touch hole: how big is too big?

[George]

But, I have been wrong before.

I can identify with that. I made a mistake, once. Back in 1951, I think it was. My memory of it is clearer than yours, though, I know my mistake. It was that I thought I had made a mistake, but hadn't. :wink:

Spence

 

[Black Jaque Janaviac]

650
It could be that the .50, though it has a longer barrel time, has another quality that overrides the advantage that the .62 has with respect to barrel time.

The question is: would you improve by shortening the barrel time of your .50?

 

[Artificer]

Too large and you start to loose breach pressure. This can result in irregular breach pressures which translates into irregular muzzle velocities and this means a loss in accuracy.

I know that's the idea, but I don't buy it. I can't think of any reason pressure would be variable because of the size of the hole. Can you?

Spence

Definitely agree with Spence on this one.

Gas pressure (created by the burning powder) expands equally inside a container. So when the touch hole is larger, more gas pressure will leak out faster and thus lower the projectile velocity. Increasing the powder charge to compensate for the larger touch hole, will ensure velocity is the same as with a smaller powder charge and smaller touch hole.

Gus

 

[hanshi]

I did make a mistake once; but it was on purpose, though, just to see what it felt like. :grin:

 

[Danny Ross]

Thing is he said with no projectile he doesn't notice the difference when the powder goes off and makes a bang, only with a projectile. So if there is no delay with no projectile there shouldn't be a perceived difference with a projectile. With a projectile traveling at let's say 1500 fps when it leaves the barrel, the time it takes for the projectile traveling down a 42 inch barrel would not be humanly noticed. DANNY

 

[smo]

I think the theory is the delay is that the split second it takes for the pressure to become sufficient enough to initially start the ball on its way down the barrel. :idunno:

I do know with a larger touch hole it is easier to get powder behind the ball if one has "dryballed".

On the off side of that it also makes a larger hole for the pressure to escape and not shoot the ball out. It will sit there a blow a flame back out the touch hole and never move the ball.

Don't even ask me how I know this... :redface:

 

[Billnpatti]

Let me give this old horse one more lick and then I'm done. :dead:

I truly do understand the idea behind what you are saying, yet, most all bench rest shooters prefer to have a much smaller touch hole claiming that they give a more consistent MV. In fact, due to this desire for a smaller touch hole by bench rest shooters, the Chambers White Lightning touch hole liners come with a very tiny hole. I find that when I use one, I have to drill out the hole to 1/16 inch.

Also, when a touch hole becomes eroded to the point that it is judged to be too big, the owner will replace it. Why? Apparently larger holes have an undesirable effect on the performance of the gun. Most folks to whom I have talked about this problem say that the accuracy declines when the touch hole becomes eroded.

Oh well, I am no expert on the subject and it really makes no difference to me at this point in my life because I don't get to shoot often enough any more to ever have an eroded touch hole. So, for me, it is a moot question. I still have a replacement White Lightning liner in my gun tool cabinet and I don't expect to ever need it....but I've got it just in case.

 

[Black Jaque Janaviac]

"Does the theory match our experience in real life?"

Now that I think about it: I know when reloading for supossitory guns they often concern themselves with standard deviation of muzzle velocity. Thus sometimes a particular load will produce inconsistant MVs in spite of the fact that all variables are controlled. I know smokeless is a progressive powder. But I seem to recall Mad Monk saying that BP will burn slightly faster under pressure, just not expoentially.

 

[Larry Akers]

Thing is he said with no projectile he doesn't notice the difference when the powder goes off and makes a bang, only with a projectile. So if there is no delay with no projectile there shouldn't be a perceived difference with a projectile. With a projectile traveling at let's say 1500 fps when it leaves the barrel, the time it takes for the projectile traveling down a 42 inch barrel would not be humanly noticed. DANNY

The "no delay" with a blank load is because the expanding gas has no work to do. It is free to expand as fast as it can so the report sounds instantaneous. With a ball loaded in the barrel the gas is no longer "free" but must do work--overcome inertia and start the ball moving and then overcome friction all the way down the barrel which results in a humanly perceptible delay in the report.

In the second part of your statement, the ball does not reach 1500 fps until it reaches the muzzle. It starts at zero and accelerates the length of the barrel. The acceleration is at a faster rate with a small touch hole because of less pressure loss.

