Technical Question / Discussion (NOT hc/pc discussion)

[roundball]

Technical discussion about "Recess / Jug / Tulip" choke designs in Smoothbore muzzleloader barrels.

Over the years I have learned how these chokes are made...have seen sketches like the one below...have looked into Jug Choked bores...and have used Jug Chokes in different barrels (Full and Imp. Cyl PLUS). And I know they work, are effective, etc.

I understand that the shot charge expands into the expansion chamber and is then forced back down to the original bore size at the muzzle before muzzle exit. BUT.....what I'm not 100% certain about is what 'physics' are at play, what 'dynamics' take place on the shot charge...that even though the shot charge is still exiting at original cylinder bore size...the pattern is more dense at longer distances.

The only understanding I seem to hang on to is that the outer layers of the shot charge begin to ever so slightly start to slow down as they expand into the larger diameter expansion chamber...while the central core of the shot charge is continuing ahead at full speed.

Then at the forward end of the expansion chamber, the whole of the larger diameter of the shot charge is tapered/necked back down on the back half/back end of the central core of the shot charge and in essence simply makes a longer shot string which pours more shot onto the center line of the shot column headed towards the target.

We know that modern "constrictor" type chokes like ImpCyl/Mod/Full/Extra Full are literally / physically smaller than cylinder bore and act on the shot charge as a forcing cone.
I also know the muzzle end of the expansion chamber in a Jug Choke does the same thing.
HOWEVER...the huge difference is that the muzzle end / exit is still at wide open Cylinder Bore.

Does anybody really know of an official description that confirms or is different from the physics / dynamics I'm struggling with?
What actually happens to the shot charge that makes it "act like" its coming out of a physically smaller Full constrictor choke size diameter...putting more shot into a smaller area at distance...when in reality it's actually still exiting a wide open cylinder bore opening?


RECESS / JUG / TULIP CHOKE IN A CYLINDER BORE BARREL

 

[Skylinewatcher]

I'm not sure about the exact physics of your question, but I am a full subscriber to the relationship of shot dispersion relative to shot hardness id est, the amount of antimony. Of course, the propellant charge plays just as an important role. As the shot seeks its exit and suffers no deformation it will retain the intended choked distribution. However, it is my belief as it is others that when soft shot seeks its exit and deformation occurs the shot "fight" for the same exit, therefore deforming the spread. Other than that, I don't know. :grin:

 

[roundball]

What actually happens to the shot charge that makes it "act like" its coming out of a physically smaller Full constrictor choke size diameter...putting more shot into a smaller area at distance...when in reality it's actually still exiting a wide open cylinder bore opening?

OR...

It may just be that a Jug Choke acts no different than a restrictor choke and the muzzle diameter size is irrelevant.

With a modern smoothbore the barrel diameter is "Fixed" diameter all the way to the constrictor choke, and different size chokes can be installed to get the choking effect.

With a Jug Choke, the reverse is true...the exit diameter is "Fixed" at cylinder bore and the internal size of the barrel (expansion chamber) is what's changed to affect the degree of choke.

So in that regard, the choking operation is the same as if it was a modern constrictor choke.
If so, then the question is, what are the actual physics / dynamics that act upon a shot charge as it necks down through a modern constrictor choke that puts more pellets into the center of a target.

Same answer?
The forcing cone effect changes the shape of the shot charge, won't let it all exit evenly, a squeeze play occurs that creates a longer shot column, hence a longer shot string, more shot pellets on the center line to the target?

 

[George]

I've never seen a jug choke. What is the distance between the muzzle end of the recessed chamber, the "forcing cone" part of it, and the actual muzzle?

Spence

 

[Skylinewatcher]

From your diagram I would suggest the shot are able to settle themselves before the choke area. This is where I feel the shot hardness would come into play. I think a certain manufacturer acknowledged this line of thinking by developing the cube shaped shot.

 

[roundball]

I've never seen a jug choke. What is the distance between the muzzle end of the recessed chamber, the "forcing cone" part of it, and the actual muzzle?

Spence

From reading articles associated with diagrams like this, and looking at actual bores, the front end of the expansion chamber forcing cone tapers back to normal cylinder bore size about an inch in from the muzzle.

 

[roundball]

From your diagram I would suggest the shot are able to settle themselves before the choke area. This is where I feel the shot hardness would come into play. I think a certain manufacturer acknowledged this line of thinking by developing the cube shaped shot.

Soft shot / hard shot is not really what's being sought here...yes, hardness will affect the degree of a pattern...but this is looking at the physics of how a pattern gets choked, period.

Soft vs. hard shot pattern efficiencies would be a separate discussion...like once its learned how a combustion engine / transmission / differential operation works to turn the wheels of a vehicle, then the discussion could turn to what gives the best traction...soft rubber, hard rubber, etc.

 

[George]

.... the front end of the expansion chamber forcing cone tapers back to normal cylinder bore size about an inch in from the muzzle.

That's the part I find confusing. If the recess was positioned so that the muzzle end of the forward taper ended exactly at the muzzle then that would be very similar to the regular constricted bore type of modern choke.

