early mornings and powder chambers

[Don Krag]

While getting ready this morning we heard someone knocking on the back door at 6:20am. To my delight, it was a buddy from down the street who works in a CNC machine shop. He bored out a 24" x 3" tube to 1.723" with a fine surface finish. I was really impressed. Golfballs float slowly down the tube with a slight hiss of air around the edges.

Now for the powder chamber. In addition to golfballs, I plan on shooting some special solid projectiles. If i go with a .75" dia chamber long enough to hold ~300gr of powder, will the airspace pose issues when only shooting golfballs with 50-75 gr? The chamber area will have 1 1/8" walls just on the main tube, plus an additional 1/4" from the re-enforcing ring(medieval bombard style). If the airspace could pose an overpressure issue, could I put sawdust or newspaper wadding over the powder to fill the void?

[lance]

 you don't have to fill the whole void, a small wad of paper over the powder will do the trick. you could roll some paper or foil cartridges, with a paper wad at the front.

[Double D]

300 grains in a .75 bore is a massive over load, it is three time the recommend charge for a .75 bore.  Maximum recommend load for a .75 bore is about 100grs.

If you are shooting 50 to 75 gs. with a golf ball you want even less with a heavier projectile.

If you have a windage in your projectiles the air space is not a big issue.  Only in a tight fitting load that has greater than 30% air space is there a need for concern.

[lance]

 Double D, you did bring up an important point, and i would just like to add to my first short post. i meant the paper or foil cartridges for the 50 to 75 grain black powder and golf ball loads. not the 300 grain, and solid projectile, home made bomb killer loads.

[Don Krag]

I guess I was going by projectile dia when looking at loads and misunderstanding cannon physics. This cannon was originally going together with some medieval plans where a long, narrow, thick-walled, powder chamber is used with a much larger bore.

Edit: Guess I missed the big bold *Addendum* for howitzers.

So going back to the charts, if I want to push a 1.75" projectile with a max load of 200-300 gr, do I then work backwards for chamber dia(ie 1.1 - 1.25" chamber dia and adhering to projectile weight limits)? It seems like using a smaller chamber dia with a much thicker wall:chamber ratio would be stronger than opening up the chamber dia, for a given bore. 

Another question...
Does a sub-chamber automatically designate something as a howitzer regardless of borelength to dia ratio and such, for NSSA?

I know...questions, questions and more questions. I'd rather ask a hundred questions and bore/drill once, rather than end up with some pretty paperweights!

[accuratemike]

Don, the way I figure, based on the "1 caliber thick at the breech" rule, you divide the the OD of the tube by three to get the max chamber diameter. In your case, 1". My GB mortar's chamber is about 1"x1" and the mortar is about 3" OD around the chamber. In a deeper tube, you might have a hard time getting the charge in the chamber. There is a scoop-looking thing on a stick for just such occasions. I would run a 1" drill in there and follow up with a ball end mill to radius the bottom of the chamber. I radiused the transition from bore to chamber freehand in the lathe. That would be hard at the bottom of a longer tube. Maybe a radius spade drill cutter would be better. Regarding the airspace issue, I would do foil cartridges with a wad at one end. Mark the wad end . A 1" cartridge might be easier to get in the chamber than a longer .75" one. Sounds like a nice chunk. Keep us posted. MIKE

[Don Krag]

It'll be getting a re-enforcing band shrunk on around the rear 8" (similar to a parrot rifle style) plus a 1/2" wide band every couple inches for the bombard look. If I used 3/8" wall for this, I could bump up the chamber to 1.1-1.2" dia. If I go with that and radius/chamfer the opening, that would probably be MUCH easier to get seated!

This one's still a ways down the road. I'm working on the Sikh standing mortar carriage right now. I need to find a friend with a large ripping bandsaw! Working down seasoned oak logs with a chainsaw sucks. Truing the faces up sucks even worse! I've at least enlisted help for the wheels!

[Double D]

A 1.75 inch bore falls into the gray area.  It's not realy a large bore and it s not realy a small bore.

It's off Switliks chart for small bore but  it isn't 2 inch either.

I once did an extension of Switlik's chart and it comes out 2 oz per inch over 2 inch bore.  So use that.

1.75 bore times 2 oz of Cannon grade per inch equals 3.5 oz.   3.5 oz times  437.5 equals 1531.25 grs max load.  200-300 grains would be reasonable for  a 1.75 inch bore.  Safe breech  wall thickness is one caliber or in you case 1.75 inch.  Safe minumum breech diameter for a 1.75 inch bore would be  5.25 inches.

[accuratemike]

  Safe breech  wall thickness is one caliber or in your case, 1.75 inch.  Safe minumum breech diameter for a 1.75 inch bore would be  5.25 inches.

Yes, but I think he is sub chambering. I was saying that the maximum chamber bore for a 3" OD chunk would be an inch.

I found a black powder density chart here:http://www.tbullock.com/bpsg.html. "F" has a density of 235grains/cu in.. 1531 grains is 6.51 cu in of powder (!). A 1" cylinder is .785cu in per inch of depth. That would require a powder chamber 1" dia x 8.3" deep(!) A 1" dia x 1 5/8" deep chamber will hold 300 grains. Looks like my mortar holds ~185 grains.

Food for thought, MIKE

EDIT So, while I was out getting lunch, I had a thought;
On chambered pieces the max load would be dictated by the diameter of the chamber (not the bore). I think I might have asked this before, but i can't remember the answer (or if there was one). We are probably really talking about 1-1.25" bores (max load wise)for chambered golf-guns.
Then I looked at the chart at the S.L.C.P. stickie. I see the addendum (thanks DD). And ~180grains is a max load for an unpatched ball in a 1" bore. Maybe I figured that out when I was building the mortar (I suffer from CRS sometimes).
Using the NSA's 2oz/inch for blanks, It would be nice to fit 2-2.5oz of powder in the chamber. 2oz of F would be 3.72cu in, or 4 3/4" deep in a 1" bore. 2.5 oz of F would be 4.65 cu in, or 3 3/4" deep in a 1.25" bore.
Sorry, I had some spare time

[Don Krag]

Yes, this is sub-chambered. Looks like the tooling available says the subchamber will be 1.125". After polishing it'll end up around 1.15". Based on that, I'll go with 3/8" additional banding which will give me an overall wall thickness of 1.3". That gives me a max load of 240 gr. If I go with a 1" deep chamber, that gives me ~233 gr, pretty much right in the middle of the range I was shooting for. Sound workable?

[Don Krag]

Here's the conceptual drawing. It's based on some Swiss designs from the 1550-1600 era. I might lengthen the 48" dimension to more like 60" and mount the pivot point about half-again closer to the axle. According to the historical groups over there, a nice burgundy color is proper. I'll try and resist the temptation to put some Texas A&M logos on it.

[Rickk]

One possible reason why the 2 oz per inch rule does not scale into smaller calibers is that Cannon grade powder is not really practical with small calibers. It does not measure well, does not burn well, and in really small ones it just does not fit.

In a 50 caliber for instance, 2F would be pretty much standard behind a ball, with 3F for a blank.

I personally use 1F for balls and 2F for blanks in my one inch bores. I cannot quote a source that recommends that, or anything else in fact, but that is what I use.

With smaller grain size, the peak pressures are going to be higher for a given charge weight. That could be a consideration with respect to not generalizing the 2 ounce per inch rule with smaller bores.

Since my 1" barrels tend to be fairly overbuilt, exact loads I will keep to myself. The 1 to 2 inch bores are sort of in a no-man's land with reguard to generalized recommendations. There are a few 1" shooters on this board that do trade load info in private, but it is generally barrel design specific and we sometimes get the barrel manufacturer in the loop.

[CU_Cannon]

I like the design. 

Why did you choose 3” stock with separate bands rather than larger stock turned down?

[Don Krag]

Several months back I got a heck of a deal on several drops. The largest was 3". I was really wanting 4", but I couldn't pass on the price on what was available. I got some 1 3/4", 2", 2 7/8" and 3" all in 1045 and 4140 steel. The smaller are being used for gonnes and use turned imitation rings. The middle sizes are cut into trunions and various armour making tooling stake heads.

My *regular* noisemaker is a tiller-mounted 1.1"ID x 17.5"L. I also use F for balls/shot and FF for blanks, pinecones, and frozen cherry tomatoes.