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

Ah, the old vector hypothesis again.

I believe the observed speed differential refutes this.

The venturi does not accelerate all the pellets to the same speed. If it did, we would observe a cylinder of pellets the same diameter as the choke I.D., just going faster.

Stop action photos show an elongated shot column, which can only come from a speed differential.

The inward vector of the outer pellets forces the inner pellets out of the way, imparting both forward and backward momentum resulting in the longer shot string.

The 'sideways' energy is thus cancelled, the result being the observed speed differential.

Aero effects take it from there, the dispersal being delayed as compared to a cylinder.

That aero effects predominate is supported by the fact that very hard pellets (steel) don't require much if any 'choke' constriction to shoot very close patterns.

Thus, it would appear that deformed pellets (lead) fly straighter in turbulent flow caused by the leading pellets.

If there's any 'vector' physics involved it would appear to be 'drag' vector, trailing pellets flying in apparent wind different from the bore axis caused by the wake of the leading pellets.

Just my Humble Opinion Don. I didn't stay at a Holiday Inn Express last night either.

It is noted some rather extensive testing was done many years ago in England. A pressure barrel was fitted to a pendulum gun so recoil, pressure & velocity (Observed over 20 yds) could be measured simultaneously. A single barrel was used with two interchangeable extensions for a total length of 30". These two extensions were very precise fit with the bores lapped in to that of the barrel's bore, one being a true cylinder & the other carrying a full choke,
Choke of course had no affect on the max chamber pressure but was included to assure all loads fired were of regular ballistics. Various levels of loads were fired using several variations of powders available at the time.
Two things stood out in these tests, one the full choke gave a consistent higher velocity over the 20yds. Tow the full choke gave slight but consistent "Lower" recoil. It is well noted that higher velocities with lower recoil are not normally compatible.
Explanation Please!!

It could only be:

1. Higher velocity of the leading pellets, which is what the chronograph would measure.

2. Lower average velocity of the ejecta as a whole.

The creation of the 'shot string' therefore must require energy. Frictional heat in the choke and the energy required to deform the pellets would seem to be likely candidates.

SHERMAN BELL DID AN EXTENSIVE ARTICLE IN THE DGJ WINTER 2001. I HAVE PDF
OF IT IF YOU NEED A COPY. PM ME WITH YOUR E MAIL

The words of newspaper editor Arthur Brisbane to the Syracuse Advertising Men's Club, in March 1911: "Use a picture. It's worth a thousand words."

Ed Lowry, ballistician and the director of research at Olin-Winchester, spark shadowgraphs. See "The Effect of a Shotstring" American Rifleman, November 1979.
I do not have the original, and would be very interested to know if shot speed was also measured

Remarkably, 3" extra travel in 6 feet is 50fps at shotgun velocity.

This is a wonderful topic for further investigation.

Certainly, photographic technology has advanced since Lowry.

If we could figure out a way to 'tag' a few individual pellets, perhaps with a contrasting color or reflective index, then we could determine where the pellets went in the cloud compared to their initial position.

Do the rear ones in a choke pattern catch up with or over run the front ones? Which ones diverge from the string first and by how much?

I'd like to see a cloud from a Cutts with a spreader tube, a Tula, and both WS1 and the Beretta skeet choke too.

Perhaps we could obtain a grant from the National Science Foundation. Oh... wait...

AN INTERESTING STORY ABOUT "SHORT CHAMBERS"
ONE OF MY NEIGHBORS CALLED ME LAST YEAR AFTER HEARING I WAS A SHOTGUN ENTHUSIAST. HE SAID HE HAD NOT OWNED ANY SHOTGUNS, BUT HIS UNCLE HAD LEFT HIM ONE A FEW MONTHS AGO.
HE SAID HE HAD BOUGHT SOME AMMO AND WAS KEEPING IT FOR PROTECTION AT THE HOUSE. HE WANTED TO BRING IT OVER AND LET ME CHECK IT AND ALSO SEE IF HE HAD BOUGHT THE RIGHT AMMO FOR IT.
WHEN HE BROUGHT IT OVER IT WAS A WELL WORN EARLY M 97 WINCHESTER AND HE HAD IT LOADED WITH 3 (THREE) INCH STEEL # 2.!

WHAT WOULD HAVE HAPPENED IF HE HAD FIRED IT.? I AM AFRAID TO GUESS.!