doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Pressure/Speed related to shorter/longer chambers?

Question for smarter people then me.

Not sure how it relates to shotgun chambers, but when I reloaded rifle rounds (and I reloaded a lot years ago). The more chamber pressure generated directly meant more speed for the round headed downrange. Any powder you looked at in your reloading books had a correlating x amount of powder provides this speed and x plus amount of that same powder gave more speed, but with increasing amounts of pressure. What some folks did was look at the maximum speed one could achieve (because folks were speed driven) at a certain pressure to speed ratio and then back off a grain or two and check for flattened primers, sticky bolts etc., which would show you were generating too much pressure. Folks also figured out if you happened to load rifle rounds that were too long and they touched the grooves of the barrel, pressure would spike big time. There had to be a bit of distance to where the bullet started to accelerate before it hit the rifling.


What I don't know is if 2 3/4" shells are fired in a 2 1/2" chamber does the opening of the crimp hit the shorter chambers and thereby cause increased pressure? I think from what I've read it does indeed.

What I don't know is if that increased pressure results in additional speed for the shot charge headed out the barrel? More speed, more recoil even though the shell is essentially the same, but simply fired in a shorter chamber?

Be curious what folks think?

Has anyone chronographed the same 2 3/4" load fired in a shotgun with shorter chambers and longer chambers and seen what the results are from a velocity standpoint?

First & foremost note one thing well, "Maximum" chamber pressure is "Not" the final determination of muzzle velocity in any firearm. Pressure in any firearm is always in a Curve. Upon ignition the pressure rises very quickly & then as the charge begins to move giving it more area it begins to fall off.
Loads requiring more velocity or a heavier load require a slower burning powder to achieve the goal without reaching excess Max pressure.
To use your rifle experience as example if you note the charge tables well you will see that as the powders get slower, the charges are increased & can give higher velocities at similar pressures to a lighter charge at a slower velocity of a faster powder until the case capacity is reached where no more powder can be added. Thus a 300 Win Magnum will burn a lot more of slower powder & give a higher velocity than can be reached with a .308 Win even if they operate at the same max pressure.
To put it briefly & simplistically, it's the overall "Average" pressure of the entire curve & NOT the max pressure which determines the velocity. I have never seen it demonstrated but understand that it would be possible by use of an extremely fast powder such as a detonating powder to actually burst the chamber of an ordinary rifle without actually moving the bullet more than a minute amount, the pressure simply rising faster than the bullet could move enough to relax it.
I realize this does not exactly answer your question, but hope it is of help in understanding how the pressure reacts. I will also note the recoil is not determined by the Max pressure either but by the overall ballistics of the load. Thus a heavier load or a higher velocity will give a higher recoil.
Although I have not done any actual chronograph tests I have serious doubts the firing the same load from both a 2 3/4" & a shorter chamber in the 2 3/4" hull is going to make enough difference in the overall ballistics that you could ever "Feel" a difference. There might well be a very slight difference in the pressure curve in the come area but not likely much in the overall average.

If I remember correctly Sherman Bell did test with a 2 3/4" shell in a gun with 2 1/2" chambers and pressure went up about 500psi. I don't believe that would make hardly any difference in velocity, at least not what the average shooter would notice. With a 30" pattern, maybe a inch difference in lead to center it.

�Long Shells in Short Chambers�, Sherman Bell with technical assistance from Tom Armbrust in �Finding Out for Myself� Part V, Double Gun Journal, Winter 2001

The test barrel was a �heavy� Krieger barrel with piezo-electric transducer at 1� from the breech and an Oehler velocity gauge 2 5/16� from the breech.
Tests were performed:
1. 2 1/2� chamber with a 7/16� forcing cone � �British chamber� (BC)
2. 2 1/2� chamber with a 1 1/4� forcing cone � �Modified British chamber� (MBC)
3. 2 3/4� chamber with a 1� forcing cone � �American Chamber� (AC)
Each load was tested in each chamber at least 5 times.
No significant change in velocity was noted between the 3 test chambers

Scroll down about 1/4 here for details
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit

And with due respect to Bros. Bernoulli and Venturi
Shotguns: big tube/low pressure
Rifle: little tube/high pressure

Thanks folks. I'm comparing apples to oranges to a large degree. However, its nice to see what the folks who frequent this board a lot think. Thanks in particular to Drew. It would be interesting to shoot some current shells in the different gauges and see what they measure post firing. Nothing stays the same for sure.

Chamber pressures are not directly related to velocity in shotgun shells. Be clear on that. Go look at Hodgdon reloading site and look at loads for Longshot in 12 gauge. They will give very high velocity with much lower than expected pressures. Alliant 300mp, in the .410, is the same low pressure for high velocity type load. But just as in rifles, the fastest loads did not produce the tightest groups, the fastest loads do not always produce the best patterns. Nothing is free in ballistics. You trade one thing off for an improvement in another.

Shooting longer shells in short chamber increases pressures. Worse pressures in longer factory shells goes up by length. So your 2 1/2" factory shell might be 8k, your 2 3/4" might be 10k, your 3" factory shell might be 11k and your 3 1/2" shell might be 12.5k. If you get an extra 1k per different shell what coluld the effective pressure be if you shot a 3 1/2" shell in a 2 1/2" chamber? 15k or more. Who knows but nothing I'd be will to risk in my gun.

I thought that as well, that pressured jumped off the charts, but the link that Drew provided showed the jump from 2 1/2 to 2 3/4" in their short chambered test gun only went up by 500 psi. I expected much more of a spike then that frankly.

Here's what the results were copied from what Drew provided:

�Shooting 2 3/4� shells in 2 1/2� chambers does make them produce more
pressure-but in most cases it is less than a 1000 psi increase. I see no reason,
related to safety, to modify an original 2 1/2� chambered gun to shoot
2 3/4� shells, if the 2 3/4� load you intend to use would develop pressure
that is safe in that gun, when fired in a standard chamber!�
7. �We found that lengthening the forcing cone in a 2 1/2� chamber usually helps
mitigate the pressure increase that comes from shooting 2 3/4� or 3� shells in
the short chamber.�

Factory loads tend to go up with length.

B&P Competition One 7/8 oz. 2 3/4"; Fiocchi 2 11/16"; Fiocchi 2 3/4"; Federal (and Estate) 2 11/16"; Remington Gun Club 2 3/4"; Winchester 2 3/4"; Herter's (Federal) 2 11/16".



All 2 11/16" - Nobel Sport Low Recoil, Remington STS Premier, Remington Nitro, Federal Top Gun, Federal Top Gun



Estate (Federal) 2 5/8"; Fiocchi 2 11/16"; Nobel Sport 2 3/4"; Winchester 2 5/8"; Winchester Universal 2 5/8"; Remington Gun Club 2 11/16"

The average increase in pressure in Bell's tests, long shell fired in a short chamber, was indeed several hundred psi. Although there was at least one load he tested that showed an increase over 1,000 psi.

The typical takeaway is that it's safe to fire slightly longer shells in slightly shorter chambers (which the Brits have been doing for some time) IF you reduce the pressure of your load in order to offset the likely increase due to the longer hull.