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Posted: 7/2/2026 11:38:20 AM EDT
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I was wondering if anyone could help me out here. I am planning to build a close approximation of the Tredegar Mountain gun. I just don't have the means to transport a bigger gun right now. I have Parrot, Ordnance and Napoleon blueprints. I have done a lot of research on Tredegar scaling and have a close idea. I want to overbuild the barrel. I am planning on machining from solid steel and rifling it. I just wanted and idea of what type of steel to use and where to get it? I am planning to HT 4140 CM 1/2 thick liner around a mild steel outer sleeve. The only thing is I am not sure 4140 can be easily rifled. |
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4140 is a great choice, but the problem with it is the wide range of elements in it & the wide range of SFM those elements bring with them. Your best bet will be to start at the low end of the guesstimated SFM range & work your way up from there. Also, you will face work-hardening risks. Don't be too scared of it - with the proper carbide selection you can get the job done. Rigidity is paramount. What bore Dia. are you looking at? The larger you go the easier it will be to keep the chatter & vibration down. Will this be a button-style rifling method, or a single-point tool where you will index it & cut it like a multi-lead thread? Be sure to not use too much of a crush on the OD liner.... The Machinery's Handbook should be able to get you close to what you need for a minimal Force Fit obtained with heating one & freezing the other. |
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4130 = 4 Molybdenum alloy class 1 1% Chromium & Molybdenum by mass 30 0.30% Carbon content plus miniscule amounts of Sulfur, Manganese, Phosphorus etc etc It should be good enough for a cannon if you stiffen the "breech" end appropriately. I can't recall the diameter to length rule of thumb for that at this moment. |
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Oh, that's good to go, then  A twist rate of 120" linear distance per 1 revolution is going to be difficult to cut on a manual lathe unless you get lucky with a wide range of back gearing selection on the lathe. What barrel length are you looking at? What geometric shape will the projectile be? You might consider spherical round-bottom rifling rather than a hook-toothed bottom rifling to lessen the lead build-up...... |
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That's almost 4' long.... Impressive !!! You will be best served by a mid-sized or bigger CNC lathe that can bore that deep; Less is possible if the bulk or half of the barrel length can be positioned inside of the spindle throat. The boring bar itself would have to be at least 50" to 5' long to give it sufficient stability to cut your twist using a feed rate of 120.0 IPR..... You could probably get by with 10 RPM. Ideally, you would be able to have a setup where you can check each end of the barrel for circular runout before cutting the rifling to ensure accuracy. Also, make sure that your barrel tubing is not seamless welded. You have one heck of a project going on for sure !!!! 
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Dextra gladius, laeva scutum, corpore honor
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The rifling could be cut with chemicals and a silicone mask, or EDM that is slowly pulled through the bore. If you make a slightly smaller tube with a channel cut into it at the same rate as the rifling you need then you can set up a jig to hold a post and at the mouth of the bore it will turn the tube as it is pulled out. A spring held cutting head can hold the pressure so it is cutting as it is pulled out. |
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I have a solid bar of chrominized steel that is 5.5 inches in diameter. The bore would be 2.25 inches. I am not sure if its is 4140 or just a high carbon steel that 1045/1050. I believe it is used to make hydraulic parts for heavy equipment. If it is 4140 or something, I should not even need a liner. |
The barrel can have the rifling cut using a 3D printed, simple ECM setup. Here is a video of someone making an air rifle barrel using this technique: Making barrel rifling using water |
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