doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Measure those barrels if you can.

The wall thickness gauge Bill Henry made for me (only using a drill press - incredible skill) is shown here
http://www.picturetrail.com/sfx/album/view/24488932
Dennis Potter had one on his table in Vegas made out of copper tubing and it was very light.

I measured a high grade barrel in Vegas that had been eccentrically honed and had a wall thickness of only .017 several inches back from the muzzle.

David,

I have also read where it is difficult to get repetitive readings, but I have a few thoughts about how I might modify tension inside of the bore. I am hoping to at least have a margin of error with which to work. Combined with my bore caliper, I have to think I can at least identify a potential problem. I'll just pass if things start getting marginal.

I agree regarding the pressures you listed inside the barrel after the first 3 inches, but I don't know what your numbers mean without reading the article. My primary question is in regards to where 3" is located? 3" from the breech? 3" from the end of the chamber? 3" from the center of the chamber? 3" from the breech? I've read in several places that pressure is highest IN the chamber, but how much more vague can you get when the chamber itself is often nearly 3" long? Is pressure the highest behind, even with, or in front of the powder charge?

I shot placed birds yesterday with a friend. I was making a point of making sure I could see the ground through the bore before placing the next shell in the chamber of my gun. I have not been vigilant about this practice in the past. My challenge is making sure to do it after every shot. Would everyone agree that complacency is the real enemy in regards to obstructions?

This is the whole thing that I copied from the site.
The 3 inches is from the breech.
Following is the best explanation of pressure and velocity versus
length in a shotgun barrel that has ever been posted here.

Almost all shotgun powder burns within the first 2 inches
of the chamber. A typical fast burning powder will burn
in the first 3/4 inch of the chamber. This is where
maximum chamber pressure is attained. A typical fast
burning powder like Red Dot might give a chamber pressure
with a certain load of about 10,000 psi at that point.
This is why a chamber is very thick on a shotgun barrel
back at the chamber and gradually becomes thinner as
it gets further from the chamber.

After that first 3/4 inch, the pressure and tempurature
starts reducing. When the shot has travelled twice as far,
then the already burned powder which is now gas has
twice as much space available and the pressure therefore
drops to 1/2 what it was at 3/4 inch travel.

So when the shot has traveled 1 1/2 inches the pressure
drops to 5,000 psi if it was at 10,000 when the shot
had traveled 3/4 inch and was at max chamber pressure.

Likewise when the shot travels twice as far again, to
3 inches the pressure drops to half again. So when the
shot has traveled 3 inches the pressure is about 2500 psi.

This can be carried out on down the barrel and the
pressure keep reducing by half as the shot travel
doubles onward down the barrel. Here is a table giving
shot travel vs pressure.

inches:pressure

3/4 : 10,000
1 1/2 : 5,000
3 : 2,500
6 : 1,250
12 : 675
24 : 338

So as you can see as the shot travels further down the
barrel the pressure reduces. Since the pressure is
very high early on, the shot accelerates rapidly early
on. With 10,000 psi back initially it is accelerating
very fast. Once it gets out to 24 inches though the
pressure is only 338 psi, so the shot is accelerating
very little after that point.

Adding barrel length does give extra velocity, but its
not a lot once you have a barrel length approaching 30
inches. If you had a 2 inch barrel and increased it
to 4 inches though your velocity increase would be
very large. Increasing from 28 inches to 30 though
might only give you about 10 feet per second.

If you make the barrel very long, say something like
8 feet the pressure has dropped so much that the
gun hardly makes any noise. This barrel length thing
is why an 18 inch riot barrel is so much louder than
your 30 inch trap gun. The pressure wave let loose
upon the air at 18 inches would be substantially
higher than the pressure wave let loose from a
30 inch barrel.

The above explanation is not entirely accurate,
because barrel pressure also drops with temperature
as well as with getting more space available, and
the powder is also cooling as pressure drops, but
the above explanation is accurate enough to think
of it that way as a close approximation.

