doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Chamber Lengthening Wall Thickness Trial

I believe I had put up two pictures on that thread prior to it being deleted. Looked through my files & I know one of these was what I used, but not positive about the second one. Anyway here goes;
This first one is of the unmodified chamber.



2nd pic is of a lengthened chamber with the blacked in portions being the metal removed in the process. Upper side shows a 5� per side angle to the cone. Lower side shows the cone cut parallel with the outer wall. On this particular drawing that gave a cone length of approximately 1�". This would, of course, vary on individual guns.


Normal taper for the chamber itself is about 005" per inch. The barrel's OD will universally have a steeper taper than this in the chamber area so any lengthening of the chamber will result in a thinner wall at the junction of the chamber with the cone. Whether or not this creates a dangerous situation depends totally on both the barrel steel & its dimensions.

It is as patently Wrong to say that all chambers can be safely lengthened as it is to Say that None can be. It simply has to be determined by a knowledgeable person on an individual basis.

PS; You can click on the thumbnail pics & enlarge them for easier viewing.

Thank you Miller.

This is a No. 0 "Good 2 Iron" 16g Smith with chambers lengthened to 2 7/8" with end of chamber WT of .102" right and .105" left, that still bulged; with what load is unknown



X-ray of bulge




Part of my confusion was related to my 1906 12g Armor Steel Smith that I discovered to have chambers of 3 1/4" after forcing cone lengthening. The end of chamber WT was still .118" right and .112" left. Clearly the pre-lengthening WT was substantial.

Looked some more in my records and found a Parker 12g Grade 3 2 frame with D4 barrels with chambers extended from 2 5/8" to very slightly more than 2 3/4" - end of chamber WT .100" right and .114" left.


As said over and over, lengthening British 2 1/2" game gun and U.S. maker's small bore chambers should be done only by a barrel expert and with great caution.

It might be interesting to measure the barrels of some 20-gauge Browning Superposeds from the days when they were only made for 2 3/4 inch shells and after they began chambering the hunting models for 3-inch shells.

I. Have not read the whole thread as I am on a very small computer at the moment, but is it proper to consider that the pressure at 2 3/4� is less than at 2 1/2� in a normal shell ? If so the calculation of the safe stress at each distance should be made, and that seems to say the wall thickness at the 2 3/4� distance could be thinner than at the 2 1/2� and be equally as safe in each area.

Ancient pressure-distance curves





Modern, but computer generated



Clearly modern fast powder have a higher 1" pressure which then falls more rapidly than old Bulk or "progressive burning" powders, even with the same total area under the curve

A pressure curve that shows higher pressure at the point of the standing breech than at the point in the chamber where the powder charge is situated in the shell should be viewed with extreme skepticism. After all, it is the rapid burning of the powder, which sits between the base wad and the under shot wad, which generates the pressure. Peak pressure will obviously be generated very near that point. We don't see examples of early pressure test barrels with lead crushers at each and every point along the full length. So it should be obvious that there was considerable interpolation and conjecture involved in the early pressure curves. And we would also have to assume that even the modern computer generated version relies on some unknown technology that would provide continuous pressure data at every point as the charge passes down a 30" barrel. We would similarly need to have piezoelectric transducers installed along every point of the barrel to know precisely where the shot charge was at a given time in the pressure curve. Multiple strain gauges could be installed along the full length of a barrel, but they are not as accurate as piezoelectric transducers which require a hole in the chamber wall. As it stands, we rely on the ideal gas law to know approximately where the shot charge is at a given point in time. But the pressure is variable, dependent upon the burning rate of the powder in a confined space of continuously variable volume.

well yeah keet, what you state above is obvious...but you forgot to factor in the phase of the moon...

Historically, "breech pressure" using crushers (LUP) was measured at 1", as are the piezo transducer SAAMI and CIP standards today

I really learn a lot here. Checked Amazon for that Bernard dehydrated water, but it must be out of stock or discontinued. Sure would make those deep forays into the Sandhills easier without that water bottle banging against my back in the game bag.

Mike

Yeah, but you have to reconstitute the stuff with goat's bile