doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Wall thickness recommendations

Thank you all. That is what I was looking for.

I shoot a Hammer gun that's .018 near the muzzle at doves and don't have a problem. I do shoot lite loads.

From "Modern Sporting Guns" by Christopher Austyn: "There is no legal minimum thickness for a shotgun barrel, although the British Gun Trade Association RECOMMEND 20 thousandths of an inch as a general minimum." (Emphasis the author's.)

As noted above, there are plenty of British game guns that left the maker with MBWT well under .035. As also noted above, the main issue with barrels (assuming good condition) as they approach .020 is what happens if you dent them. Likewise, the location of the thin spot is also important. Like Jeweler, I wouldn't be overly concerned with a gun slightly under .020 close to the muzzle. Pressure certainly wouldn't be an issue in that case.

Just to clarify, the British .20" recommendation refers to 18" from the breech and beyond. Published data shows even a 3 Dr. Eq. of dense smokeless (Ballistite) runs less than 1500 psi at that point.

Dr. Drew,

.20 is for a .308. Just kidding. We know you meant .020.

Best,
Bill

I believe that I got this from Shotgun World under the Reloading Forum. There was a whole story on pressures and psi and the gentleman that wrote it did a lot of homework. Makes sense to me.

"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."

There was a lot more written.

3/4 : 10,000
1 1/2 : 5,000
3 : 2,500
6 : 1,250
12 : 675
24 : 338
He stated that every inch multiplied by two the pressure dropped in half.

This statement would be true I believe if the volume exactly doubled "& IF" the temper stayed constant. In real life of course neither of these are true. The internal diameter of the hull will normally be of a larger diameter than the bore, plus the forcing cone will have more volume than the same length of bore. Although I do not have the means of measuring them I am quite certain the temperatures of the burning & expanding gases are not constant for the length of the bore. Pressure is relative to both volume & temperature.
Max pressure is of course reached at the point where increasing volume of the chamber exceeds the expansion of the burning gases, but all the powder has not been burned at that exact point. The pressure therefore would not be expected to have fallen to half of the max at a point where twice the volume was reached from the point where the max occured. A look at recorded pressure curves of various burning rate powders will quickly show these exact ratios simply do not exist in the real world.

This may be the thread David referenced
http://www.shotgunworld.com/bbs/viewtopic.php?f=13&t=405370

Most of the graphs are calculated/predicted numbers. I believe the pressure/time curve was interpolated to distance, not measured as such.

Miller, he did mention temperature as one of the factors. Here is the whole thing that I copied and pasted for my records.

"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 temperature
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."

Drew, that is a good post also but I think someone on Shotgun World got it from another site and posted it there.

Yes the full explanation is much closer to actual facts. The main point I would still disagree with is that total combustion of the powder charge has been reached at the point of max pressure. While the above graph is based on time rather than distance the curves are very similar to those whch have been shown related to distance. In fact as best I recall I was the first one to mention these pressure curve graphs & lacking a photo program at the time scanned in a copy of the duPont one made in the 1920 with Black, Dupont Bulk Shotgun, Infallible & DuPont Oval as I recall which I sent to someone to post for me. This graph has surfaced many times since.
"IF" that ratio of twice the volume = half the pressure were always the case then the load giving the lowest max pressure woulds "Always" give the lowest velocity. I think we all understand this is not the case.
The "Fly in the Ointment" here is that the P:V ratio is not an exact constant & varies with the burning rate of the powder. As can be seen in the curves the slower powders continue to release more gases (Still Burning) for a bit longer past the point of max pressure than do the faster ones, though all do so to some extent.