As mentioned in another thread recently Sir Gerald Burrard has a very extensive section on barrel bursts in his "The Modern Shotgun". To simplify his conclusions with all ordinary powders useful for shotshells, "IF" the burst does not occur in the chamber area it is not a high pressure problem. In fact he gave an approximate measure down the barrel at which if the pressure could be brought up progressively a normal sound barrel could actually contain a closed cell pressure of the complete powder charge. An obstruction of course does not bring up the pressure progressively but creates a spike at the point of obstruction. Other than excessive pressure, which will occur in the chamber area or an obstruction which will be at some point down the barrel, there is a third cause of burst barrels. This is from an internal flaw within the metal itself. The most common occurrence of this is in steel barrels & is the result of a void formed as the steel solidified. In subsequent working of the steel to form a barrel this void is drawn into a long slender void & on rare occasions can end up being entirely within the wall of the barrel, thus invisible. Even rarer it may withstand proof firing. This accounts for those steel barrels we see occasionally with a long slit running longitudinally down the barrel with no sign of the tell-tale ring bulge of an obstruction.
JDW, all of this is in very close agreement with the SS Mag data & the conclusion you drew, with the addition of the occasional flawed barrel. An obstructional burst is indeed far more common & accounts for the vast majority of those bursts which are not in the immediate chamber area.

Miller, I thought that article was a good one to post as per Drew's post. From what I saw in those figures on how the pressure dropped rapidly, my thought was that if a barrel ruptured 12" from the breech, it had to be either an obstruction or something that I didn't think of, an invisible void that you pointed out.

This comment here is probably the most important, and the one most likely to prevent a barrel burst, in this thread. With our doubles, we have the advantage that it is so very easy to just make a habit of peeking through the bores before we load and reload. Also much easier for us to stop and check if we slip in the mud or snow. Several years ago, I grabbed an old Stevens 20 ga. that I leave out in my garage, to shoot a groundhog in my garden. I was ready to pull the trigger when something told me to stop and check the bore. Several inches of the barrel was packed solid with a mud dauber wasp nest.

But this might be a good place to ask... I know what the nominal bore sizes are for the various gauges, but what are the maximum diameters we could expect to see in unhoned vintage American doubles? Since I bought a Stan Baker gauge, I've measured a couple that seemed dramatically undersized, and I've measured a couple that seemed quite a bit oversized... yet there is no indication or clue that they have ever visited a gunsmith.

Just to set the record straight, and no offense taken, but Dave Norin was not in Las Vegas, but Dennis Potter was, and had the copper pipe gauge. It was meant as kind of a fun project, if one chose to built it. I was telling people who asked "I walking through Home Depot, I found this wall thickness gauge"

I forgot to add, that it is only suitable for 12 gauges, due to dimensions on the tube, but 12s are the most prone to problems anyway.

Keith, you bring up a very valid point. I can only speak about L.C. Smiths made in Fulton, N.Y. There is no material for bore sizes of the 10 and 12 gauge guns made in Syracuse, N.Y. (also a few 8 gauge guns that are not listed in catalogs)

Bore sizes of Hunter Arms, Fulton, N.Y.
10 gauge. .775 chambers .855"x841*
12 ga. 729-732 (I have measured a few of my hammer guns at .726)
chamber .809"x795"*
16 ga. pre-1939 .650 chambers .745"x732"*
16 ga. post 1939 .662 chambers same as above
20 ga. .615 chambers .698"x685"*
.410 .410 chambers .4765"x461*

The taper for the forcing cone was 1 7/8" in 12 1/2" in all gauges.

The 10 gauge was always 2 7/8" unless ordered different
12 gauge was always 2 3/4" unless ordered different
16 ga. was 2 9/16" until around late 1939 after this it was 2 3/4" unless ordered different. (there are a few early 16's with 3" chambers marked on barrel flats.
20 gauge was 2 1/2" until the early 1930's after this 2 3/4"
.410 early ones were 2 1/2" then 3" later.

It would be nice if other chamber and bore sizes were published here.

The link wants a log in. You are pointing people to the editor page.
http://www.picturetrail.com/members/edit?p=20&gid=24488932

Pete