Interupted screw breach

[jeeper1]

I have been following this discussion just waiting to see how long everyone would overlook the obvious. Obvious at least to me, because I spent 8 years working in an aerospace machine shop mostly as a handfinisher using a die grinder. A die grinder, for those who don't know, is an air powered version of a dremel, larger, much more powerful and using attachments.
You just cut the threads both internal and external, make sure the parts fit together properly,  then carefully measure, mark,  then grind away the threads in the proper location on each part.
In fact if you want, you can go with a  60 degree turn instead of 90 degrees.   

[GGaskill]

Maybe a double or triple start if I wanted to spin it off faster....

You can't have too steep a thread or the internal pressure will have a tendency to unscrew it for you, requiring a strong lock to prevent unwanted rotation.

[Cat Whisperer]

I have been following this discussion just waiting to see how long everyone would overlook the obvious. Obvious at least to me, because I spent 8 years working in an aerospace machine shop mostly as a handfinisher using a die grinder. A die grinder, for those who don't know, is an air powered version of a dremel, larger, much more powerful and using attachments.
You just cut the threads both internal and external, make sure the parts fit together properly,  then carefully measure, mark,  then grind away the threads in the proper location on each part.
In fact if you want, you can go with a  60 degree turn instead of 90 degrees.   

Doing the outside thread is easily accessible, inside could be a chore.  (Love to have your comments on the techniques.)

[Cat Whisperer]

The issue of strength is very similar to the strength of multiple lug bolt action (or rotating bolts in semi-auto actions).

The three lug design lends itself well to very strong actions, as the lugs are massive and can be wider at the bottom and narrower on the top and still fit well together (with a 60 degree rotation).  The strenght is roughly proportional to the area of contact between the mating lugs.  I was involved with the design of a custom bolt/barrel extension in an M16 derivitive belt-fed machinegun a few years back.  There were more issues there than just the number of lugs and spacing - namely the use of a lug to strip a round out of the magazine or from a disentigrating belt.

I can see one very great advantage to breech loading cannons - the abiltiy to load a well fitted bullet into engagement with the rifling.

[Thorn]

Lug's ? Come to think about it i do have a pic of a small breach loading signal cannon (i think its a winchester), that uses a simple breech with two lugs. Of course it uses a shotgun shell.

So, mabe a three lug with a mushroom head for "bag" charges might be the ticket. Anyone see any problems with this?

One might even take the lug design from say a .50 like the madi-griffin (sp), and super size it.

[jeeper1]

I would use a rolled sandpaper cutter to remove the unwanted parts of the threads both internally and externally making sure to radius the ends of the threads. An inside radius at the bottom of the threads and an external radius at the top on both parts. Sort of a laid over "s" curve.

[GGaskill]

Actually, it's no big deal to make a breech loading cannon if you use a plain interrupted thread instead of the Welin-type breech block.  Even cutting away the threads on the internal part is not a big deal if the internal threads are done in a separate piece that the barrel then screws into in the fashion of a rifle receiver. 

With smaller guns, the block is going to weigh only a few pounds up to maybe ten pounds, which is easily handled, unlike the corresponding pieces of really large guns.  Assuming a reasonably good fit, even a multi-layer aluminum foil powder bag would probably seal effectively at black powder pressures.

And, as mentioned above, you could easily put a properly fitting projectile into a rifled barrel.