Minimum wall thickness for soda-can bore?

[Terry C.]

I've gotten my hands on a couple more of the grease cylinder rams. These are the same hard-chromed 1045 steel rods that I used to make my golfball Coehorn.

A Confederate-pattern mortar will most likely be my next project. I already have three Union-pattern Coehorns (.69 paintball, ¼-scale 12 pounder, and golfball bores).

I would like to make it a soda-can mortar. If this were going to be Union-pattern tube, with bands, this stock would definitely be too small in diameter to accommodate a soda-can bore. But since this will be a straight-sided tube, I'm thinking it be possible.

The OD of the stock is 100mm ( 3.937"). I'll need to cut away the chrome and any pitting, plus a little extra to get past the hard case layer just underneath the plating. I'm figuring about .050" per side.

That will put the OD of the finished tube at about 3.837". Possibly a little larger but I can't guarantee it. 3.837" is a safe number so that's what I'm using for my calculations.

The bore will be 2.625". That gives a wall thickness at the bore of .606". The chamber will, of course, meet or exceed the 'one caliber' spec.

Do you guys think that is sufficient? I know the pressure is very low out in the bore, compared to the chamber, and I've seen other mortars with bore walls that are much thinner in proportion.

If there is any doubt, I won't do it. I'll reduce the bore to something smaller than a soda can but larger than a GB (2¼" maybe?).

[Double D]

Pen and pencil Math tells me maybe.

I don't have a Federal pattern scale drawing.  I do have the Confederate  12 and 24 PDR. 

Here's my Math.

12 PDR CSA OD 7.75, bore 4.62, wall 1.565
Scaled to  popcan bore using your dimensions.  4.62/2.625= 1.76.  1.565/1.76= .889 wall for pop can to scale.

24 PDR CSA OD 8.75, Bore 5.82, wall 1.465
Scaled to  popcan bore using your dimensions 5.82/2.625 = 2.217.  1.465/2.217= .660 wall for pop can to scale.

I'd go for it!

[GGaskill]

The US 24 pounder Coehorn had a bore of 5.82" and an external diameter of 8.65" in front of the chamber and 7.65" for the majority of the distance to the muzzle.  Using 2.65" for a beer can bore, the scale ratio is .455.  The original wall thicknesses would be (8.65-5.82)/2=1.415 and (7.65-5.82)/2=.915; times the scale ratio gives .644" and .416".  I have made one to those dimensions with a 1" hemispherical bottom chamber and think it is fine.  In fact, it is the one I submitted for the calendar.



Although is looks like bronze, it really is steel, and that is what gives me confidence.

[Victor3]

 I wouldn't hesitate to make your mortar. A correctly machined bbl of that material at those dimensions will easily contain well over 20,000 psi. Your bbl is never going to see pressure nearly that high at the bore using appropriate black powder loads with a cement filled soda can.

[Terry C.]

I've been playing around with the proportions a bit, it seems to be very doable. 

One question: Which would be more 'correct' for a CSA pattern tube, the full-radius or the slightly flat-bottomed breech?




The CSA mortar is a very simplistic-looking tube. That's the primary reason I've made my other mortars Union-style. The shape and details of the Union Coehorn make it appealing even to people who have never seen a mortar before.

The flattened breech end would be easier for me to make, but it would be worth the extra effort to make the full radius if that's what the originals used.

[Double D]

Full radius

[Terry C.]

The CSA mortar is a very simplistic-looking tube.

After re-reading what I wrote, I didn't like the way it came out. I wanted to make it clear that this was not a criticism of the design, just an assessment of the construction details. These mortars were manufactured under difficult circumstances and their spartan design reflects that.


With that said, DD, is that drawing from an original set of plans? 12 or 24 pounder?

[Double D]

That is a drawing from the West Bros. and yes it is considered a copy of the Original

[lance]

 Terry, i don't know if this will help, but here's another pic of an original: