CW: I was considering 4130 but it wasn't available in the size I needed unless I bought a full length ($$$$$) - the original intent was to make the whole barrel out of 4130.
I did a lot of research on 1018, especially after someone expressed concern over its "notch toughness". The "bottom line" on 1018 was that there had been one particular batch from one particular foundry (La Selle) a number of years ago that had a problem.
1018 is "a graded steel" (meaning the foundry has to watch the composition and lab-test each batch), low carbon, with very good yield strength in the unannealed state, often used for structural members and "rifle barrels" is listed as one of the recommended applications.
With a wall thickness of 1/2" and a breech plug depth of 1" it should have lots of strength. The 660 bronze barrel with a wall thickness of 3/4" (at the breech) won't add much strength but the leaded bronze should contibute some to the impact strength of the steel.
I used a figure of 30,000 PSI for my calculations - a figure that came from chamber pressures in testing of blackpowder rifles - and the 1018 with 1/2" wall checks out pretty good. If anybody has chamber pressures from an actual cannon, please let me know!
I do LIKE the idea of making a plug gauge and watching the breech diameter - I will do that!
I considered a powder chamber but decided against it for a couple of reasons: #1 - a chamber creates a "minimum powder load", being the amount of powder required to fill the chamber; #2 - I was concerned about charging the powder - how to ensure that the chamber was always full and there were no air spaces behind the ball; #3 - I don't have a lot of faith in my ability to drill thru the barrel, thru the sleeve, and hit the chamber in exactly the right spot - I am not a very good machinist - without a chamber, I only have to be accurate in one plane while drilling the fuse hole (to meet the junction of the bore and the breech plug).
To fit the liner into the barrel, I am aiming for a "press fit" of 0.001" per inch. The sleeve is 2.000" diameter and I am reaming the bronze to 1.998". Again, because I don't trust my own machining abilities, I am going to run a pin thru the bronze immediatly behind the sleeve to ensure it doesn't creep backwards in the bronze (the pin will go thru from the bottom, where it is out of sight, and will be blind at the top). I also know that the bronze will expand more than the steel with heat so my press-fit will loosen off as the cannon gets hotter.
I considered lots of different ways of retaining the sleeve in the bronze but don't have the tooling to do anything "fancy".
Double D: I WISH I could have done the 2" ream in the lathe but I ran into 2 problems. #1 - when I mount my home-made V-block on the cross-slide of my old South Bend, the center of the 4" diameter bronze is above the center of the chuck - not enough "swing over cross-slide"; #2 - if I turn the reamer any faster than about 20 RPM, it chatters in the bronze and my lath will not go slow enough.
You guys are GREAT! It's good to have folks to chat with about my project, especially folks who know something about cannon construction!
I am looking forward to finishing the reaming of the 1.998 hole so I can move on to other, more FUN things, like turning the trunnions and the barrel!
CJ