Cannon wall thickness ratio???

[Brad]

Has anyone any idea of the ration of cannon wall thickness related to bore diameter??
I have a nice 30" long piece of 3" diameter STEEL that I am going to turn to a swivel gun (small cannon barrel.
I have a friend who will drill the bore for me to what ever diameter and depth I want.
So now I.m going to design the barrel profile. I will keep it full sized at 3" or very slightly under that at the Breech area, but want the bore as large as I can and still be SAFE. I will not fire gigantic load sin it, probably no more than 100 to 150 grains of FFG ever. But there is no way of knowing what some future idiot might do.
I'm thinking about a 1-1/4" to 1-3/8" bore. But maybe someone knows of a ratio that builders use for all STEEL barrels??
Thanks

[Gus Fisher]

Brad,

I'm not so sure you will have a problem with barrel wall thickness as long as you keep to period contours, after all, they were made for iron. The PROBLEM can be what kind of STEEL you are working with and what the metallurgy of the alloy is. I'm sorry I don't know enough about steel to tell you what would be a good alloy or not, but I would personally would not make a swivel gun out of an unknown piece of steel.

[Chris W.]

The steel is everything. If you have 4140, 4130, 8620, stress proof, all of these will take a larger bore than a chunk of 1018 - 1020 steel. If you can't posiitively ID the steel used, stop, don't do it. Using mild steel ( 1018-1020 ) might not hold the pressure and end up being a bomb. And, you said it, you might just use lite loads, next person might not. Next is the breach design. That design gives max yeald to the threaded breach. Threaded breach ( acme or square threads, acme is best ) or a blind hole. Remember, if it's a threaded breach, make the OAL of threads at least 1 5/8 X the dia of the bore, no less. My small cannon on a field carrage has fired over 500 rounds so far. Barrel is 4140 steel with a 416 SS plug with acme threads for cleaning. Bore was 1 3/8" Dia until I got tired of only getting 5 shots per lb of BP. Made a new barrel with a 1 1/16 bore to shoot patched 1" ball bearings. Barrel OD is 2 5/8". Still hits reasonably well at 500 yards and out. After 1000 yards, the liter ball bearings seam to loose energy fast and drop off. You can do the math and see that my wall thickness runs .800 or so. But dealing with these home made BP cannons, a safe person treats these like they are a bomb every time you light the fuse. I highly recomend getting some training before making or using one of these. One mistake can kill lots of bystanders. Properly used, can be fun.
Chris

[Brad]

I have determined that the steel I have is 1045, which is 82KSI.
I am not going to have any threaded plugs or welding at the breach.
I have a friend who will actually drill me the bore size in from one end, and stop at the desired depth. So there will be no seams, threads or welds.
I will be welding on the trunions, but they are far from the breach.
I have spoken to a black powder rifle barrel builder and he says he can see no problems. Consider that allot of 50 cal rifles are 7/8" across the flats. Thats over half the dia as taken up as bore. (not considering the added octogonal steel).
I know I can get a .54 cal rifle in 1", thats over half also.
And a tightly patched ball in a rifled barrel has to increase the pressure over a smooth bore.
I welcome all thoughts, warnings, etc. And value them.
Thanks,
Brad

[Chris W.]

1045 is somewhat softer than say 4130. That said, if it was mine and I was building another, I wouldn't go much over 1". Why ?? Shouldn't have to worry about the pressure problems if used properly. Second, cost per shot. My 1 3/8" got about 5 shots per lb. of BP. The 1" ( 1 1/16) gets double that easy and goes over 1200 fps. !" ball bearings are cheap ( $12 for 25 through Enco ) With a 500 gr BP load, that 1" ball bearing will have enough power to punch a hole through 1" steel plate. I take it your friend doing the work is another machinist ?? Have him leave you 3" of breach, or only bore 27" in a 30" barrel. I've watched quite a few of these fail, mostly where someone has welded on the barrel. Welded breach plugs never last long. I'd recomend a design where the trunions are threaded into the barrel without heat or welds, seams to be the most successfull design. Remember, you don't get to make even 1 mistake with these and not have damage or death.
Chris

[Brad]

Thanks for the input Chris.
This is why I am asking so many questions and taking so much time.
I really want to get it right and be SAFE.
I was even thinking of having the bore at an 1-1/2 or so, but having him bore an actual chamber of
1". But I'll bet that would be even more dangerous if the chamber was not completley filled with powder?
Feel free to offer any more thoughts?
Brad

[Chris W.]

Very unsafe. You want the ball or wad, depending on what you are doing to take up all of the air space. This goes for all BP firearms as well. Thinking 1 1/2" gives you 3/4" wall. With the higher powder charge you will need for 1 1/2", pressure will go up as the wall size goes down. I'd build it smaller bore, but that's just me. Also, if your friend the machinist, that is doing drilling, has a reamer to finish the bore, it will work better. Have him drill the bore 1/32" small and finish with the reamer. If you were to do a 1" or so bore, that would leave a 1" wall to bore and tap for your trunions as well. A machinist could drill and tap a flat bottom hole, tap with a flat bottoming tap, and fit the trunions flat in the hole to hold the pressure, and in that wall, for about 1 1/4" trunions. ( use NF threads, this is a shear load on the trunions ) I'm just a old retired machinist that used to make BP cannons.
Chris

[Chris W.]

One think I forgot to mention. That if you considered a 1" bore, you could taper the barrel from the trunions to the muzzle, give it a nice appearance and reduce some weight. I'd be considering 1 1/16 bore due to the ease of getting projectiles and having 1/16 for the patch. This is quite easy to do if your machinist friend has a tapper attachment on his lathe. Still leaves full thickness at the breach where the pressure is.
Chris

[Brad]

Just what I been considering.
Thanks

[holdover]

"Welded breach plugs never last long.

I shoot N-SSA and most the barrel liners are threaded and welded. These are full size guns, both rifled and smoothbore and have been shot for years without problems. They are built to strict tolerances and inspected. Any firearm and especially a cannon needs to be made by someone who understands the process and the risk if not done correctly.