stop to think about it.