I have to respectfully disagree with you and say that a person used to shooting a gun with a large touch hole can definitly tell a difference in "ignition time" when switching to a small touch hole.

 

[yonderin]

I will suggest that the pressures don't necessarily vary from shot to shot that much, but any tiny variations in resistance such as fouling or differences in lube consistency might become more pronounced (although not to our perception on firing). Yes, that inconsistency could noticeably affect accuracy.

 

[Artificer]

I will suggest that the pressures don't necessarily vary from shot to shot that much, but any tiny variations in resistance such as fouling or differences in lube consistency might become more pronounced (although not to our perception on firing). Yes, that inconsistency could noticeably affect accuracy.

Once one finds the ball size, type and thickness of patch, lube and type of powder plus the amount of powder, type and "setting" of the flint that the gun likes best; then it is a matter of uniform consistency in loading that will have a lot to do with what I call mechanical accuracy or what Spence has referred to as (I think) the accuracy of the gun itself.

Then it is up to how accurately the shooter shoots the gun.

I prefer to spend more time experimenting to find what the rifle or gun likes best, than perhaps some others do. Then I know any accuracy problem is with the way I shoot the gun.

Gus

 

[yonderin]

Roger that. My point is that the ballistics would remain the same from shot to shot if everything is identical, so size of the vent would affect the amount of gas escaping and therefore the vertical path. A larger vent would allow a wider range of gases to escape through the vent if there was difference in resistance (from shot to shot) for some reason, affecting the forward motion of the projectile. I think a smaller vent would better provide for consistency.

As an extreme, if simply using a large vent with identical conditions from shot to shot resulted in pressures that were too inconsistent and resulting loss of accuracy, recoilless weapons would have likely never had much application for that reason. I will concede that they are likely less inherently accurate than closed breech systems.

Accuracy is the visible result of consistency of variables, both internal and external, and we generally can’t absolutely control all of them all the time.

Hopefully this makes sense. I sometimes don’t express myself well. When push comes to shove, another way of acknowledging what others have said.

 

[Artificer]

You made some good points in the post above.

One thing I have to politely disagree on is that recoilless rifles actually prove that 1. The vent hole does not have to be perfectly round to be consistent for accuracy and 2. Until the Vent hole gets so large that it allows too much gas pressure to escape, so it does not consistently overcome getting the projectile moving and down the bore, accuracy will be just as good as with a smaller vent hole. (I am just old enough to have worked on 106mm recoilless rifles in my early years in the Corps. These were every bit as accurate as fixed ammunition guns in the projectile and barrel, but could suffer less accuracy when the gun mounts were not as stable as on regular artillery carriages/mounts.) Gee, never thought about that until you brought it up and I've been shooting flintlocks since the days I still worked on recoilless rifles.

Of course as the British Infantry found out during our period, a worn/too large vent hole would allow a jet of flame that was injurious to the soldier standing on the immediate right (in formation) of the musket with too large of a vent hole.

Gus

 

[yonderin]

I agree that shape of the vent likely makes no difference but reason that the larger flash hole will vent more gas through it and reduce the velocity of that shot if resistance to the bullet increases. Lower velocity for any reason equals longer time in barrel resulting in more time for shooter error (flinch, etc) and barrel harmonics to come into effect. The expanding gases will attempt to follow the path of least resistance and a larger vent will provide less resistance than a small one; therefore a smaller vent will hold the gases in better and better nullify the differing amounts of resistance.

To belabour the point, the more that all variables are held to a constant is what contributes to accuracy. Everybody has their own combination to get to what they find acceptable, whether it’s one hole groups at 100 yards or “minute of deer”. Change any one thing in the combination and results may change noticeably (for better or worse). And it may be internal or external factors. We mere mortals may not be able to perceive the differences except for the terminal results down range.

Wish I hadn’t been such a sweathog in school, a ballistics lab suddenly seems like a fun place to work.

Respectfully.

 

[Zonie]

I have no doubt that increasing the size of the vent hole improves a flintlocks reliability of ignition.
I've seen too many cases of increasing the size of the vent greatly reduced slow ignition or "flash in the pan" failures to fire.

That said, I think the effect of enlarging the touch hole on accuracy depends greatly on the size of the bore the gun has.

Just out of interest I poked my button box and calculated the following. It shows the bore size, followed by the area of a .078 diameter vent hole divided by the area of the bore, in percentage.

The larger the percentages allow a larger percentage of the powder gas to escape so the change in velocity will be more noticeable.

.078 diameter vent
.31 = 6.33%
.36 = 4.69%
.40 = 3.80%
.45 = 3.00%
.50 = 2.43%
.54 = 2.10%
.58 = 1.81%
.62 = 1.58%
.75 = 1.01%

I then calculated the percentage of the bore area with the area of a 1/16" (.0625 diameter) vent:

.0625 diameter vent
.31 = 4.06%
.36 = 3.01%
.40 = 2.44%
.45 = 1.93%
.50 = 1.56%
.54 = 1.34%
.58 = 1.16%
.62 = 1.02%
.75 = 0.69%

To figure out how much the increase in vent size area would be when going from a .0625 diameter vent to a .078 diameter vent I subtracted the percentages shown above from one another and arrived at the following effects.
This shows how much drilling out the vent would have on the amount of gas leakage.

Increase in vent area from enlarging a .0625 diameter vent to .078 diameter

.31 = 2.27%
.36 = 1.68%
.40 = 1.36%
.45 = 1.07%
.50 = 0.87%
.54 = 0.76%
.58 = 0.65%
.62 = 0.56%
.75 = 0.39%

I have no proof but I suspect that a percentage difference of less than 1 percent would have minimal effects on the amount of gas leakage or in velocity of the projectile.

This rational explains why I have no hesitation about using a .078 diameter vent on rifles .50 caliber and above.
The .45 and smaller may show more noticeable effects if the vent size is increased.
This is why I would hesitate to enlarge the vent hole this much.
IMO, a vent hole of around .070 diameter would be about the limit I would use on the smaller .45 and I would not go beyond .0635 (#52) on a .36 or .32 caliber gun.

 

[yonderin]

I agree that enlarging a flash hole would increase reliability of ignition. I agree that if everything remains constant from shot to shot the vent size itself is not a point of concern. Artificer confirmed that recoilless guns can be every bit as accurate as a closed breech system. Even though, I’ve read, up to 90% of the propellant is expelled through the venturi. Astronomical inefficiency compared to a similar charge in a conventional gun but in itself no problem.

Difficult for me to explain; A hydraulic cylinder that freely bleeds off fluid while being pumped full. Under ideal conditions it’s a constant and the results can be predicted. Increase the size of the bleed off port, no matter because it can be predicted and compensated for. Now do something to change the internal dynamics (something creates additional resistance or a seal starts to leak). If this is something that varies from application to application, that’s where the loss of repeatability comes in. A larger bleed off port (vent) would allow a greater range of inefficiency than a smaller one.

Not trying to advocate against larger vents. Merely suggesting that the results of any internal inconsistencies may be more pronounced with a large vs small vent. If everything is kept the same from shot to shot (swabbing, patch lube, whatever) I have no reason to think that a larger vent in itself would create an accuracy issue. Looking at it from another angle, a large vent that produces good accuracy might be a better indicator of more constant application of variables than a small vent. IF my theory is correct.
One day at the range I used a larger vent liner and switched back to a smaller one. I don’t recall any loss of accuracy but certainly it didn’t impress me enough to keep to the larger one. I thought I did notice a larger flash on ignition but wasn’t surprised by that.

Boy, I can see a summer of experimenting coming up.

 

[Larry Pletcher]

I appreciate your math and would like to throw in another wrinkle. Builder and CLA member, Allen Sandy had a beautiful rifle at CLA with 2 small vents instead of a larger single vent. Fred Stutzenberger wrote it up in Muzzle Blasts. After testing his conclusion was that the small double vent lost less gas through the vent than the single vent (assuming they both had the same area with differing sidewall friction).

Steve Chapman and I timed two comparable vents - a 2-hole vent with .054 diameters vs a single vent at .070. I hope I remember the numbers correctly. Anyway, vents were drilled so that the sum of the area of the smaller holes equaled the area of the larger hole (as best we could).

Our concern was that if as Fred noted, the smaller holes limited gases exiting the vent, might they not also limit gases entering the vent from the pan. After testing we did find that ignition by the two hole vent was slower than the single vent.

I should point out that Fred and I tested two different things. Fred noted an increase in velocities with the 2 hole vent. His conclusion was that the increased velocity was caused by more friction in the 2 hole vent. That vent does have more circum. than the single vent.

Steve and I measured ignition speeds and concluded that the same increase in side wall friction caused less gases entering during pan ignition and therefore slower ignition.

