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

Generally, someone who has 'only skimmed through' at least looks at the pictures.

Oh, I did that and a bit more and the pictures certainly don't answer my question at all.

Choke works thusly:

Upon bore exit, a shot column will disperse due to aerodynamic effects.

With a cylinder bore (no choke), the drag acts on a wide front of pellets the diameter of the bore. The pictures show radial velocity starting shortly after muzzle exit. The classic 'mushroom'. Dispersion is fairly rapid as the front pellets slow and are pushed to the side by the rear ones.

With a choke, the 'nozzle effect' causes a speed differential, shown as elongation in the pretty pictures. The column is stretched out, and aero forces don't immediately jam pellets to the side as they do with a simple cylinder.

And that's about it.

The WS-1, the Tula, and the Beretta skeet chokes have the interior profile of a converging-diverging nozzle and undoubtedly add an outward radial vector by design.

Inward vector effect is controversial, to say the least.

How did I do, prof?

Brent,
A lot of people have tried to help you here, but they seem to be missing the validity of your question. Ignore the rudeness.
It is indeed possible to achieve greater than cylinder bore choke without any internal constriction whatsoever. As long as you go from a larger diameter to a smaller diameter you will achieve choke. It is called swaging and gunsmiths have been doing it for years. Whether it would achieve the same degree of pattern reduction as an internal choke remains to be seen. Historically fully swagged barrels were made before internal choke was commonly used. Your example where the bore diameter stayed the same would not achieve the desired result. Whether a gun with a small cylinder bore diameter would actually throw a tighter pattern than one with a large bore diameter is something that would need to be tested.

Do tell how a conical profile is 'no internal constriction whatsoever'.

Brent, the bore size isn't much of a factor, within limits. The convergent constriction causes the pellets to accelerate as they pass through. The only energy available to support acceleration is contained in the internal pressure of the shot column (air pressure and pellet to pellet contact pressure stored up from the trip down the barrel). If mass flow rate is not maintained, we have a "restriction" (as opposed to a constriction) and the barrel blows due to increased pressure. Failure to maintain mass flow rate occurs with shot bridging and obstructions.

During choke constriction passage, the pellets are rearranging themselves further forward, which means higher velocity. The instantaneous velocity and directions at muzzle exit (forward and sideways) predict the ballistic trajectory for flight in a zero gravity vacuum. Aerodynamic forces and gravity alter said trajectory in reality.

Zero choke (cyl bore) pellets do not experience the acceleration and pressure drop as above. Therefore, the front row of pellets are encountering aerodynamic drag while the trailing pellets are still at muzzle velocity. The cyl load experiences considerable pellet to pellet jostling at muzzle exit. Sideways velocities gained from this jostling will remain with the pellets all the way out.

Note that the muzzle diameter does not play into the above explanation. The factor is amount of constriction. While we state choke in diameter reduction, the real effect comes from bore area reduction. It isn't quite that simple because the area reduction must be at or near the muzzle. In a parallel bore, the wall drag will reduce velocity which will rebuild internal pressure. If the foregoing were not so, we could establish choke with the forcing cone.

Does that help. BTW, you have to think in fluid flow, not in simple mechanics.

DDA

The purpose of a converging-diverging nozzle is to achieve higher velocity of the ejecta. The converging section is capable of accelerating the flow mass only to sonic speed. The divergent section of the nozzle will accelerate the mass on past sonic speed. So, what is the sonic velocity of lead shot air mixture? I dunnnno.

DDA

RocketMan
Referring back to something mentioned in a previous post:
Several experienced shooters
have told me that a cylinder bore 410 had a longer killing range than a cylinder bore 12 gauge. I never tested it, but I take it from your computations they were wrong.

Brent, don't even bother thinking of physics. Rather, think of it this way: If bore diameter absent constriction had anything to do with making a pattern tighter, then we'd all be shooting smaller bore guns in order to get tighter patterns. That little .410 with no choke would produce way tighter patterns than that big .730 diameter 12ga. But in fact, if both the .410 and the .730 have no constriction at all, then they will both produce very open patterns that are very similar. The only advantage a smaller bore has over a larger bore when the concept of choke is applied is that a .410 requires less constriction to produce a full choke pattern than does a 12ga. That's because it's not the AMOUNT of constriction involved that gives a tighter pattern, but rather the PERCENTAGE of constriction compared to the bore diameter. .010 constriction in a .410 is thus more significant than it is in a .730 12ga.

I would ask them to prove it.

SRH