touch hole: how big is too big?

[M. De Land]

I have an unproved thought that may or may not work out to have any merit over the long term.
I have noticed flash hole erosion that eventually burns the port out of round may have more of a deleterious effect on accuracy than previously thought.
My thinking is that the venturi effect of escaping gas from an irregularly eroded vent makes internal barrel pressure fluctuations that effect and influence both harmonics and velocity.
Possibly the irregular hole also effects ignition uniformity from pan ignition.
My guess is that the shape of the flash hole may be more important than the size.
Beveling,chamfering or funneling a flash hole has advantage in the speed of ignition but also presents less metal wall thickness to be eroded out of round. Platinum plating would seem to make a good deal of sense to me in maintaining flash hole integrity and thus consistency of ignition once the fire is lit.
Kicking around some thought generated by a flint pistol I have void of a liner and the hole is beginning to eroding out of round.

 

[Artificer]

My thinking is that the venturi effect of escaping gas from an irregularly eroded vent makes internal barrel pressure fluctuations that effect and influence both harmonics and velocity.

I have been thinking about this and have to say I do not understand how in irregular shaped hole would cause internal barrel pressure fluctuations from shot to shot. If all else is equal in the loading process, gas pressure acts equally throughout the container. This means the gas pressure would act equally on the vent hole, no matter if it were perfectly round or not. So I have to say I don't understand where a fluctuation would come from.

Gus

 

[Larry Pletcher]

I have been thinking about this and have to say I do not understand how in irregular shaped hole would cause internal barrel pressure fluctuations from shot to shot. If all else is equal in the loading process, gas pressure acts equally throughout the container.
Gus

Gus, I tend to agree with you on this. I believe that regardless of the hole shape or even size that the resistance to gas flow determines the amount of escaping gases and therefore the velocity of the ball. I also believe that the resistance to gas flow is primarily related to the circumference of the vent and length of the vent - similar to the way a garden hose limits flow by resistance in the hose. I believe that the relationship between circumference and area are important.

These ideas do not help explain shot to shot variations, but do explain changes experienced by a shooter who replaces a large vent with a smaller one or installs a small 2-hole vent as I reported in an earlier link.

For shot to shot variations experienced with larger vents, I would look for an another variable perhaps caused by the larger vent. For instance does a smaller vent "self-clean" better that a large vent? Could a shooter with a large vent diameter lessen the variations by cleaning the vent after every shot? I don't know the answer here, but am pushing a different way to think about it.
Regards,
Pletch

 

[M. De Land]

I think once the hole has changed from round to irregular the rate of erosion is increased and the internal pressure change is linear with this eroding vent hole change.
I also think that the ignition from pan flash back the other direction would be effected by round to irregular vent shape change.
A round hole will vector discharge at a constant rate and direction that an ever changing eroding hole will not.
It looks to me like once the hole is no longer round that the erosive effect of hot gas is increased negating uniformity of internal pressure and pan ignition.
Kicking ideas around.

 

[Artificer]

How many shots have to be fired before the vent hole is no longer round? How many shots have to be fired before the vent hole enlarges enough to affect the normal variation of gas pressure from shot to shot in a single string, or even a days shooting and change the group size from earlier groups?

How much variation is added by not weighing the primer pan charge of powder? How much variation is due from the shape of the flint as it erodes from shot to shot? How much does the amount of sparks thrown into the pan cause variations in the speed of setting off the main charge. (Pletch used a heated wire to get rid of many of these variables in his testing, as I recall.)

Though I understand with enough shots fired that the gas pressure will be reduced in the barrel when the vent hole erodes and thus cause less accuracy in a group than many shots fired before, I don't see how an out of round vent hole is going to change shots in a string or even in a days shooting from the rifle. With all the other variables that can cause poor ignition time and other changes of gas pressure in a barrel, I am not even sure how we could quantify how much an enlarged vent hole reduces accuracy over short term and until the hole gets very large and out of shape?

Gus

 

[RichardPitts1]

There are a number of difficulties accurately modeling the behavior of a gas moving through the touch hole. Many of the liners I've seen had a cone machined into the inner surface, effectively turning the liner into a venturi type nozzle. It's counter intuitive, but the pressure inside the throat of the nozzle is much lower than the pressure on the intake side due to the acceleration of gasses moving through the orifice. This effect actually draws gas into the flow channel.

Now the difficulties come in when you try to model molecular adhesion to the wall of the flow channel i.e. gas friction, as well as the fact that you are definitely not in a laminar flow condition inside the orifice. Add to that, the fact that pressure is not stable inside the chamber, instead that it builds until the ball starts to move, then immediately starts to drop as the volume behind the ball increases and you have a very difficult system to model.