I believe your idea of thinking about the diameter of the recess as the bore diameter is on the right track. Then I run out of steam. I keep wondering what would happen if you put that recess 12 inches behind the muzzle instead of 1 inch.

Spence

 

[Capt. Jas.]

Roundballs
I think the jug and modern constriction have commonality in operation. In a jug barrel the muzzle is really not cylinder bore but a constriction because the jug behind it is expanded. The jug depth in effect, becomes the cylinder bore size. Difference between the bore size (the deepest point of the jug/ bore size on a standard barrel) and the constriction becomes the amount of choke.

 

[Skylinewatcher]

I disagree, you are focusing only on one side of the equation. You must take into consideration the nature of the substance being forced through your choke. Kind of like toothpaste from a tube vs water from a hose. But whatever, the problem really isn't worthy of loosing sleep over. You either kill game with your rig or you don't. Hope you figure it out. :grin: :v

 

[CoyoteJoe]

It is the constriction from the recess back to bore diameter which determines the choke effect, just as if the recess diameter were the full length bore diameter. The shot enters the recess and doesn't remember what the bore diameter had been before the recess.
As to how far from the muzzle the recess should end, most high grade shotguns of old included a "parallel" about one inch long between the taper and the muzzle. I'm speaking here of conventional taper chokes in breechloading guns. It was believed that the parallel offered some improvement in the consistency of patterns but that has pretty much been dispensed with. By dispensing with the parallel one can use the same reamer for any degree of choke, just running it farther out the muzzle for lesser degrees of choke. On new guns today most chokes are formed by swagging rather then boring so the parallel is omitted to simplify production. I have seen removable choke tubes which retain the parallel section at the muzzle end but don't know if it really improves patterns.
In cutting jug chokes I have always ended the taper 1/2" to one inch back from the muzzle, leaving the aforementioned parallel. Based on chokes of modern shotguns I suspect it would work just as well if I brought the taper right up to the muzzle but I never tried that, it would require modification to my tools and I never saw a reason to do so. As to how far back the recess could be located, again I've never experimented to find out. I certainly think it would loose it's effect if too far from the muzzle but I don't know how far would be too far.

 

[leslie]

I think you should look up information on the Venturi affect,understanding the physics involved might help in understanding whats going on.Now days I lean more to the idea if it works I do not care why,its been 15yrs since I quit working as a CE and I don't miss anything.I do admire your wish to know why and good luck in finding an answer.

 

[CoyoteJoe]

Understanding how a thing works is the essential first step toward making it work better. :haha:

 

[Fred_Dwyer]

Not gonna be so bold to say I understand how chokes work; but the way I heard it explained that makes sense. The taper of the choke force the outer layers of pellets inward, this forces the inner core of shot to accelerate forward since shot being not so compressible. There isn't much acceleration in the last inch or so of bore so there's a percentage of pellets that move ahead of the shot column. The parallel section lets the column stabilize in its' new configuration before exiting the muzzle, otherwise the movement inward of the outer shell of shot would disrupt the column and it would be a spreader muzzle rather than choked.

It could be likened to squeezing toothpaste from the tube, the center always bulges forward ahead of the main column of paste. So if you cut off that first bit and sent it on the way to a target it would have a hotter core.
Now that's a bold face lie. Toothpaste shape comes from other effects, laminar flow and boundary layers yadayada. But it paints a good picture of what's going on with shot.

 

[roundball]

In another discussion it was speculated how interesting it would be if some slow motion film could be made as the shot charge is going through the contortions of being choked...and particularly so if different layers of shot could have different colors.
Bet that would be something, just knowing how revealing the slow-mo photos / videos are that Larry Pletch has produced about Flint lock ignition...

 

[roundball]

The simplest explanation I've been able to find on the Internet comes from one of the major choke manufacturers...explained using the analogy to a common garden hose.

Turn the hose on and lets say water is coming out softly in an arc just a couple feet long before it curves down towards the ground.
Then as we know, if we hold a thumb down over part of the hose end (muzzle) we're restricting the size of the opening the water can come out of.
But with pressure still pushing it, it travels a much farther distance in a smaller tighter column before it starts to slow and spread out from air resistance.
Shot charge acts the same way...put a squeeze play on it with some choke and it squirts out further before slowing down and spreading from air resistance.
The explanation didn't get into the actual dynamics of "inner layers vs. outer layers" but had some value.

I remember years ago when Lowell Tennyson, Iowa Jugged my first .62cal, he tried to explain that it caused the shot to squirt out like a fire hose but I still didn't understand the dynamics of what was actually happening to the layers of shot pellets

 

[Fred_Dwyer]

Nevermind.

 

[Fred_Dwyer]

OK, maybe this source works?

You can see the nose of the column made up of the slightly accelerated shot. If a square column of shot encounters air I'd expect it to mushroom like a slug hitting water. But the shot column is backwards from that. There are even a few "rabbit" pellets that were further bumped ahead of the main shot column.

