threaded breach cap nessecery?

[maggot]

I'm intereted in making my own cannon about 1 1/2 dia.I was just going to weld the breech cap on,but I,ve seen that many of you thread yours.Is that needed or just for convinience?

[Calamity Jane]

The strength and security of the breech plug is of critical importance since a filure of the plug could well result in distruction of the cannon, property damage, personal injury, or death.

Whichever method is used, one must ensure there is sufficient strength to withstand the forces of firing and a 0% chance of failure.

For a person who's knowledge of welding techniques and who's skill level is not up to certification level, welding is chancy. It is far too easy for the amateur to end up with inclusions, lack of penetration, or voids in the weld, all of which cause the weld to be far weaker than assumed.

It is easier for most people to cut a set of matching threads (of sufficient lenth to provide enough strength) with enough integrity than to weld.

Whichever method is used, it is wise not to trust a single method alone. Add one or more cross-pins of sufficient size the withstand the firing forces if the primary method fails.

I welded my own breech plug (after preparing the plug and sleeve with a 15 degree bevel 80% the length of the plug) with 3/32 6011 rod. 6011 is a "deep penetrating quck freeze" rod. It is was a very slow process and took a lot of rods, but ensured good penetration, no voids, and the 6011 allows you to "float out" all the slag as you go.

If you haven't tested your welds for voids and inclusions in the past, take the assembly to a pressure welder and have it done professionally - it's worth it! Then take it home and add 1 or 2 pins.

[Double D]

From the N-SSA rules on liners and breech plugs.
http://www.n-ssa.org/NATIONAL/Rules/010.pdf
 

10.2 ELIGIBILITY
Actual or exact scale replicas of Civil War artillery pieces may be fired. The term “Civil War” applies to any artillery piece whose model antedates April 26, 1865. Replicas of artillery pieces must duplicate original pieces.
All reproduction barrels must be made of iron, steel or bronze. All reproduction barrels and those original barrels failing inspection must
be lined with a bore liner of extruded seamless steel tubing of a minimum ANSI standard and of a minimum 3/8-inch wall thickness.

The liner must be closed at the breech end with a steel plug, sweat-fitted into the liner and welded. The breech plug must have a radius of at least 25 percent of the bore radius and be at least 1 inch thick at its thinnest point. (See figure 10.1). All reproduction barrels manufactured after March 1, 1986 must have pictures of the liner and breech plug before
and after welding. No reproduction barrel shall be approved after March 1, 1986, which does not have one caliber's thickness of metal surrounding the bore at the breech. (See figure 10.2 for example.) Liner may be affixed by casting barrel around the liner or by other approved methods such as bonding with high strength adhesives. The method of locking liner in barrel shall be approved by the Artillery Ordnance Officer. The gun and its crew must pass the inspection specified in Section 18. A gun crew shall consist of a minimum of 4 members of the organization. Effective 1 February 1996, all artillery pieces must be originals or full-size, exact replicas to be approved.

That being said there is another school of thought .  A group of west coast Cannoneers recommend thread and pinned breech plugs. Their reasoning is that the Breech plug can be pulled for cleaning.

The Cannons I have now are all drilled from solid shaft. No breech plug.

I have a piece of Seamless out in the shop in which I am going to make a cannon and intend to use a threaded breech plug. This gun is going to be a Mountain howitzer.  Howitzers have a smaller powder chamber with large bore. . I am going to make the chamber in the breech plug.  Then put it in the gun.  This will give me my 1 caliber wall thickness at the chamber and  will allow me to have a bigger bore.  The model will appear  more howitzerl ike .

Beside for me it is just simpler. I can't weld very well.

[Cat Whisperer]

Lots of options - figure out what you want it for - fun or perhaps competition - that will sometimes limit your options.

Ammunition availility is often a driving force on picking the caliber too.