Safety question, superballs as fodder

[pyro_tek]

Has anyone here tried using superballs, ping-pong balls, or similar inexpensive balls in mortars or cannon? I came across some thick walled (o.d. >= 2 x i.d.) steel tube scrap at a local hydraulics repair shop. They gave me several different sizes and lengths free of charge out of the recycle bin. Until I have the proper ball molds I have nothing but blanks to shoot in the models I'm trying to build.
The local mega-mart sells 12-packs of ~1" superballs for about a dollar. There is also a ready supply of these same size balls just about everywhere I've seen a rack of 25 cent gumball machines (more expensive but awfully handy when you run out of ammo far from home). These balls roll smoothly in one of the pipes I was given. Patched, these balls are snug but slide with little effort.
On light powder loads, in a cool tube, should superballs be safe? Would coating the balls in fine powdered graphite for lubrication and a little fire resistance help or be necessary?
Thanks for the help, p_t

[Cat Whisperer]

Superballs dropped from the 7th floor of a dorm (how would I know?) will rebound to about the 4th floor.  They won't do it more than once or twice.

That should give you a starting point.

[CU_Cannon]

I have thought of trying supper balls but figured it would be unsafe.  The ball will deform from the pressure possibility expanding.  If it expands too much it will get stuck and make your cannon a bomb.  I would stick with the "standard' projectiles.  You may have to wait a little bit but it is better than blowing your self up.

[pyro_tek]

[Superballs dropped from the 7th floor of a dorm (how would I know?) will rebound to about the 4th floor]

In my own testing my nine year old niece and I discovered that a superball dropped from the fourth floor of the Mall of America bounced back up to the third floor. Our sample size was small due to poor planning and mall security however the dorm testing seems to confirm our results.

[The ball will deform from the pressure]

Would pressure build up to the bursting strength of the metal before hitting the pressure required to push the deformed superball from the tube? The pipe I'm working with is quite substantial, seamless with an outside diameter just over twice the inside diameter. I've not cut it to length yet but my tentative plan would have the length at ~5 x caliber for the longest tube.

As always I want the maximum possible level of safety built into this project. I'm working with unknown pipe material alloy and only visual and tactile assurance of pipe integrity. Keep the input coming.

[GGaskill]

I can't answer all your questions from either experience or first principles.  That's why we fire some test shots with the gun in a safe place and ourselves in a different safe place.  

Since the super balls don't weigh much, they will not offer much resistance to the powder gas.  Try to measure both the ball diameter and the inside tube diameter.  If there is enough difference, there should be no problem.

[Cat Whisperer]

[Superballs dropped from the 7th floor of a dorm (how would I know?) will rebound to about the 4th floor]

In my own testing my nine year old niece and I discovered that a superball dropped from the fourth floor of the Mall of America bounced back up to the third floor. Our sample size was small due to poor planning and mall security however the dorm testing seems to confirm our results.

[The ball will deform from the pressure]

Would pressure build up to the bursting strength of the metal before hitting the pressure required to push the deformed superball from the tube? The pipe I'm working with is quite substantial, seamless with an outside diameter just over twice the inside diameter. I've not cut it to length yet but my tentative plan would have the length at ~5 x caliber for the longest tube.

As always I want the maximum possible level of safety built into this project. I'm working with unknown pipe material alloy and only visual and tactile assurance of pipe integrity. Keep the input coming.

Here's another observation on firing flexible bullets.
When I started (1974-75) with my first mortar - beer can caliber, I just filled the cans with water - didn't have anything else, nor a clue to pressure.  Cans would do well.  At the top end as I would increase the charge I noticed the can would (from hydraulic pressure) expand in the middle of the sides and touch the inside of the bore.  More powder and the can would split along the side.  It was good that I started small and worked the charge up.

You've got the right attitude of wanting maximum safety.  The risk is that at some point - perhaps caused by a build up from firing each round - that the pressure will increase exponentially.  This is all theory at this point.  Testing will certainly give you more knowledge.  Do you have a really BIG tree to get behind if the tube goes?  That's the attitude I've grown up with, knowing that for years I'd never see 21.

[Mark 42]

Keep in mind that the TURE Superballs are not very common anymore. They were extremely dense rubber, and bounced far more than the common "bouncy balls" that you find at the dollar store these days.

I have thought about using the bouncy balls (if I ever finish my cannon)... I found a package hidden in our bedroom (I assume they are hidden from our kids, but can't be sure). They appear to be about the same diameter as the bore of the cannon I'm building. I probably will measure one tonight. I suspect I'll want smaller ones so I can use a patch to avoid creating a plugged up barrel condition.

[pyro_tek]

Sounds like you are working on the same test that I was thinking of trying. My plan included a very clean bore and a patch, possibly impregnated with fine powdered graphite to act as an inert lube.

I've gotten sidetracked so have not followed up on my tests. Please keep us informed on your progress.

[Cat Whisperer]

Just a word of caution.

You're moving into uncharted waters playing with superballs (or near equivalent) from solid rigid projectiles.

Common practice has determined powder charges for specified diameters of bore with common materals (some work some don't) and specific rules for amount of windage - NO patching (for cannon caliber projectiles).

IF you want to be safe, as you go into this project, trying things that have not been proven by common practice, I would strongly recommend on planning on the cannon exploding.  If you take the precautions for that you stand a better chance of living longer than if you don't.  Then IF the cannon doesn't explode you have discovered a combination of charge, projectile, and design that worked once.

That's a long-winded way of saying LOOK AT WHAT OTHERS HAVE DONE SUCCESSFULLY and copy that, or plan for the worst on the off-chance it will happpen.

[GGaskill]

If you are talking about the balls sold by WalMart with 8 or 10 small ones and a bigger one, they are not very round and are greater than 1" in diameter.  Shootable in the right gun but not a 1" bore.

Where did you buy yours?

[pyro_tek]

The Malwart was the first of several places I found "superballs". Vending machines, dollar stores, and pretty much every five and dime are other sources for virtually identical balls. Not only are they imperfect spheres but when pressed in a vise they are substantially more than 1 inch in diameter. They are far from ideal to use in any bore.