Author Topic: BB Mortar (Read 1984 times)

191701 · « **on:** June 25, 2010, 11:58:28 AM »

Anyone want to learn me a thing or two on these o2 cylinder bowling ball mortars? Is everyone using the powder chambers? Seems like most are using just the cylinder tapped out for a trailer ball and a fuse hole. My cylinder measures 8.74" -8.75" inside. wall thickness .25" Seems like allot of clearance for a 8.5" ball. I am a machinist and know some good welders. Are there any drawings on these. Thanks and great site.

Cat Whisperer · « **Reply #1 on:** June 25, 2010, 01:00:24 PM »

191701 -

WELCOME to the board.

(do a search on BB mortars and read the details of how to build the powder chamber).

The powder chamber is built for saftey! By making the ID equal to the wall thickness it will have enough strength to withstand the peak pressures generated by the black powder. The pressure right after the powder chamber is MUCH less, hence within the strength of the cylinder itself.

If you don't have one, sometime down the road you will have a catestrophic failure when the metal fatiques and cracks. You need to KNOW when you light the fuse whether it's a cannon or a bomb.

smokemjoe · « **Reply #2 on:** June 25, 2010, 06:16:05 PM »

I would like to add that the balls I have checked are 8.560, The 3-2400 Argon tank I have has a ID of 8.750, The fit is nice, DD and the fellows here told me the specs. on making the powder chamber, I fired it last week in and after a few shots I was very glad I had the 6 in. X 5 in. steel chamber, When these go off and you see that big ball high in the air and the noise your be very glade to have made a safe mortar. I got pics. from sawing the tank,machinceing, to making the sled and firing, I just got to get someone in here to help me get pictures going. Joe

js2346 · « **Reply #3 on:** June 27, 2010, 01:32:23 PM »

Whats the make and model of the cylinders that fit bowling balls. I know its got to have 29 1/8" circumference. I read one post on the forum that says its a 300 CF, DOT 3AA2400 made by Taylor Wharton. Can anyone confirm this so I can make sure I buy the right tank! Thanks

smokemjoe · « **Reply #4 on:** June 27, 2010, 06:28:23 PM »

My tank didnt say Taylor Wharton, but it did have 3AA-2400 on it and that was the right tank, 8 3/4 inside.

191701 · « **Reply #5 on:** June 28, 2010, 01:19:54 PM »

I have several feet of 4" 1018 and a piece of 6" diameter 4140 that is only 4" long. Will either work for the powder chamber? Is there a safe load to shoot this thing without a powder chamber?
Thanks

Cat Whisperer · « **Reply #6 on:** June 28, 2010, 01:45:51 PM »

Both are excellent steels for this application.

1018 is weldable and is one of the preferred steels for cannons.

4140 is of higher strength, but poses some issues when trying to weld it. Some folks consider 4130 to be on the edge of weldability; but talk this over with a CERTIFIED welder. (It has to be preheated and it has to be done right (technique, rod, etc.))

191701 · « **Reply #7 on:** June 28, 2010, 03:12:09 PM »

If I use the 4" 1018. What are some ways to do it. Could you cut 3-4" of 1" -14 threads on the powder chamber and screw it into the valve hole from the inside. Less welding. Maybe a 3/4" piece welded to the outside and going to the powder chamber to bridge the gap for a fuse hole? learning here so please correct me or make suggestions. Hole in the powder chamber 1 5/16" id? how deep? Maybe a radius to fit the ball? The powder chamber needs at least 1 5/16" solid all the way around and bottom. right? Thanks

GGaskill · « **Reply #8 on:** June 28, 2010, 03:55:04 PM »

I see no reason you couldn't contour the back of the chamber piece and bolt it in through the hole in the neck. You wouldn't need any welding then, at least regarding the powder chamber. You could cut off most of the neck as it won't add anything to the equation.

Double D · « **Reply #9 on:** June 28, 2010, 06:00:13 PM »

Quote from: 191701 on June 28, 2010, 01:19:54 PM

Is there a safe load to shoot this thing without a powder chamber?
Thanks

No.

191701 · « **Reply #10 on:** June 29, 2010, 12:24:33 AM »

How does the pressure in the cylinder compare with the pressure in the powder blast chamber? Is the blast chamber taking 90+ % of the pressure? I guess allot would depend on powder type, ball weight and size of chamber.