I did a little math and though the taper on a choke seems gradual with a shot charge passing at 1100 FPS that passage take about only 0.00015 seconds. That is like that choke slapping the sides of the column and is seems readily apparent this "slap" squirts some shot ahead.

 

[roundball]

An excellent series of snapshots...stands to reason something has to give and stretch out...can't fit 10lbs of potatoes into a 5lb bag.

What's so revealing from the photos is that the choke didn't just 'taper-spread' the shot charge in a linear fashion...there is definitely that spurt of lead out front...some kind of squeeze play at work there for sure.

 

[paulvallandigham]

Please note that this set of pictures shows shot fired out of a modern shotgun shell with a plastic shot cup, and NO OS cards in place.

I do believe that Jug Choke does SQUEEZE the shot forcing the column to elongate( forward) in the center, much as you see that core area shot pushing out in front of the rest of the shot in mid-air.

However, I also think that the 1 inch of Straight bore allows the OS cards to straighten out( if they tipped from the core pellets pushing on them from behind) and for the column of shot behind the OS card to regain its cylindrical form before exiting the muzzle. I don't believe that short- 1"-- section gives sufficient time to defeat the choke effect on the shot load, and that the load probably is still a bit elongates at the time it leave the muzzle.

I agree with Roundball that much of our questions would be answered with time lapse photography of a BP ML shotgun pictures as the shot leave the muzzle- comparing a cylinder bore barrel, to one with jug chokes, and also to one with a Screw-in muzzle choke tube being used.

I don't have a jug-choked barrel. I have never fired one. I have wanted one for more than 40 years, but just never saw a sufficient use for such a barrel to justify having it done. Only in the past 10 years have enough wild turkeys been introduced around here that we have a season now on them. 30 years ago, when I was more physically able to do that kind of hunting, you had to drive many hours from here to even find turkeys to hunt.

I do have experience with elongating the throats of shotgun barrels with long tapered reamers- these being breechloaders. And I understand the practice, and concept of BackBoring shotgun barrels- either the long tapered( from chamber to muzzle)- or those new barrels that begin as 10 gauge, and have choke tubes near the muzzle that choke the 12 gauge shot loads down to 12 gauge again.

All these efforts are variations on a theme. I can't see there being much difference in how the chokes effect the shot passing through them, unless the choke is short enough and at a steep enough angle to damage shot pellets as they enter that short taper. Damage to modern shot pellet loads is now largely mitigated by the use of those plastic shot cups.

Damage is what happens in factory guns with short tapered throats. I long-tapered my barrels on my 12 gauge 30 years ago, and have no targets shot before and after now. However, the improvement in the ROUNDNESS of the patterns after elongating the throats was clearly apparent, and pattern density After the throats are elongated is much more uniform.

A now deceased friend of mind, Jim Gabbard, had a working relationship with some of the Execs. at Remington, who provided him with a limited run barrel for his Mod. 1100, that had a long tapered Muzzle choke-about 3" instead of 1". That barrel shot far better patterns than any other factory barrel he owned, with any load he shot in it.

He won a lot of matches with that barrel. He kept a closet full of before and after patterning sheets for that gun to show other shooters who visited his home and shop.

Sadly, Remington didn't ever offer that kind of choked barrel in its commercial line, even in its "Trap Grade" guns. Jim got that barrel just before Screw-in tubes became the rage, and I suspect that affected the decision by Remington bean counters to go that way, rather than making a more expensive long-choke Trap barrel.

So, until we find someone willing to take pictures of shot loads leaving the barrels of BP MLing shotguns, I think we all have to wait for the true answers. I don't think we are far off in our educated guesses as what is happening.

Thanks for the pictures, BTW. :hatsoff: :hatsoff: I only wish the pictures indicated the distance from the barrel for each picture shown. My own tests, done with Jim, showed repeatedly, regardless of choke used, that shot loads did not begin to expand in diameter beyond bore diameter until they are approx. 9 feet from the muzzle. That is meaningless in the field, but is rather important when considering using a shotgun INSIDE a home for home defense purposes.

Oh, I also think that Using slower burning black powder( like Fg vs. FFg, or FFFg, will affect patterns, simply because we generally are dealing with shot loads leaving the barrel at under the Speed of sound. Modern shotgun load RARELY leave the barrel at less than the Speed of sound, altho almost all have dropped down through the sound barrier in the first 20 yards in front of the muzzle.

As BrownBear has been reporting recently, his use of Fg powder in his smooth bores has produced better( tighter) patterns than what he gets using either FFg or FFFg powders.

Not only is the velocity achieved with the same volume of powder LESS, but the burn rate is SLOWER, giving less of a " bump" to the shot when its gases first overcomes the inertia of the shot sitting in the barrel. That should mean less distortion of pellets in the loads, when they leave the muzzle, and when they pass through any Jug choke, or muzzle choke. :hmm: :thumbsup: That is why I intend to acquire a pound or 2 of Fg powder my next time ordering, and do some of my own testing using this powder in both my 12 and 20 gauge smooth bores. :idunno: :hmm: