Making a Half Pound Cannon
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  1. #1
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    Default Making a Half Pound Cannon

    I am planning on making my 5th cannon sometime this year but I wanted to get some advice from a knowledgeable crowd to make sure my design will be sound. It will be quite a bit larger than the previous ones I have made, with the largest one being 0.750 x 11 inch bore in 1.50" diameter 4140 stock and fired with FFFg. There are a few specifications that I am set on so far and some that I have a rough idea about.

    The barrel bore will be 1.50 inches which will primarily be firing chrome steel ball bearings of the same size(8.0 ounces) using Black Powder.
    Body length is going to be somewhere in the range of 24-36 inches depending on how much center to center I can squeeze out of the lathe I will be using.
    Planned material for the body is either 3.00" or 3.50" diameter annealed 4140 steel rod drilled out to size and depth. Any extra attachments(such as trunnions) will be installed via shrink fit so as to leave the barrel as strong as possible.
    I intend to outfit it with a Cap Lock firing mechanism(my biggest question here is should I make the fire tube/nipple removable or no?).
    Will this design be of sufficient strength to withstand firing larger loads and/or charges with black powder(what kind of safety factor)? What about smokeless?

    Last question regards the legality of a cannon like this in California. I have looked high and low regarding this type of device and all sources point to it being legal as long as it is a primitive muzzle loading device fired with black powder. Anyone here know if I would need to get it registered or missing something? Or am I fine within the stated parameters?

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    I wouldn't mess with smokeless powder, I believe around 150 grains of 1F powder would work fine, I would also check the legality of making a canon of this size. How do you plan on firing the percussion caps?

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    So the legality of it that I have found after much searching is that it is neither a firearm or a destructive device. It gets categorized as an "Antique Firearm" which is it's own category of device. It falls under category C of California antique firearms as best as I can tell. The destructive device page of Wikipedia even gives muzzle loading cannons as an example of exemption.

    For firing the percussion cap I plan to use a hammer and lanyard set up of some kind mounted behind the detonation chamber.

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    Is this going to be a blind hole barrel or a through hole with a breech plug? If your shot are truly 1.500" in diameter, you should make the bore at least 2.5% larger so the shot won't get stuck in the fouling when loading. 1 17/32" is probably OK. Your wall thickness is on the thin side, even for a high quality steel. The simple thick wall hoop stress formula gives 33 Ksi at 20 Ksi chamber pressure. A percussion cap nipple makes cleaning the vent difficult and likely to be skipped, which is not good practice.

    If you haven't visited already, the Graybeard Cannon Forum is a good place for help.

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    The barrel will be drilled out blind, I don't trust my welds for something like this. With my smaller cannons I have not had issue with the close fit so far(though I will be reaming the barrel slightly oversize), I haven't used any wadding with the ball bearings. The steel in question has between 80 Ksi-100 Ksi tensile yield and modulus of elasticity of 29 Ksi.

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    Hi, Sounds like a good project, but I would not like using steel ball bearings, if things get stuck on firing something has to give and steel on steel??. I would go for lead balls only, they are soft, can give and black powder only. Have fun but be careful. Homebrew.357.

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    A lot of the NSSA shooters are using cast zinc balls in their full sized cannons and mortars.

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    M1841 Six Pounder (Bronze) was over three inch wall for 3.67 bore at chamber

    Other thumbnail is Mass Effect chart for 4142 - showing the nice improvements thru the various levels of heat treat - all of which are readily machinable
    Attached Thumbnails Attached Thumbnails 4142.jpg   six-pounder.jpg  

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    So in terms of design, the consensus I am hearing from you guys is that I should bump up the OD of the cannon to something closer to say 4 inches(1.25 inch wall for 1.5 inch bore), correct? I can just get a bigger piece of steel if I need to. Would shrink fitting a piece on at the back be of any benefit, or should I just stick with the whole one piece plan? Steel rod 4 inches in diameter looks about the biggest I can easily source/afford for this project.

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    Have you got lots of insurance????? And a real good lawyer???? A cannon blew up not far from here, killed 2, last I heard the builder / shooter was still in the slammer.

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    I'm sure he did something to cause it to blow up like it did, I personally prefer the path of overkill with things I build and that is why I am here asking about this. With the suggestion of making the body bigger taken into account it pushes the max barrel pressure into the 60Ksi range before exceeding the yield strength of the material. That's a pretty hefty safety factor.

