Pressure diff from choke?

[LBL]

Today I took a handful of my new 16ga skeet loads to the club and did test shots at the pattern board and using the club’s chronometer. I tested using my 16ga VHE though it’s not choked for skeet because I don’t shoot untested loads in my DHE which is choked Skeet/Skeet. Generally AOK achieving the lower speed (lower pressure) range I was seeking. The really interesting and unexpected result was the difference between the two barrels of the VHE. The RH barrel with Mod choke clocked 1040-1060 fps (4 shots). The LH barrel with Full choke clocked 1140-1160 fps! (3 shots)

If you have insight on this pressure difference due to choke I'd sure like to learn about this topic. For example: Is choke a factor to take into account with reloading (or any shot shell decision) when trying to limit the pressure that a Parker is subjected to? Or is this increase in fps with the full choke barrel just a manifestation of the same pressure going through a narrower orifice? If that’s true then one wonders why this “choke effect” on fps isn't highlighted, and what choke is used for all the fps specifications for ammo and reloading recipes?!?!


Thanks!

[J. Scott Hanes]

The chronograph records velocity, not pressure. It takes a different type of equipment to measure pressure.

[Dean Romig]

But the question remains - what accounts for the 100 fps difference between chokes?


I believe it is only the constriction under the same pressure.
Put a nozzle on your garden hose without changing the pressure and the water stream goes faster and farther than without the nozzle. Physics 101.



.

[J. Scott Hanes]

The chronograph was designed to test velocity of rifle bullets, a solid mass. They are "ok" for testing a shot pellet load but the density of the mass does affect the readings. At what distance did you shoot the test patterns? Of course the full choke would emit a slightly denser mass than the modified choke. The chronograph reads the first recognizable "mass" it "sees" to trigger the reading, therefore the tighter grouping may read as 'faster'. (fewer pellets spreading ahead of the mass)

Also, statistically, it would be best to perform 3 groups of 5 shots to get an average. Were the shells factory or handloads? It is not uncommon for factory loads to vary +/-10% for field loads. If handloaded, due to the slower process, shells tend to be more equal. Not to worry about factory loads; you can't physically tell the difference and the velocity difference on a target or a game bird makes no material difference, at all.

[Bill Murphy]

J. Scott Hanes implies, correctly, that certain chronographs or types of chronograph, may give somewhat incorrect results when used to measure shot load velocity. Not much we can do about that except to use a chronograph made specially for shot load measurement.

[Aaron Beck]

The bore id in both barrels may be different accounting for a variety of factors such as gas seal and or friction. I would assume by the time the load gets to the choke, the ballistic variables of chamber pressure and velocity are pretty set, but i could be wrong.

[J. Scott Hanes]

Pressure is a force that is exerted in all directions in an enclosed area (the chamber being most important - hence the chamber on a firearm is thicker than the barrel needs to be). As the volume of space (down the barrel) increases from the shot load moving at its velocity [in one direction], the pressure is lessening all around the barrel and continues to dissipate until it is released by the shot load exiting the barrel. If the pressure is too great for the barrel's wall thickness {think over-bored after factory specs}, a failure can occur as can happen if the muzzle is plugged. Disaster most likely.

[LBL]

Originally Posted by Dean Romig

But the question remains - what accounts for the 100 fps difference between chokes?


I believe it is only the constriction under the same pressure.
Put a nozzle on your garden hose without changing the pressure and the water stream goes faster and farther than without the nozzle. Physics 101.

.

I think Dean’s “Physics 101” explains the velocity difference between the Mod and Full choke barrels in firing these tests of a new reload recipe. As the concern re older Parkers is pressure, and pressure is only relative to velocity in the same constriction (and undoubtedly other barrel attributes) that raises my last question again, if anyone knows:
What choke (and other barrel attributes eg length?) is used for the fps specifications published for shotshell ammo and reloading recipes?

Further: Are there perhaps any formulas for translating shotshell velocity published specifications of a given gauge and shot mass into the pressure to which the gun is subjected? (Seems like pressure could be a more consistent specification on commercial shotshells than velocity.)

Thanks again for your consideration of this topic.

[Double Lab]

The difference is in the choke constriction. Dean is correct and it is called the Venturi effect. I certainly would not worry over 100fps delta. Worry about finding ammo.