Barrel length and powder chamber dimensions for a BP mortar/cannon?

[Aragon]

I am doing the layout for a BP muzzleloading cannon or mortar (haven't decided which yet) I hope to build and I have a question about barrel length and powder chamber dimensions?

The nominal OD of the projectile will be 2.50" and it will not be patched in any way. The nominal ID of the barrel will be about 2.75" Plenty of "windage." The piece will be charged with between 50 and 500g of F BP.

Given the fairly loose clearances will barrel length still make a big difference? I have a fairly wide latitude of how long I can make the barrel and I'm curious how big of a difference a barrel say twice the length of another will make in this application?

I'm also curious about powder chamber sizing. I have seen diameters for pieces of this sizing ranging in diameter from 1 to 1 5/8" and from 1 to 1.75" in depth. Any insight to how differing dimensions affect performance would be appreciated. Thanks.

 

[ofitg]

Aragon, the book Round Shot & Rammers lists the bore diameter of the U.S. 6-pounder cannon at 3.67 inches. For what it's worth, a "dug" 6-pounder ball in my collection measures 3.50 inches in diameter (it might have been slightly larger before it spent 100 years underground).
I'm thinking that 0.25 inches of "windage" is too much for your proposed application.

I believe this is an accurate sectional depiction of the Gribeauval mortar barrel - you might be able to scale the chamber dimensions to your particular barrel -

 

[Aragon]

Aragon, the book Round Shot & Rammers lists the bore diameter of the U.S. 6-pounder cannon at 3.67 inches. For what it's worth, a "dug" 6-pounder ball in my collection measures 3.50 inches in diameter (it might have been slightly larger before it spent 100 years underground).
I'm thinking that 0.25 inches of "windage" is too much for your proposed application.

I believe this is an accurate sectional depiction of the Gribeauval mortar barrel - you might be able to scale the chamber dimensions to your particular barrel -

Thanks for your insight...

 

[BCRider]

I'm sort of thinking that it's likely that around 40 to 60 thou rusted off the ball over that time. So I concur that .25" of windage is too much.

Thinking off the top of my head I think I'd want to have a wide enough windage that the projectile falls easily into the bore. But that it should be small enough that it falls firmly but with a damped "thud" when it hits the end. The gap should be small enough to act as a restriction so the projectile doesn't just fall unhindered and to "CLUNK!" when it reaches the end of the bore. And certainly .25" larger is going to be in the "CLUNK!" category.

Ofitg, thanks for the drawing. It led me to do a search and now my sub directory for "golf ball mortar" designs has a lot more material in it...

 

[Aragon]

I'm sort of thinking that it's likely that around 40 to 60 thou rusted off the ball over that time. So I concur that .25" of windage is too much.

Thinking off the top of my head I think I'd want to have a wide enough windage that the projectile falls easily into the bore. But that it should be small enough that it falls firmly but with a damped "thud" when it hits the end. The gap should be small enough to act as a restriction so the projectile doesn't just fall unhindered and to "CLUNK!" when it reaches the end of the bore. And certainly .25" larger is going to be in the "CLUNK!" category.

Ofitg, thanks for the drawing. It led me to do a search and now my sub directory for "golf ball mortar" designs has a lot more material in it...

I don't think .250 of windage is ideal from a ballistics standpoint. However many thousands of well-performing soda can mortars have been built with such clearances.

There's nothing wrong with a projectile that can be lowered "unhindered" all the way down so that it's touching the powder chamber. Many BP mortars, howitzers and cannons allowed just that by design.

Anything much less windage wise in practice, and cleaning between shots becomes an issue.

 

[CANNONMAN]

I decided to try your thoughts with a cannon of mine 2.5" bore with a 2.25" projectile. This makes a great impression when fired. As in a lot of smoke from propellant burning out of the barrel. This shot was not chronoed but the difference in sound impact at 150yds was significant in both time and impact. For what ever that's worth. Next I patched the thing. Much better results. 5oz cannon powder per shot. I find much more pleasing ballistics with the shot that measures 2.45 w/o patch. I was going to shoot a mortar using similar undersized shot. But I believe it would be a waste of time. W/o some kind of resistance, a mortar just "coughs" up its shot. A cannon has some time to build pressure but much is lost around the edges. What are you shooting? Can you mold shot?

 

[Aragon]

I decided to try your thoughts with a cannon of mine 2.5" bore with a 2.25" projectile. This makes a great impression when fired. As in a lot of smoke from propellant burning out of the barrel. This shot was not chronoed but the difference in sound impact at 150yds was significant in both time and impact. For what ever that's worth. Next I patched the thing. Much better results. 5oz cannon powder per shot. I find much more pleasing ballistics with the shot that measures 2.45 w/o patch. I was going to shoot a mortar using similar undersized shot. But I believe it would be a waste of time. W/o some kind of resistance, a mortar just "coughs" up its shot. A cannon has some time to build pressure but much is lost around the edges. What are you shooting? Can you mold shot?

Thanks for your input -- I really appreciate the practical data.

I've done a fair amount of research on "soda can mortars." Many seem to be constructed with off-the-shelf tubing having nominal IDs of 2.75" and 2.625", while soda cans have nominal ODs of 2.5" Both IDs work well based on conversations with people who actually own and shoot such mortars. I was told that those with 2.625" bores could become difficult to load if they are not kept clean.

Keep in mind the projectiles are commonly re-used and lose their once "tight tolerances."

Someone suggested that windage should equal of 1/40 of the bore diameter which is fairly close to the projectile diameter in this case. Using that formula, the bore diameter of a mortar for a soda can having a nominal OD of 2.5" would be about 2.56.

Good luck loading that mortar! LOL! Bring a bigger hammer!

What did you use for patching? Your experiment sounds very interesting...

FWIW, your 5 ounce loads are a world away from the 60-300g loads I am looking forward to...

 

[ofitg]

Aragon, how many shots do you anticipate in a single session? I used to shoot this little mortar - 2.25" bore, 2.20" projectile - eight or ten times in an afternoon (after that it started getting boring) and I never had any problem seating the projectile down against the chamber..... I did experience some problems with the vent hole getting clogged.

 

[Aragon]

Aragon, how many shots do you anticipate in a single session? I used to shoot this little mortar - 2.25" bore, 2.20" projectile - eight or ten times in an afternoon (after that it started getting boring) and I never had any problem seating the projectile down against the chamber..... I did experience some problems with the vent hole getting clogged.

Not more than a dozen shots per outing. It's interesting to know that you could get by with a clearance of .050" Did you clean between each shot? Did things tighten up near the end of the day?

I would also be interested to know the length of the barrel, the shape/material of the projectile and if you had to ram it home? Thanks.

 

[ofitg]

Aragon, the mortar is a reasonable half-scale copy of the 18th-century English 12-pounder Coehorn. Here's a sectional depiction of the barrel -



Since the rear of the bore is curved/contoured to accommodate a round ball, I didn't want to use a flat-bottomed projectile (which would leave a significant air gap between the projectile and chamber). Here's what I finally settled on -



The projectile is a plastic pipe coupling filled with concrete, including a cast concrete hemisphere on one end. I cast a small loop of wire into the "muzzle" end, in case I ever wanted to lift a projectile out of the barrel.
The projectile weighs 11 ounces, and not including the wire loop, overall length is 4.2 inches. When loaded, the forward end of the projectile was pretty much "flush" with the muzzle.

I did not clean the bore between shots, and I loaded the projectile by sliding it into the barrel by hand. I don't remember encountering any resistance from fouling, so I would say it was not "significant".
With the fixed 45-degree launch angle, 300 grains of powder would provide about 200 yards of range, but I had just as much fun (and recovered more projectiles) using 150 grains of powder.

 

[ofitg]

Aragon, I thought I might add, I have also shot a blackpowder mortar which DID exhibit the fouling/loading difficulties you refer to....

This other mortar had a 2.42" bore, and the barrel was 23 inches long. It had a rather shallow powder chamber at the rear of the bore, but the bore/chamber junction was a flat "shelf", so we used flat-bottomed projectiles.

The projectile was a 3" section of PVC piper, 2.37" diameter, filled with concrete (flat on both ends) and weighed 14 ounces.

For some reason, powder fouling made this mortar difficult to load after 3 or 4 shots. We finally resorted to running a wire brush down the barrel after each shot.
The "up side" with the longer barrel was increased range for a given amount of powder. With a 45-degree launch angle, 300 grains of BP would lob the projectile 300 yards.

So.... since both mortars have "roughly" the same bore size, projectile type, etc, with 0.05" windage, why does the long-barreled gun exhibit the "fouling" problem while the short-barreled gun does not?

...

 

[CANNONMAN]

Oftig, really kool! I signed you up for two design "ATTABOYS". What's the mold look like?

 

[ofitg]

Thanks, Cannonman, but if you're referring to the mold to cast that bronze barrel, I cannot take the credit. I got lucky and found that mortar on Gunbroker several years ago.

 

[Aragon]

Aragon, I thought I might add, I have also shot a blackpowder mortar which DID exhibit the fouling/loading difficulties you refer to....

This other mortar had a 2.42" bore, and the barrel was 23 inches long. It had a rather shallow powder chamber at the rear of the bore, but the bore/chamber junction was a flat "shelf", so we used flat-bottomed projectiles.

The projectile was a 3" section of PVC piper, 2.37" diameter, filled with concrete (flat on both ends) and weighed 14 ounces.

For some reason, powder fouling made this mortar difficult to load after 3 or 4 shots. We finally resorted to running a wire brush down the barrel after each shot.
The "up side" with the longer barrel was increased range for a given amount of powder. With a 45-degree launch angle, 300 grains of BP would lob the projectile 300 yards.

So.... since both mortars have "roughly" the same bore size, projectile type, etc, with 0.05" windage, why does the long-barreled gun exhibit the "fouling" problem while the short-barreled gun does not?

...

Fascinating information -- thank you so much.

Obviously the powder had a great deal more time to burn and to accelerate the projectile with the longer barrel. I suspect that also equates to more time for the burning BP to deposit its gunk under pressure on the barrel's walls?

 

[ofitg]

Obviously the powder had a great deal more time to burn and to accelerate the projectile with the longer barrel. I suspect that also equates to more time for the burning BP to deposit its gunk under pressure on the barrel's walls?

That sounds like a plausible explanation.

A 2.75-inch-bore mortar could be a good choice to experiment with. Rummaging through my kitchen cabinet, a "Fanta" soft drink can measured 2.60" and a 10.5 oz condensed soup can measured 2.65" diameter. You might find other cylindrical containers which would provide a tighter fit.

 

[CANNONMAN]

I'd like a few pics of your projectile molding techniques. Please.

 

[ofitg]

Cannonman, there's not much to it. The plastic pipe coupling has an internal rib, halfway along its length - this rib keeps the plastic coupling and the concrete core locked together -



The "hemisphere mould" is an aluminum dome cut from a fence post cap -



Just smear some Vaseline on the interior surface of the dome and attach it to the plastic coupling with masking tape. Insert a loop of wire into the other end of the plastic coupling.



Place the dingus into a suitable container to keep it sitting upright, and fill the coupling up to the brim with concrete.
Let it sit for 1 or 2 days, then pry that aluminum dome off the end. Wait another week or so for the concrete to cure, and the projectile is ready to use.

...

 

[CANNONMAN]

That's pretty darn kool! The cap radius would work with several charge pockets. Would you mind trying to place a "straw" in at an angle? I'd like to see if you can get one of those to whistle. My mortars are fun, just not as much as the cannons. Never really cared for accuracy with the mortars so adding noise could be fun. I'm currently toying with firework additives to my BP for rather interesting night shooting. Thanks for sharing.

 

[ofitg]

Cannonman, I don't know when/if I would ever get around to it. The summer heat is closing in on us here in Arizona..... I was hoping to get out to the desert at least once more before the temperature tops 100, but I got rained out the past two weekends. I might have to wait until October or so, and then I've got some "modern" gun projects which will have higher priority.

What if you had a cylindrical projectile for your mortar, and you installed some type of whistling fletchings on the "muzzle" end (away from the powder)? Instead of tumbling in flight, the projectile might achieve aerodynamic stability with the fletchings oriented rearward.

 

[CANNONMAN]

I think it's a great way to make a mortar or cannon a whole lot of night fun to make a shot that whistles, is a tracer round and is shot with the fanfare of an exploding firework. Egads, the time and research this is gonna take. Damn honnydues! Who cares if the grass is dying and the plumbing don't work! Look at the night time fun! Wonder if the Wife is gonna buy this?

 

[ofitg]

A lightweight projectile with a "miniature road flare" installed in the nose (away from the powder)? The flash from the gun's propellant charge could ignite it.

I started a grass fire with my cannon on one occasion..... maybe it's your turn now

 

[OldDoggy]

I have a golf ball mortar battery. I load the four up with a 2 ounce charge of 2FG, aluminum foil to pack the powder tight and an unpatched golf ball as a topper. Then I put the shortest fuse in the farthest and longest in the closest. Light them longest to shortest and stand back to see the impacts in the pond. Barrage balls hit in a fairly close cluster about 400 to 500 yards out. Grand kids love it!!! Just Another Old Doggy, Don

 

[CANNONMAN]

Good ideas, lets go for world domination! Eh, I should start wear'n a helmet! I will make something of GREAT interest, [to someone...] and then i'll share it with ya'll. Currently in contact with a fireworks Co.

 

[Officers'Wife]

You guys are all scary. Cannonman, you are not a bad man, go ye and repent of your OMB way. Otherwise your legacy will be the everlasting amazement of your community that you managed to die of natural causes.

 

[CANNONMAN]

OK, set me straight. I used to be an intensive/critical care RN and frequently worked in the ER. I am pretty sure that death by cannon is of natural cause. [silly girls]