Author Topic: powder chamber shapes for golfball swivel gun (Read 863 times)

willysjeep134 · « **on:** December 06, 2005, 07:30:14 PM »

Hello all,

I know it's been a while since I posted here, but I have a project that I am thinking about getting started that I need some advice on. I have a peice of C-1025 round hollow stock. It has a bore of 1-3/4 inches and wall thickness of about 3/4 inch. It seems almost perfect for a golfball cannon project. I was thinking a swivel gun with a trailer hitch ball for a cascabel would be about at my skill level.

I intend to build a powder chamber/breech plug. I need to know what shape I should be thinking about for a powder chamber. I have heard that for a good margin of safety there should be one chamber-diameter of wall thickness at the breech end around the powder chamber on small cannons. So, with a 3.25" overall diameter at the breech I was thinking a 1" diameter powder chamber would be an adequit margin of safety. Should I be thinking about a truncated cone shape, a truncated cone with a hemispherical tip, or possibly a straight walled chamber with a hemispherical end? Also, should I chamfer the edge of the powder chamber or leave a square shoulder where it steps up to bore diameter?

Also, I assume a powder chamber should be just large enough to hold a standard powder charge. I bet puting more powder than the chamber can hold would be a very bad thing. About what volume of powder is a standard loading for golfball bore cannon? I might eventually make up a mold for lead or possibly brass balls, so I want my powder chamber to be as large as it needs to be to practially shoot these heavier balls as well.

Also, is underfilling the powder chamber a big problem? I know with a black powder rifle if the ball isn't down on the powder you run the risk of bulging or bursting a barrel.

Any advice you can give would be appreciated.

GGaskill · « **Reply #1 on:** December 06, 2005, 08:49:06 PM »

So, with a 3.25" overall diameter at the breech I was thinking a 1" diameter powder chamber would be an adequit margin of safety.

I would buy that. But be sure the inside diameter of the tube is truly round so the breech plug fits tightly. A light shrink fit would be even better. You may have to bore the breech end to guarantee a truly round hole.

Should I be thinking about a truncated cone shape, a truncated cone with a hemispherical tip, or possibly a straight walled chamber with a hemispherical end?

Consider what you have the tooling and ability to make. Original Coehorns had a truncated cone with radiused corners which made swabbing more effective. Since you can make the breech plug without having to work at the bottom of a deep hole, there should be fewer limitations.

Also, should I chamfer the edge of the powder chamber or leave a square shoulder where it steps up to bore diameter?

Ideally it would be radiused to match the shot, but an appropriate chamfer can be almost as good.

Also, I assume a powder chamber should be just large enough to hold a standard powder charge.

Yes, although there is no established "standard charge" for golf ball artillery. You'll have to establish that yourself.

I might eventually make up a mold for lead or possibly brass balls, so I want my powder chamber to be as large as it needs to be to practically shoot these heavier balls as well.

Forget brass; too hard to melt and too expensive. ZA-3 zinc diecasting metal is about the best choice. Lead may be too heavy (about one pound for golf balls.)

Also, is underfilling the powder chamber a big problem?

That was the method used to adjust range with Coehorns as they had fixed elevation. If should not be a problem with golf balls, although it might approach a problem with heavy shot.

Cat Whisperer · « **Reply #2 on:** December 07, 2005, 01:11:07 AM »

Cool project.

3/4" wall thickness should be sufficiently over built for most any projo.

I'll speak in terms of principles. Adapt as you see fit.

Inside corners should be rounded not sharp - to eliminate stress risers.

Powder chamber should hold your max charge. Overfilling isn't going to do much damage with your 3/4" wall thickness on the tube.

The plug should SEAL very well - not because of the pressure (although that is important) but to keep the corrosive stuff from between the two pieces of metal.

Threading (or welding) will reduce the strength by about half - so if threaded move the threads (welds) back aways from the powder chamber.

Long thin powder chambers produce greater velocity (see the research done in the references on handgonne's).

Filling the powder chamber will be an issue. Figure it out first as to how you're going to do it. You may well want to package the powder in aluminum foil - so taper the entrance of the powder chamber so the packet slides in easily. There is a cannoneer's tool for placing a scoop of powder into the powder chamber - long stick with a long thin scoop that is open on the top - insert and rotate.

