First you did good. You have a cannon. There is never anything bad about that.
On the negative side the guy that built that gun has little knowledge of cannon and motar design and construction. Bore is a bit tight and walls are to thin, trunnions to small and may be to far back.
On the Plus side and this is a very big plus, he built the tube from 4140. 4140 is also known as gun or ordnance steel. It is very strong steel and I doubt that you will have any problems with pressure when using blackpowder in that. No the plus far out weighs the negative. and you have a perfectly shootable noise maker to celebrate your coming Senior Citizenship.
The extra double big plus you found us. We can help get your gun setup and shooting. Once you do that you can start thinking about your next cannon project.
Unit's of measure. Get cooking measurements out of your mind. Your not making cookies and you shouldn't use cooking measurements. You will find your self getting into trouble real easy if you use cooking measurments. You will find no credible reloading information based on cooking measurments. It's to easy to get an overload using these unknown quantity measures.
Get yourself some way to measure powder. Traditionally black powder is measured by volume in grains. In cannons it is recommended to weigh the black powder on a scale. Smaller guns like this volume is just fine. You need to know how much powder you are using. Our safety chart is measured in Grains For now just find your self a black powder measure like this:
These these cost $10 to $15 and are cheap if you don't have a scale. This type of volume measure will work just fine for your little gun. You can use it to determine the volume of other recomended items used for loading powder such as cartridge cases and film cannisters.
Based on postion of your trunnions, this is as George says more a howitzer than a mortar. Mortars generally have a bore that is about two calibers long with trunnions at the rear. There are exceptions such as the Dictator. Your bore is 2.9 calibers and has a great long extension to the rear of the trunnions You want to look to garrison, naval and field carriages not mortar bases. Let us know what you decide and we will help where we can.
Loading and firing. Check for the maximum load on the chart stuck to the top of this board in safe loads and cannon plans. The bore size to use is 1.25 and according to the chart your maximum load is 280 grains. Use only Fg or Cannon Grade black powder. For blank charge with no ball FFg would be fine.
You are going to need to make youself some cleaning tools. You will need 2 swabs, one to clean the powder chamber and one to clean the ball bore. You are going to have to clean that ball bore meticulously between shots. It won't won't take much to jam a ball in that tight bore. Onthe other hand I'll bet you get a good crack when it fires.
Well that should get you started. Tell us when you are going to fire your celebratory shot and we will all go out and fire an answering volley...any excuse to shoot you know!
