doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Shot Distribution in Pattern

Brent, as I've tried to explain it's aerodynamic forces that spread a shot cloud.

The speed differential I refer to is pellet to pellet speed difference. All the shot is not accelerated to the same velocity by the choke. The leading pellets are going faster, and retain that differential for a good distance as evidenced by the continuing lengthening of the shot stream as shown in the photos. Even with the increased drag on the leading pellets and the following ones flying in their 'draft' it takes quite a bit of travel for the dispersal to equal what a cylinder choke shot cloud looks like at a shorter range.

Profiles of common skeet chokes are available on the interweb.

RocketMan
You are the guy with the golden pencil. We all rely on your computations as a firm starting point.

I think some of these questions relate to the possibility that what goes on in the gun before it gets to the choke determines a degree of the tightness of the pattern.

I think damage to the shot due to ignition, forcing cone damage, and/or barrel scrub possibly may factor into choke effect.

Someone mentioned the fact that it was commonly believed by earlier generations that a short chamber increased the tightness of patterns in trap guns. I think that is largely disputed, but the owners of those guns were not easily convinced. How do you feel about that?

I suppose that some mismatch between chamber/forcing cone and ammo could have led to this belief. Rather, I'd guess that some very good shot had a short chambered gun and his competitors needed to blame his success on something. I'd seriously doubt there is data (the real thing) to back this up.

DDA

This is what Bro. Compton said, which I totally do not understand

p. 129 The other variable which can be altered in the test barrel to change the internal ballistics is the chamber length. In Table 3.10 the standard deviation and pellet counts at 40 yards are given for a 36g load of #4 lead shot loaded into a 2 3/4� cartridge cases which were fired through a selection of chambers lengths. The averaged results show a definite alteration to the lateral dispersion of the shot cloud when using the wrong chamber length. The tighter pattern generated by 2 1/2� chamber length may be caused by the restrictive crimp opening acting like an internal choke.

I don't think you can get choke effect at the forcing cone/chamber. The trip down the bore is going to negate any effect there. Possibly, there is something reducing pellet damage. But, I'm still thinking about it.

OK Bro. Don; you're on

....the shot charge fired from a choked barrel lengthens between 6 feet and 14 feet then nothing will. The only possible thing that could cause this is speed differential. The trailing pellets are going slower than the leading ones....

Better than most for sure. A agree with your first and second paragraphs. Not so sure about the second but I'm not sure what differential" is in reference to - all the shot before it reaches the choke or some of the shot in the choke (the periphery vs the center)? Do you avhe a diagram of the WS-1, Tula, or Beretta skeet chokes?

Seems like folks are arguing for Venturi like effects and claiming a choke works like a de Laval nozzle. I'm not at all sure I buy that, but I see where you are coming from. However, Venturis and de Lavel nozzles are all about the speed of gases or at least fluids. And not about radial dispersion at all. In fact, in the case of the de Lavel nozzle, radial dispersion is to be avoided (so as to generate thrust). But anyway, I'm following along. I just don't see the first principles here working to create the radial spread of the patterns that, undeniably, happen.

The shot is not a true fluid, but exhibits some fulidic characteristics. A perfect de Laval nozzle will accelerate the fluid flowing through it to sonic velocity in the converging section and above sonic in the diverging section. Exit pressure would be ambient for complete expansion of the gas (there would be a straight sided exhaust plume as opposed to a bulb shaped plume)[spoiler][/spoiler].


Just as a thought experiment considering fluid dynamics (which may or may not be relevant), take your common garden hose. Cut the coupler off the end and run the water on high. What happens? You get a long, relatively solid, stream of water about the size of the inner diameter of the hose for quite a distance before it breaks up. Sort of a full-choke type of pattern, if you will.

The problem with this example is that water is relatively incompressible and has fair to middlin' surface tension to hold it in a cohesive stream. Likely the stream exited under laminar flow, too.

Now, put a constriction on the end. A reducing coupling - this acts as a choke. Water comes out faster of course (Venturi), but it also spreads much wider - much more like a cylinder choke in comparison to the first example with no constriction. Clearly, speed isn't everything. In this case, the constriction has exactly the opposite effect as the shotgun choke. But the Venturi principle is preserved, of course, because physics did not go on vacation.


The coupling is likely a poor shape for a nozzle and induces turbulent flow at the exit.

Finally, into that reducing coupling, screw a 28" straight walled tube that is, let's say, 3/4 the diameter of the original hose. Turn the water back on. Now you get a solid stream like the first example, but it is going faster and further. Again, it looks like a full choke but it is, in fact, a cylinder choke, now on a smaller bore barrel.

The smaller bore tube maintains mass flow rate by exchanging the water's pressure for velocity. It maintains laminar flow so you have a higher velocity, cohesive stream.

So these three experiments seem to work exactly opposite of what happens in a shotgun choke. We have all done the hose experiment just fixing up the gardening equipment every spring. And we all have shotguns that we have patterned with different chokes. Clearly, different things happen in each. I am not so sure I trust fluid dynamics to tell me much of anything about chokes in guns. I trust it pretty well in garden hoses.

You are mixing differing flow conditions and you will not get the same result.

DDA

It's not a venturi, a water hose, or cars at a stoplight.

It's a shotgun choke. Analogies are not exact.

If the pictures on page 3 of this thread do not convince you that the shot charge fired from a choked barrel lengthens between 6 feet and 14 feet then nothing will. The only possible thing that could cause this is speed differential. The trailing pellets are going slower than the leading ones in spite of the drag front acting on the leading pellets. The trailing pellets must have been launched at lower velocity.

If there were an endless constant stream of pellets traveling through the choke, they would all behave the same at exit. This is clearly not the case in the physical shotgun.

The 18 foot photo shows the pellets starting to bunch up and spread due to the same effects that work on the cylinder choke shot mass. That's 6 yards downrange and still only 1/3 the diameter of the cylinder choke sample.

That is 'choke effect' plain and simple and that's all there is to it.

It's caused by a differential in speed between the leading and trailing pellets. If all the pellets shown in the 6 foot picture of the choked barrel were going the same speed, the pattern would already have started to mushroom like the photo above it.

Here's a WS-1 choke, if this site will allow me to post the link to another shotgun site...

http://www.shotgunworld.com/bbs/viewtopic.php?f=60&t=365315

This is what Bro. Compton said, which I totally do not understand

p. 129 The other variable which can be altered in the test barrel to change the internal ballistics is the chamber length. In Table 3.10 the standard deviation and pellet counts at 40 yards are given for a 36g load of #4 lead shot loaded into a 2 3/4� cartridge cases which were fired through a selection of chambers lengths. The averaged results show a definite alteration to the lateral dispersion of the shot cloud when using the wrong chamber length. The tighter pattern generated by 2 1/2� chamber length may be caused by the restrictive crimp opening acting like an internal choke.

OK Bro. Don; you're on