DD has mentioned doing a crush fit on the front, thrreaded at the back - a good design. A friend did that plus welded at the back on his beer-can caliber mortar.

Be safe, keep legal and have fun. All three are requirements.

Cat Whisperer · « **Reply #3 on:** December 07, 2005, 01:16:23 AM »

Cone for powder chamber is great for strength. Cylinder may be easier to place the powder in the same place each time. Rounded bottom is very good.

I assume the tubing you have is seamless. (Seamed tubing has the issue of welding porosity and hence long term lack of strength.)

CU_Cannon · « **Reply #4 on:** December 07, 2005, 04:52:33 AM »

I think I would go with a threaded breach plug with no welding. It will make cleaning and inspection easier and will avoid possible hardening problems.

For the chamber I've found a straight chamber with a hemispherical end to be the best. It is easy to machine with an appropriate size ball end mill. The hemispherical end helps avoid stress concentrations. Alternately a drill could be reground to have rounded corners. I've never done this so IÂ’m not sure of the actual functionality of such a drill.

A 3/4" - 1" by 1Â” - 1.25" chamber would be a good starting point. My golf ball mortar with a 5/8Â” by 5/8Â” chamber will put a ball around 100 yards down range with 60 grains Fg. Once you start getting higher charges it gets harder to keep track of the ball. I like to see the impact others like the noise.

Keep us posted on your progress.

willysjeep134 · « **Reply #5 on:** December 07, 2005, 05:13:22 AM »

I was thinking about projectiles too. I know I want this cannon to be able to shoot golfball sized metal spheres. What they are made of is up for debate. I was also thinking about an ultra inexpensive canister or grapeshot.

The railroads around me ship pelletized iron ore. Basically they refine iron at the mine to about 85% purity, mix that with bentonite, and roll the mixture into little pellets about the size of small marbles. They can be shattered with a hammer, but they don't crumble in your fingers. They are like stone. They actually have a machine to remove all of the ore that falls out of cars and lands on the tracks. If they don't remove it, the rails can be forced apart! I was thinking that a near bore diameter wooden disk in the bottom of a light canvas bag, the bag filled with enough iron ore pellets to make the whole thing weigh as much as a round shot, would make a wonderful canister shot. I don't even know if the wooden disk is required. I might be able to get by with a canvas bag sewed up so that when it is filled it will be a little smaller than bore diameter. It will be definately cheaper than shooting spherical shot.

willysjeep134 · « **Reply #6 on:** December 07, 2005, 05:24:29 AM »

About the breech plug. I take it that the hardening caused by welding is what weakens the barrel? I know most welding rod filler metal is of higher quality than the steel being welded. I was originally thinking about making the whole breech plug smaller than the bore, then filling the gap all the way around with weld, then welding up 3 or so inches all the way around. Essentially there would be a sleeve of weld all the way around the breech plug holding it in the bore. I was thinking that afterwards I would have the barrel annealed.

Would it be better to go with a shrink fit or threads, or do you think a massive weld done by a professional (my dad has 30 years working experience with arc welding) and then annealing might get me by? I have the resources to turn down the outside diameter of a breech plug, heat the barrel, and weld, but not to thread the breech. I would have to get somebody to do that for me.

GGaskill · « **Reply #7 on:** December 07, 2005, 08:53:58 AM »

The primary advantage of a threaded breech is the ability to disassemble and clean powder fouling from between the plug and barrel. A shrink fit with a circumferential weld at the back would be way more than strong enough for a golf ball gun, even when fired with metal shot.

Cat Whisperer · « **Reply #8 on:** December 07, 2005, 12:01:47 PM »

Welding - very strong; but some problems.
Some steels take special care (pre-heating).
Stresses in some cases should be relieved by heating and slowly cooling.
Even the best welding will have some porosity - which because of the corrosive nature of the products of combustion should be kept well away.

That means that if welding is to be used good design and good welding techniques must be followed.

A removable breach plug is not a bad idea.

The combination of putting a shoulder in the tube against which the breach plug is forced (threads) gives a great seal and allows one to weld well back from the products of combustion - with the welding adding strength and making everything rigid.