Victor3 · « **Reply #11 on:** June 29, 2010, 01:00:23 AM »

Quote from: 191701 on June 28, 2010, 03:12:09 PM

Could you cut 3-4" of 1" -14 threads on the powder chamber and screw it into the valve hole from the inside. Less welding. Maybe a 3/4" piece welded to the outside and going to the powder chamber to bridge the gap for a fuse hole?

If you're going to have it in the lathe it to cut threads anyhow, why not turn a boss on the end behind the threads and drill the vent (fuse hole) into that rather than weld on a separate piece?

Cat Whisperer · « **Reply #12 on:** June 29, 2010, 02:04:25 AM »

Quote from: 191701 on June 29, 2010, 12:24:33 AM

How does the pressure in the cylinder compare with the pressure in the powder blast chamber?
...

The pressure curves always will show a huge spike in pressure quickly tapering off. The spike occurs early in the burn (substantially in the powder chamber). Subsequently the pressure is MUCH lower in the tube as the diameter (volume) is MUCH more.

These are GOOD questions. ALL of them need to be answered to be safe.

js2346 · « **Reply #13 on:** June 29, 2010, 04:21:20 PM »

Has anyone ever calculated the PSI generated by the blast? Maybe others have done this research? Obviously it would depend on the load you are firing. If we know the blast pressure and the strength of steel we would be able to determine the safety factor

Double D · « **Reply #14 on:** June 29, 2010, 04:24:26 PM »

The chamber is where the major pressure is, the ball portion of the barrel is an expansion chamber and pressure drop off rapidly.

GGaskill · « **Reply #15 on:** June 29, 2010, 06:27:04 PM »

Has anyone ever calculated the PSI generated by the blast?

It is almost impossible to calculate chamber pressure with any degree of accuracy in small arms. And low pressure black powder arms are probably similar. I know of no formulas that are applicable (but then I don't know everything, either.)

191701 · « **Reply #16 on:** June 29, 2010, 11:30:44 PM »

Can I make a safe and effective blast chamber with the 4" 1018? I can go bigger but I already have the 4" material. How deep would the 1 5/16" hole need to be? On another subject has anyone priced thick walled tube? I know where some 8" id 11" od material is. Will try to make a deal with the guy.

Cat Whisperer · « **Reply #17 on:** June 30, 2010, 01:42:57 AM »

Quote from: 191701 on June 29, 2010, 11:30:44 PM

Can I make a safe and effective blast chamber with the 4" 1018? I can go bigger but I already have the 4" material. How deep would the 1 5/16" hole need to be? On another subject has anyone priced thick walled tube? I know where some 8" id 11" od material is. Will try to make a deal with the guy.

The 8" id 11" od would LOOK COOL! You'd still need a powder chamber though. The weight of the tube would aid in recoil too.

191701 · « **Reply #18 on:** July 03, 2010, 03:32:38 AM »

Can I make a safe and effective blast chamber with the 4" 1018? I can go bigger but I already have the 4" material. How deep would the 1 5/16" hole need to be? Any ideas? Or should I stop being cheap and buy some 6" 1018?

Double D · « **Reply #19 on:** July 03, 2010, 04:41:04 AM »

4 inch or 6 inch is your call. Treat the powder chamber as a cannon, one caliber rule applies on the breech end.

4 inch diameter gives you 1.33 inch diameter bore. Maximum safe charge from Switlik's chart for bores under 2 inch is 250 grains. To determine the depth of the chamber bore a 1.33 hole deep enough to hold 250 grains of Cannon grade. Measure the depth of the hole and add 1.33 to get the OAL of the piece.

For 6 inch your chamber is going to be 2 inches in diameter. Follow the 2 oz. per inch rule for maximum safe load. Bore a 2 inch hole deep enough to hold 4 oz. of Cannon grade. Measure the depth of the hole and add 2 inches for over all length.

Personally for a bowling ball I would opt for the 6 inch.

191701 · « **Reply #20 on:** July 03, 2010, 01:32:06 PM »

250 grains = around 1/2 ounce right? I think the 6" is the way to go.

Double D · « **Reply #21 on:** July 03, 2010, 02:04:13 PM »

Quote from: 191701 on July 03, 2010, 01:32:06 PM

250 grains = around 1/2 ounce right? I think the 6" is the way to go.

No 1/2 ounce ounce is 218 3/4 grains. 30 grains will make a difference. Weigh all charges this large on a scale. Volume measures this large are to irregular.

If you don't have a scale get one for these large charges.