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Offline irishman

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'Speak' Repeatability ?
« on: September 21, 2005, 05:57:55 AM »
I plan to build several small bore mortars. In prototyping, I am having a problem with repeatability. I get one good loud, sharp report, reload  exactly the same as before and end up with meager report. These barrels are .177, 22, 25 and 30 cal Coehorn style, brass barrels, so they are all under 3" long. We have built several hundred larger barrels and have not experienced this problem.

I've used measured loads, 1gr./cal + more and less. Wadding is identical. Compressing the load is as close to the same as I can.

Should I change toothpaste?

                                          Irishman
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Offline Cat Whisperer

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'Speak' Repeatability ?
« Reply #1 on: September 21, 2005, 06:51:22 AM »
I assume that the difference in report is also a difference in pressure.

One variable at which to look is the condition of the bore and/or the time between rounds.

That is to say two things might-could be producing the effect.  One is a buildup of carbon - which I would think cause higher pressure.  Another thing would be the amount of moisture (acting as a lubricant) which would affect pressure.  Hence, clean between each shot for better consistancy.

Folks who use blackpowder and patched ball have much the same variability and would be able to give advice from better experience than I can offer.
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Tim K                 www.GBOCANNONS.COM
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Offline GGaskill

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'Speak' Repeatability ?
« Reply #2 on: September 23, 2005, 09:06:14 PM »
Maybe you are right at the edge of a supersonic report; sometimes supersonic, sometimes subsonic.  What happens if you use a little more powder?
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Offline pyro_tek

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'Speak' Repeatability ?
« Reply #3 on: September 27, 2005, 08:20:33 AM »
Quote
Maybe you are right at the edge of a supersonic report


I'm not sure if sound (the report) can travel faster than the speed of sound (i.e. be supersonic). Are you referring to a projectile exiting the barrel? The speed of sound is typically calculated to be in the 760 mph range under ideal atmospheric conditions. As I understand it, in a given atmosphere, at a given temperature, the speed of sound is a constant. Sound can't go faster or slower than itself.

One variable that will become more apparent as the size of your model diminishes is heat. As the tube warms with repeated firing it heats the subsequent load. Even a small increase in temperature will result in noticable changes in a given load/barrel. Think of it as the residual heat jumpstarting and excelerating the load. The warmer load will ignite and burn more quickly resulting in a shorter burn time and a more meager report.

The actual physics involved here are far beyond my abilities so all this is just conjecture on my part.
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Offline GGaskill

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'Speak' Repeatability ?
« Reply #4 on: September 27, 2005, 08:40:33 AM »
The escaping powder gasses, being at greater than atmospheric pressure, can travel (for a short distance) at greater than the speed of sound.  This may be happening in some cases and not in others, giving a sharper report or not.  They need to conduct some experiments with greater powder charges to see if they get a greater percentage of sharp reports with slightly increased charges.
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Offline irishman

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'Speak' Repeatability ?
« Reply #5 on: October 12, 2005, 04:08:07 PM »
I have found after LOTS of firing, that per CW, the difference in report is from the difference in pressure. It was a long time coming...

We started with 22 cal. barrels with differing bore lengths. 2.1", 3.1" and 3.6". All had .125 fuse holes.

Fired each set of 3, 50 times. The short barrel did not produce a consistent report so we concentrated on that for the rest of the testing.

We made another barrel (2.1") using a 5/64 fuse hole. Fired 100 repetitions.

In the short barrels, the report was consistently sharp with the 5/64 vent hole. The barrel with the 1/8 hole was inconsistant in its report, with more of a boom than a sharp crack.

                                          Michael
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Offline Powder keg

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'Speak' Repeatability ?
« Reply #6 on: October 12, 2005, 04:40:42 PM »
How much powder did you use? Any pictures of the testing? Later,
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Offline Cat Whisperer

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'Speak' Repeatability ?
« Reply #7 on: October 13, 2005, 12:55:46 AM »
Quote from: irishman
 
....
In the short barrels, the report was consistently sharp with the 5/64 vent hole. The barrel with the 1/8 hole was inconsistant in its report, with more of a boom than a sharp crack.
....


Thanks, Michael - I really like to see emperical testing!

With the larger touch hole, then, perphaps it was related to whether the fuse did or did not hang up when fired?

Perhaps a quick test with very straight vs. fuses that were intentionally bent slightly would reveal a difference?


Did you notice any difference in how clean the two burned?  I.e.: was there more crud left in the barrel from one vs. the other?  Any difference in smoke?
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Tim K                 www.GBOCANNONS.COM
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Offline Cat Whisperer

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'Speak' Repeatability ?
« Reply #8 on: October 13, 2005, 12:56:20 AM »
Quote from: irishman
 
....
In the short barrels, the report was consistently sharp with the 5/64 vent hole. The barrel with the 1/8 hole was inconsistant in its report, with more of a boom than a sharp crack.
....


Thanks, Michael - I really like to see emperical testing!

With the larger touch hole, then, perphaps it was related to whether the fuse did or did not hang up when fired?

Perhaps a quick test with very straight vs. fuses that were intentionally bent slightly would reveal a difference?


Did you notice any difference in how clean the two burned?  I.e.: was there more crud left in the barrel from one vs. the other?  Any difference in smoke?
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Tim K                 www.GBOCANNONS.COM
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Offline irishman

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'Speak' Repeatability ?
« Reply #9 on: October 26, 2005, 10:42:51 AM »
We have been able to photographically capture the difference between a 5/64ths and 1/8th fuse hole that answers many questions.  In testing we consistently use 22 grains of ffg; equally and tightly packed, no wadding.  I speculate that a fuse hole changes into a vent hole when the powder ignites.  

After many firings of these cannon barrels, one conclusion may be that the fuse hole-turned-to-vent-hole, particularly when it is more than half the diameter of the bore, has vented a some portion of the pressure(see photos below)...thus effecting the sharpness, loudness....

You can find these Coehorn Mortar at www.brooks-usa.com.  They are l isted with 6/64th fuse holes, but we would be happy to provide any fuse hole diameter you desire.  This small barrel seems to be an ideal test piece for the purpose of this experiment.

Because we cannot have any lighting for these photos, the GOEX can is barely visible.  :grin:    Michael
[/img]
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Offline irishman

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'Speak' Repeatability ?
« Reply #10 on: October 26, 2005, 11:10:29 AM »
CORRECTION After all that!!! Vent hole offered on www.brooks-usa.com in the above post is NOT 6/64, it is 5/64

                                                  Michael
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Offline Cat Whisperer

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'Speak' Repeatability ?
« Reply #11 on: October 26, 2005, 05:57:43 PM »
Assuming the pictures are typical, that's a great visual indication of the effect!

Good research, Michael, thanks for posting it and the pictures!
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Tim K                 www.GBOCANNONS.COM
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