Air between projo and charge?

[rusty barrels]

 While being too cheap to buy a 60lb bag of concrete for six pop cans I used some hydrocal that I had laying around, boy does it set up fast. Then I got to looking at the bottom of the cans and became aware that they are concaved, so with a given amount of powder I'm gonna have some air space between the projo and powder, is that a problem? At the risk of getting flamed, I know it is dangerous with blackpowder firearms, is it also true with cannons? The mortar does have sub chamber, which brings up another question, do you pour a premeasured amount directly in the chamber or use a foiled charge.
 

[gunsonwheels]

I really do not know about space between the charge and the mass of the projo but:
 
The old ordnance manual near the back of it on pressure maximums achieved states a pound of powder in its volumes achieves a maximum pressure of about 120 KSI where-as in twice its volume creates a little over 40 KSI.  I'd post a link to the manual and page but like most other things around here I don't seem to be able to find it.  Anyone else have it handy??
 
GOW

[keith44]

as long as you have proper windage the powder charge will not develop enough pressure to be dangerous in a properly designed cannon or mortar.


Others with more experience will likely be along soon with better info, but without a wadding, or patch, or something to seal the bore more than a bare projectile (again with the correct windage)  it is my opinion that it would be as safe to fire as a flat bottomed projectile

[Victor3]

 The highest pressure is going to be in your powder chamber; it drops substantially as gas escapes the chamber out into the bore (that's why we can build can-size mortars with fairly thin-walled barrels).
 
 Also, a concrete filled can is pretty light and has a lot of surface area on the base. It's gonna get moving PDQ when the charge goes off even if you have a little air space between it and the can. I wouldn't worry about any pressure spike.
 
 I pour a measured charge into my mortars from a film can.

[little seacoast]

I've been shooting the filled cans for a while with no problems. Powder introduced with a funnel and tube, tube end placed in powder chamber.

[guardsgunner]

It was British theory that (through testing Im sure ) that a charge of reduced dia. behind a std, projectile would lower pressures and increase ft/second.
 

[Artilleryman]

In chambered mortars there will be some air space between the charge and any projectile.  So you shouldn't have any problems in a properly designed and built mortar.  If you can post some photos of your mortar firing.

[rusty barrels]

 The mortar is made by HMR so I'm not worried about its construction. Since I used hydrocal instead of concrete I weighed the projo and it is 1.5 lbs. Does anyone know what a pop can weighs with concrete?

[little seacoast]

Cans 2.60"@16.2oz
        2.20"@ 9.5 oz
Concrete fiber mix

[ironball]

Rusty, what you have heard about air space in blackpowder firearms is true, because they have a tight fit between the patched ball or bullet, and the barrel. So if you have an air space, the projectile acts more like a bore obstruction, and can cause the barrel to explode. Like it was said before, with proper windage it's not a problem in a cannon or mortar, but I wouldn't want you to think you can get away with it in a rifle, shotgun, or pistol.

[jeeper1]

My mortar mix filled cans weigh 26 to 28 ounces.
I fill the cans with the dry mix add a wire loop then place them in a bucket and add the water last until the water is at least one inch above the tops of the cans. After 24 hours they are hard.

[rusty barrels]

 Thanks for the info guys and jeepers thank for the weight info. The hydrocal seemed heavier than concrete. It is pretty dense and tough stuff.

[JeffG]

Hollow based wadcutters!

[Double D]

There is a well meaning but misinformed concern about airspace in black powder.  Despite the warning airspace by itself is not hazardous in black powder or smokeless powder arms. 

There are specific parameters  required to create the conditions needed to ring, bulge or burst barrels.  Type and burn rate of powder, position and method of retention of powder, amount of air space,  metallurgy and contsruction of barrel all have a bearing on the phenomenon.  Air space by itself will not do it. 

Follow the safety standards for gun design, maximum loads and safe loading procedures- including using windage in cannons and you will not have any concerns.

If you wish to learn more about the phenomenon read the works  of  Paul Veille.  Some of his work is translated from the French in Major Sir Gerald Burrard work on the Modern Shotgun.  More modern writing  by the  Charles Dell also describes the problem.  See his book the Modern Schuetzen rifle.  Dell was able to create and eliminate at will the phenomenon.