Clearance between bullet and bore in cannon and mortars

[Cat Whisperer]

The question arose on what the bore diameter would be good for the bowling ball mortars.  This raises the question of what the clearance should be between bore and bullet in the larger bores, as in cannon and mortars.

I have some limited experience with two or three different diameters.  

Let me relate some observations; later in life we might find some good rules of thumb, and if we get genuinely professional about this we should come up with some principles that will produce good safe designs.

My first mortar is of the old beer-can size - pre pop-tops.  It is still the size for soup cans, although they're a little shorter.  When I was turning it on the lathe, I simply made it a little larger - for a loose slip fit.  When I fire it I will put one layer of a 'kim-wipe' dampened with water over the muzzle for a patch and load the can.  This keeps the buildup of fouling down.

Then I watched someone fire a 12 lb mortar with a 4.5" 'bullet' made from PVC pipe filled with concrete.  There was about total of 1/2" clearance.  It still worked, but the comment was that with a tighter fit it worked better.

I had my 4.5" mortar made to 4.550" bore diameter to fit a 4"PVC pipe (which is 4.500" in diameter).   That's .050" clearance on the diameter.  In practice one needs to wipe out the bore every or every-other shot to keep down the fowling buildup.

I used a little less clearance (0.035") on the soda-pop 16-20-24oz size.  This worked very well.  The tube was MUCH longer requiring less powder and had much greater range - and had much less need for cleaning.

So bigger diameter - more clearance.  Lots of clearance will cause a decrease in velocity and probably inaccuracy.  Too little clearance could be unsafe, but certainly makes loading tough.

Shorter barrel - the temptation is to use more powder, hence more fowling and needs more clearance.  Longer barrel - less powder, better ignition/burning of powder, less clearance needed.

Heavier bullet, better burning of a given powder charge, better range and cleaner burning.

Re-enactors often will add flour for more smoke.  I've never tried it.

So if the bowling ball is 8.5" in diameter, and the bore 8-5/8 that would be 1/8 or 0.125" clearance.  That, from the easy-chair, sounds reasonable.  

So how about some observations from practice about what works (or not) in the larger bores?

Thanks,

[Double D]

The first mortar I made was for the old steel beer cans. We  had to use soup cans.  We made it slip fit.  Drop the can down the bore and it slid down to the bottom.  Put your finger over the touch hole and it would stop.  We actually fired it the first timed with an air hose.  Made a dent in the ceiling in the shop. Thought it would be easier to get the can out  that way, well it's faster that way.  

I don't remember the dimensions, but we came came up with it by measuring the can.  I don't know how the machinest did the bore as I had it done.  The other three I drilled partial and bored the rest of the way to the aluminum pop top can dimension, then honed with a hone for motor cycle cylinder until the can would slip.

[WLB]

:-)

DD, when you said "hone for a motorcycle cylinder" you really got my brain a going.  I bore and resleeve hundreds of motorcycle cylinders a year.  Do you think that a cast iron sleeve shrunk into a thick aluminum tube would hold the pressure of a mortar?  Years ago I had some real thick liners that we used to reduce the bore size to make an engine fit a particular racing class.  I know that L A Sleeve will make most any size sleeve but I doubt that you could get one much over a foot tall.  This method would require a breech plug.  
Bill

[Double D]

NO!

No cast iron, it will break.

Some modern made cannon barrels are cast.  The tube is then sleeved with a seameless steel tube liner.

It's done for strength.

I like you idea though it shows you are out there thinking.  You could do a seamless tube liner in the aluminum.  Machine a threaded or press fit base plug and pin it in place.  No welding.

The only questionable in my mind is the light weight of the aluminum tube. The gun might jump around to much.  With a high angle gun like a mortar this might not be as noticable as flat shooting tube on a cannon. the mass of a steel or iron tube helps reduce recoil.

Trust me these guns full size or scale model recoil!!!!

[JeffG]

Boy, it is nice to have some experience to share!  Some scattered points, I would like to make:
Bore length should be 2 1/2 bore diameter, that point was made correctly earlier.

Baron Menno von Coehorn was an early (16th) century designer, his country failed him, (Holland) so he went to work for the Austrians...he found this out:

a mortar is best served on a bed, where the entire surfaces provides friction to the GROUND...wheels don't work.  60 degree elevation is optimal for recoil also, adjust range with powder increments.
 Iron sleds were ok if the BARREL was very heavy too.  In both cases, he found the barrel trunions had to be secured very well to prevent barrell movement during recoil, a shot straight up sucks!
Our prospective barrels are going to be light.
 Mortars operate best with a powder chamber, which is tapered and has  1/2 the bore's size.  This makes them very efficient with powder.  This also works with the proposed shape of the manifold end of a gas tank...

The bore's pressure is  greatly affected by projectile density and shape.  Balls were found to work best because of projectile velocity from a mortar.  In our case most bowling balls are filled with cork and rubber, or concrete.  I cut bowling balls open all the time for knife making material...This makes them safer pressure-wise that iron or lead....they tend to be just big resin balls...this is good.

Man you got me fired up, you guys!

Gotta go check the scrapyard!!
Later!!