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    A shrink fit reinforce over the powder chamber area plus a couple of inches would be fine. Same idea as the Parrott rifle of Civil War days.

    There is risk in all human endeavor, even watching the NFL (heart attack from excitement and lack of exercise.) There are too many people making "cannons" from water pipe or other inappropriate materials; they work for a while without overt problems but, in truth, are pipe bombs waiting for their time. Eventually the owner wants a larger boom and adds that last increment of powder and the bomb explodes. You can't fix stupid.

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    I assume you are already aware of them but just in case you aren't:

    National Safety Rules of the American Artillery Association

    I always feel more comfortable getting information from the people and groups that have been at it for years.

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    Quote Originally Posted by RoboChair View Post
    I'm sure he did something to cause it to blow up like it did, I personally prefer the path of overkill with things I build and that is why I am here asking about this. With the suggestion of making the body bigger taken into account it pushes the max barrel pressure into the 60Ksi range before exceeding the yield strength of the material. That's a pretty hefty safety factor.

    News reports stated that it was made of stainless steel. Brilliant! with every shot it would be harder and more brittle.

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    one would think that's going to weigh more then 1/2 pound 4140 isn't light

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    1.5 inch ball bearing weighs 8 oz or half a pound. Cannons are typically named for the weight of steel ball they fire.

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    1.5 inch chromium steel ball='7.952 oz. I would call it 1/2 pounder.

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    So, I'm back with some progress reports. I had a long and arduous journey drilling out the bore on this thing to a depth of 33 inches. I put a 1-15/32" silver&demming on a series of shaft extensions and modified the drill bit to allow for through coolant. I was unable to modify the flutes in such a way that they could clear chips past the bit so I was forced to peck drill, all 33 inches, 0.2" per pass, dragging the tailstock back and forth each peck. I tried to use my adjustable reamer with the extensions under power but the setup wasn't able to deal with the torque and was forced to leave it at about 1.47" ID until I could do the final reaming. After that the removable percussion cap vent was added easily enough. I turned up some projectiles to fire at the current bore diameter and got to totally destroy some cans of water. I ended up running about 800 grains of FFg black powder, but still feel it was under powered which may have been due to not having a great seal. The 2 aluminum slugs that were fired had little damage after going through 2 coffee cans of water and hitting a concrete tube I had as a back stop, it also didn't have as much of a boom as would be expected.

    Next I will be reaming out the last bits of the ID to probably 1.515" followed by a nice through run of cylinder honing to get it nice and smooth so it's easier to keep clean. Still need to finish the trigger assembly for it. Planned out how I am going to shrink fit my trunnions. Then I will need to make a cart/carriage to mount the cannon to that will be able to fold up enough that I can stick it in my trunk or back seat. After all that I should be good for a long time.

    Now I am aware of the standard wisdom that Smokeless powder should never be used in a black powder device. However I just cannot give that wisdom the weight it would normally deserve considering just how massively over-built this thing is. My engineering shows that the barrel is capable of handling up to 60 ksi before beginning to yield, a region that many rifle barrels operate in. Now operating without a safety factor is a stupid idea, so lets assume the most I would want to operate the barrel at was between 20 and 30 ksi. Is that pressure even realistically possible with black powder? How would I even go about calculating my barrel stresses based on a particular powder selection. Could I calculate the stress that a particular smokeless powder would exert on the barrel to determine if getting it to somewhere in the operating range of a handgun is a possibility? I hope that some of you can give me some insight into this. I may be a bit crazy, but I'm not an idiot, I merely want some education in regards to the math involved because I can see what it can handle capacity wise but don't know enough to work out said math without someone pointing me in right area.

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    The fundamental problem with calculations for something like this is that there are no relevant pressure tests to start from. So without making a pressure testing device of some validity, how are you going to determine what pressure any load has reached? Calculation has proved to be unreliable for small calibers and really is not used professionally. Experiment using pressure measuring devices is the preferred method for small bores. The "it hasn't blown up yet" school is not really an appropriate way to establish the safety of a load.

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    I was kind of afraid that would be the case. I would be able to mount a sensor to the cannon for testing purposes, but I don't know what specific kind I need to properly get a reading from it. It's usually some kind of strain gauge? I could just keep testing shots with some amount of powder and chrono the projectile to gauge how fast it was accelerated, but that only gives me the average chamber pressure which doesn't give me a complete idea of what is happening over time. No way to know if the pressure is spiking dangerously or not.


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