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

One key factor in this is the shell must have clearance for the crimp to open normally i9n order not to get a drastic pressure rise. Loading a 3�" shell or even a 3" one for that matter could well have the crimp jammed into the cone which would put resistance on the opening of the crimp. This could cause, & likely would, a much higher rise in Max pressure.
Older SAAMI standards for the 12 gauge chamber gave a minimum length of 2.616" for use with 2 3/4" (2.75") shell. Also listed was a max taper to the cone of 5� per side. This was deemed safe even with a 2 3./4" roll crimp shell. As the "Loaded" length of a roll crimp shell is longer than a fold crimped one there is not as much leeway in chamber length for them. That 5� per side taper in the cone with a chamber end of .798 & standard bore of .729 would give a cone length of .394".
"IF" you have a gun with an Extremely short cone or step then it becomes extremely undesirable to fire any shell with a ""Fired" length longer than the chamber. No gun should "Ever" be fired if the "Loaded" length is longer than the chamber proper, not including the cone.

What Miller said

12g 12 1/2" chamber British double
Left - 67mm Eley 28gm; Right - CompX 65mm 21 gm.



Left 20g 67mm case; right 65 mm case

2-p, I know you know that "area" should be "volume." I mention it only because there may be some that don't understand that a change in volume will cause a change fluid pressure. Statically, increasing volume equal decreasing pressure. In the case of fluid flow, decreasing area will cause an increase in flow velocity and a decrease in pressure.

AS the shot/wad moves down barrel the volume containing the powder gas is rapidly increasing and the gas pressure rapidly decreasing. As the shot/wad transit the forcing cone, the pressure drops and the velocity increases. Ditto for the choke constriction.

DDA

Those old pressure curves speak volumes.

Rocketman;
I have no idea why I slipped up & said area there. You are certainly correct I meant Volume, thanks for pointing out my Slip-up.

We've wandered into choke rather than forcing cones, but a study by Neil Winston comparing shot speed and choke constriction
http://www.claytargettesting.com/study2/Study2.2.pdf
Full choke is 25-50 fps faster than cylinder at 4 FEET

Neil did not measure, nor can I find 40 YARD data, but the difference would be very small

Note SAAMI Shotshell ballistics are reported through a test barrel with .031 constriction

This is as it should be to keep Bernoulli happy. Note that the extra velocity comes at the expense of lowering pressure within the choke section --- that is, lower pressure at the muzzle for full choke. Lower pressure within the shot column means lower dispersion force on the pellets.

DDA

Don, do I understand correctly that there are actually two reasons for a tighter constriction choke yielding a tighter pattern? One - the actual physical constriction of the shot load, and ..... Two- the lower pressure within the shot column lessening the dispersion?

SRH

I am working on creating a pdf of the Sherman Bell article from Winter 2001. Send me a PM with your e mail if you would like a copy.

Stan, I doubt that the physical/actual constriction of the shot column actually does much it the way of choke effect. The effect, IMO, comes from lower pressure within the shot column. This lowers "sideways wind" (good bit of air within the shot column) that gives the pellets an initial sideways velocity (low velocity, but sideways none the less) and lowers the "spring apart" of the pellets (due to compression).

DDA