The point of this rambling diatribe was to quantify the "unknown variables" mentioned earlier in the thread. You have a pressure wave, a dynamic gas flow problem, and a turbulent flow through an orifice going on simultaneously.

Honestly, variations in your powder load, atmospheric pressure, and the thickness of your patch/friction of the lube you chose will have a much larger effect than the shape/diameter of your touch hole. Maximizing efficiency could come from experimentation or computerized finite element analysis. The problem is that real world variation makes your calculations irrelevant most of the time, and you're left with the simple question: Does the fire stick go boom? good.

 

[M. De Land]

Good counter argument guys! Your thoughts are logical and make a great deal of sense.

 

[Artificer]

That was a very impressive post, thank you.

Though I was never educated as much as you in the science involved, I was involved in a career where gas ports were at least partially responsible for extremely accurate suppository guns and that left something that probably applies to muzzle loaders as well. There were quite a few small things that we found had made some rifles shoot more accurately, but not all the rifles and not all the time. Some things only made a very small percentage of rifles better for accuracy. Since we could never know what would make each individual rifle shoot better, we put all the small points into every rifle, as we found those points over the years. What that did was eliminate some to quite a few variables, but we could never get rid of all the variables. Still, it made for building ever more accurate rifles over the years.

We were also extremely fortunate to have some real mechanical engineers who had a great deal of experience with firearms, including some from H.P. White Laboratory, and were kind enough to answer many of our questions. Some of the things we came up from testing were explained by them on why they worked and some other things even their answer was, "Too many variables to even explain it, but if it works, it works."

Gus

 

[Black Jaque Janaviac]

To report back:
I finally got a chance to get out shooting. It appears that the new liner with smaller touch hole (whatever size comes with a WL) does not remedy the problem. There is a slight delay.

It Really forces me to pay attention to follow-through.

Firestick still goes boom, just not as quickly as some flintlocks I have shot.

 

[Larry Pletcher]

To report back:
I finally got a chance to get out shooting. It appears that the new liner with smaller touch hole (whatever size comes with a WL) does not remedy the problem. There is a slight delay.

It Really forces me to pay attention to follow-through.

Firestick still goes boom, just not as quickly as some flintlocks I have shot.

On a freshly installed WL liner, one of the last steps is to file the liner flush with the barrel. This may leave a sharp edge around the vent. I like to spin a counter sink by hand to clean off any burr that is there. I have also more aggressively used a counter sink to leave a mild exterior cone.

IIRC, Chambers sends the liners out with a .055" hole. I like to increase this to 1/16" (.0625").

Currently I'm playing with a .58 flint hunting rifle with an internally coned vent made with a Tom Snyder tool. (This is designed to copy a Chambers WL interior.) We cut the interior to about .020" of the barrel surface. The remaining "web is .067". Then we cut an exterior cone with a shape similar to an intake horn on a hotrod carb - no edges, rounded instead. This last came from an engineer who dealt with fluids. He said that the fastest way to get fluid, gases into a hole was with a bell shaped opening, at least that was my non-engineer interpretation.

Regards,
Pletch

 

[smo]

To report back:
I finally got a chance to get out shooting. It appears that the new liner with smaller touch hole (whatever size comes with a WL) does not remedy the problem.
There is a slight delay.

It Really forces me to pay attention to follow-through.

Firestick still goes boom, just not as quickly as some flintlocks I have shot.

Drill it out... :idunno: If you can distinguish any loss of power from the enlarged hole... Add more powder!!! If it gets out of round or too large ... just replace the liner and start over... :shocked2: :stir: :dead: op:

 

[M. De Land]

I have always been interested in the interior mating of liners and barrel interior radius.
I think perhaps I will try reaming the barrel interior ahead of the plug to groove diameter after the liner has been fitted.
This should leave a very smooth, no corner, combustion area that will be very easy to keep clean.
I have never liked the idea of and internal cone any way as it does not hold pressure as well.
Carbide reamer should work nicely to cut the harder than barrel steel liners.

 

[Black Jaque Janaviac]

Thanks Smo,

Well the problem with the delay seems to be unfixable. The old liner already had a larger touch hole.

I will probably drill out the hole to 1/16" just to eliminate the occasional flash in the pan. But my hopes of solving that slight hesitation are dashed.

 

[Billnpatti]

Love the number crunching, Zonie. It has produced an excellent premise for an experiment to test your reasoned hypothesis. Use a gun with a removable touch hole liner. Start with a very small hole, fire a few shots (3 to 5...more would be even better) and record the MV and calculate the std. dev.. Then enlarge the hole slightly and re-do the test. Enlarge it again and repeat the test until you have ended up with an oversize hole. It would be interesting to see how well the test data matches the theoretical data. :thumbsup: