Building a Mountain Howitzer

[C2ND]

Hi, I have been lurking for about a month and have found a lot of great info. on this sight!  I am looking at building a Mountain Howitzer and would like some advise on wall thickness.  The tube is DOM 3" inside, 5" out, 1"sidewall. Is this heavy enough to fire concrete filled soup cans or even lead balls? What size powder chamber should I use for this?
Thanks,
C2ND

[Cat Whisperer]

C2ND -
First, let me say WELCOME!

A 1" wall thickness is "right-much strong" in DOM.  Engineering can be done in at least two ways.  One, run the numbers based on proper theory, education and experience (qualifications as an engineer in the area).  OR, since most of us don't fit into that category, look at what others have proven by experience over a long time and build it stronger.

If I'm not mistaken, the N-SSA requires that cannon used in their competitions must be steel tube (seamless) lined and the lining be 3/8" wall thickness (check it out in their regulations, don't trust my memory).  That should tell you something right off the bat on the relative strength of 1" wall thickness DOM tubing.

DD has some references (as the More Complete Cannoneer - if I remember the title correctly) that give some good rules of thumb as to powder charge, size of powder chamber, windage and such.  Those are good starting places.

With DOM, you have the issue of how to seal one end - threaded with crush fit shoulder and then welded further back (not welded in the area where the porosity of the weld would come in contact with the products fo combustion.

This is a start on the answer to your question, others will undoubtedly pitch in.

Good luck, keep us informed and post some pictures!

[Cat Whisperer]

.... The tube is DOM 3" inside, 5" out, 1"sidewall. Is this heavy enough to fire concrete filled soup cans or even lead balls? What size powder chamber should I use for this? ....

I have a mortar bored from a 5" diameter round of 4140 to just under 3" ID (fits 16-20-24oz soda pop bottles)  Has a powder chamber of about 1" x 1" (should be bigger).  It has no trouble shooting two 24oz bottles of concrete over a couple of ounces of FFg.  Bore is about 15" deep on 19" tube.  I'd consider 1-1/2" x 1-1/2" powder chamber were I to do it again.  In fact, it may just get modified to become a mountain howitzer if I can find a chunk of bronze/brass to slide it into.

[C2ND]

Thanks CW,  
A question on the chamber/breech plug. Should the face of the plug be cone shaped to the powder chamber or is a flat surface better to minimize air space between bottom of projectile, {can of concrete} and the powder charge?

[GGaskill]

chamber was cylindrical with round corners and 3.34" diameter by 2.75" length.  That connected to a frustrum of a cone (piece of a cone with circles at top and bottom) with a small diameter of the 3.34", a large diameter of 4.62" and a length of 3.25".  Scaling all that down to a 3" bore would give 2.17" for the 3.34 dimension, 1.79" for the 2.75 dimension, and 2.11" for the 3.25 dimension (the 4.62 dimension obviously goes to 3.00".)

The original chamber was designed for round shells or canister, both with the powder bag pre-attached with iron straps to a wood sabot between the projectile and powder bag.  If I were doing one for flat-bottomed can projectiles or round shot, I would go with a 2" diameter by 2" long chamber with a 120º included angle to bore diameter.  Filleted (round) corners at the bottom would be a good thing.

[Cat Whisperer]

Thanks CW,  
A question on the chamber/breech plug. Should the face of the plug be cone shaped to the powder chamber or is a flat surface better to minimize air space between bottom of projectile, {can of concrete} and the powder charge?

The idea behind the powder chamber is two-fold.  One, it provides greater strength because of greater wall thickness; and two, it (when not filled entirely) provides CONSISTANT air space from round to round.  That doesn't answer the question of little vs. a large air space, but I think it is more important to be consistant.

[Double D]

Airspace in black powder guns is a real issue in bores with tight fitting bores.  It's less an issue in cananon, but it is an issue none the less.

I don't think we have ever talked about marking your ramrod to show the depth of a properly load charge and a properly load ball. When you load you need to see that the ramrod align with these mark or find out why not.

[C2ND]

I have finish internal machining on tube, and finished Breech plug.



Breech plug has 3 1/2" x 4 threads, it is 6" long with a 2" x 2" powder chamber.  Plug will tighten down against a shoulder cut in the tube, then it will be welded in place.