Need your advice

[Judson]

I am a custom rifle builder living in St. Albans Maine.    I am thinking of building a 1/2 scale field howitzer with a 1.5" bore out of mechanical tube with a 3.5" OD, and a 1.5 ID.    I was thinking of using a threaded breach block would this be a problem?    I am new to this game and figured I would ask you guys.    With modern firearms and BP rifles a threaded breach is no problem but this canon stuff is alittle different and I was wondering what you think.    By the way the tube and breach is 4140 so I know it has the strength.

[Double D]

Just build it like you would build a breech plug for a BP rifle and you should be fine.  Jusy mak some provison to pin the plug.

There is some good guidance on bulding cannons in the Complete Cannoneer.  it won't hurt to buy that book!

[Cat Whisperer]

1/2 scale eh?  That would put it at 3" full scale - 75mm perhaps?  Blackpowder I assume.

4140 prehardened has the strenght you need and as you know machines well enough.  4130 is better if you have to do some welding - as it's at the limit for weldability.

Threaded breach - permanent or designed to be opened?  If permanent design in a shoulder that will provide a crush fit such that the elements of combustion won't have a chance of getting into the threads - then weld or pin into place.  If removeable, do the research on the strength of threads you need.  Our 155mm howitzer has an interrupted thread breach-block.  If I remember right it takes about 1/3 of a turn to lockup.

The concept is good for a threaded breach.  A friend of mine built a 50BMG single shot from a M98 action and welded onto it what looked like a pipe cap threaded for the 50 barrel.  Added a huge muzzle brake - kicked less than an M1 Garand.  Only problem was that it lit the sand bag rest on fire EVERY time we fired it.