I am looking for information and discussion as it relates to the selection of materials for the home production of artillery pieces of calibers 75mm and below. I find my lust for guns turning toward ever larger specimens. No longer able to find satisfaction in .50 BMG I am seeking to go bigger. I could drop the $10k and get an Anzio Ironworks 20mm, but I'm exploring my options. WWI-era light artillery (pic very much related) piques my interest greatly.
Actual period pieces are out of my price range, generally going for $45k or more. I seek to design and build my own. I've settled on a horizontal sliding breech block as a simple and robust solution to the mechanism. I am still looking over older recoil mitigation mechanisms to come up with a simplified design, but that is a part I can figure out later since it isn't entirely necessary to the function of the gun.
In terms of caliber I am still deciding. One of the factors in deciding is the availability of casings. I don't have a large enough lathe to turn casings (this will be doubly important later) so must use something off the shelf or that can be modified to suit. On the low-end the 20mm Vulcan is a suitable stand in if one is building a light anti-tank gun of the early-WWII era. Not quite what I want, but it is a simpler proposition. This can be expanded to the 25mm and 30mm cartridges currently in US service and available on the civilian market. A step up is using the 37mm Hotchkiss. This round is decently common on the collectors market and load data is available as the Hotchkiss revolving cannon is still in use with several enthusiasts. The next option involves basing the design around the 40mm BOFORS which I would likely cut down to reduce case volume and remove the neck. This would allow me to simplify the chamber design. The last option, and that which is both most attractive and most difficult, is a 75mm gun. This would be the most expensive materials wise but really meets my desires for a true field gun. Casings can be had quite easily. Full size brass casings go for $25 on GunBroker, and as the cartridge has had a long service life there is plenty to use. I'm also considering producing a lighter projectile and using the short blank casings for this project. They are very cheap to the tune of $7 each or so when I find them.
My propensity to short for caliber casings stems from my desire to reduce operating pressures and maximum range. Smokeless powders of the proper size for use in artillery is nearly impossible to find in the open market so I am looking to load whatever I make with black powder. The exception is the 20mm through 30mm cartridges. Surplus 20mm powder is available and would likely be suitable for load development in most any of them. The 37mm Hotchkiss did have black powder loads during its service life so that's another advantage for it. By using black powder I hope to lessen the materials requirements by lowering pressure in the barrel. I'm hoping I can take the 75mm short blank casings, stuff them full of Fg, and seat a ten or so pound hunk of lead on top and blow it out the (smoothbore, rifling such a thing is beyond the capabilities of my equipment) barrel.
That's the background, here's the question. What the hell do I make the barrel from, and how should I go about it? My first thought was 4140, which isn't a bad thought. 4150 would of course also be acceptable. The problem here is that I do not have access to a lathe large enough to bore round stock out for this project, and I can not find true seamless tubing of the proper dimensions for the 75mm barrel. True seamless is a necessity as welded tube can not be trusted not to blow if I was loading for smokeless. If I'm thinking of working with blackpowder though, would not I be able to get away with less stringent materials? The Austro-Hungarians successfully fielded a bronze barreled cannon firing smokeless ammunition pre-WWI. There were several century's worth of black powder cannons using bronze, brass, and cast iron barrels. My short 75mm would be about 800 cubic centimeters not counting the portion of the case taken up by the projectile. Although not quite true, it is often taken than 1 grain of water is equal to 1 grain of black powder. It's close enough to work with. This leads us to 800 cubic cm of space = 800 mL of water = 800 grams of water = about 12,000 grains of BP or over 1.7 pounds. The 12 pounder Napoleon of the US Civil War fired a 12 pound (who would have guessed) round ball using 2.5 pounds of powder from a bronze barrel and did it with gusto. Modern reproductions of these cannons often use a cast iron barrel with a steel liner. I feel like I may be severely overestimating the requirements of this project. The only real difference between my project and a muzzle loading cannon is which end I'm stuffing the black powder in.