Regards,
Pletch

 

[Zonie]

Makes sense to me Pletch.

While the two vents have an open area that is equal to each other, that only applies in a static condition when nothing is moving thru the hole(s).

When something dynamic is happening like gas is flowing thru the hole(s) a different condition comes up.

Because the gas (or fluid) right at the wall of the hole is basically not moving while the gas towards the center of the hole is, there is a transition zone where the gas is moving slower. This creates an "effective" open area of the hole where the gas/fluid can pass freely is less than the actual area.

The amount of this dynamic effect depends on the density of the gas/fluid and the velocity it is traveling thru the hole.

The greater the velocity is (or the more dense the fluid), the more that the dynamic reduction in the effective hole size becomes and I believe the velocity of the escaping powder gas thru each of the smaller holes would be greater than the single hole vent.

This basically causes the smaller diameter two hole vent to "choke down" or become effectively smaller than the single hole vent which explains why the velocity of the shot was higher with the two hole vent.

During the time when the pan is flashing the velocity of the expanding gas is much lower than the gas velocity during the main powder charge burn so I would expect the "choking" effect to be smaller.
This, in theory, should cause the two hole vent and the single hole vent to be nearly identical during the ignition phase of the shot.
(Yes, I saw where you found the two hole vent was slower than the single hole vent but you should notice I said, "nearly". :grin:

(As an aside, if the one hole vent was .070 diameter, the two holes in the two hole vent would be .0495 diameter to give the same open physical area.)

 

[Larry Pletcher]

Makes sense to me Pletch.. . . . .

(As an aside, if the one hole vent was .070 diameter, the two holes in the two hole vent would be .0495 diameter to give the same open physical area.)

I should have gone back to look at my article. The double vent had 2 holes measuring .052". The single vent was .073". We debated about the size of the single vent and felt that the next size up was too big for a good comparison.

Here is the article as I posted on blackpowdermag:

BPM Link

 

[Artificer]

I agree that shape of the vent likely makes no difference but reason that the larger flash hole will vent more gas through it and reduce the velocity of that shot if resistance to the bullet increases. Lower velocity for any reason equals longer time in barrel resulting in more time for shooter error (flinch, etc) and barrel harmonics to come into effect. The expanding gases will attempt to follow the path of least resistance and a larger vent will provide less resistance than a small one; therefore a smaller vent will hold the gases in better and better nullify the differing amounts of resistance.

To belabour the point, the more that all variables are held to a constant is what contributes to accuracy. Everybody has their own combination to get to what they find acceptable, whether it’s one hole groups at 100 yards or “minute of deer”. Change any one thing in the combination and results may change noticeably (for better or worse). And it may be internal or external factors. We mere mortals may not be able to perceive the differences except for the terminal results down range.

Wish I hadn’t been such a sweathog in school, a ballistics lab suddenly seems like a fun place to work.

Respectfully.

First, I definitely agree that barrel harmonics affect accuracy.

I learned long ago that every muzzleloading barrel (rifled or smoothbore) has two loads where the harmonics align for best accuracy. One is a light load that can be used to great effect at short range accuracy and another with a greater charge that can be used for hunting or longer range shooting. Now what powder charge that is all depends on the caliber, length and thickness of the barrel, as well as the type and amount of powder of the main charge, the type and thickness of patching plus the lubricant, the size ball, how uniformly it is loaded to include ramming the ball, etc., etc. For example, when all else was equal except for the amount of powder, my hand built Douglas Barreled .45 cal. flintlock had a lighter accuracy load of 42 1/2 grains and a heavier hunting/long distance load of 80 grains, where the rifle threw almost identical tight groups with both loads at short range, though the groups were in slightly different areas of the target when using the same point of aim.

The interesting thing was this also was true with Minie' Bullet WBTS guns. Though the service load of the Springfield was 60 grains of powder, they shot their best at a smaller charge of 28-32 grains for a short range accuracy load and often a bit higher than the 60 grain load for long distance.

Now if ANY thing other than the powder charge was changed, one or both of these accuracy loads went out the window due to barrel harmonics.

I am sure there is a threshold where the vent hole being worn or enlarged large enough would also negatively affect the harmonics of the barrel and therefore cause accuracy to suffer, EXPECIALLY if the shooter had already experimented and found the most accurate two charges for his/her rifle or smoothbore.

Gus