You kind of have to determine what style gun you want before asking for plans. For example, a four foot long Napoleon is 2/3 scale which means about 7.33" diameter and something over 300 lbs barrel weight. A full scale mountain howitzer is about 37" long and would weigh about 225 lbs.
So take a look around the various cannon websites and see if you find a design that talks to you, then try to note the full size dimensions and weight so you can figure what the small one would weigh. And remember, your friends lathe is going to need to be about twice as long as the barrel if you are going to drill it yourself.
Or a full scale 3 pounder. They are 36" long (measured the way shown in Mueller's) and ~42" overall with backplate & cascabel. They also are a bit over 7" in diameter. At 2.913" bore, they do not hold to the wall=bore rule (and there are a bunch out there on the reenacting circuit with full scale bores). The iron repros are usually sleeved and 2 1/4" bore. There are a couple good drawings of these in Caruana's book: Grasshoppers and Butterflies: The Light 3-Pounders of Pattison and Townshend. To bore one of these your machinist buddy is going to need a lathe (probably) 10' long. Or you can work from tube, this opens up a whole new can of worms. Good hunting, MIKE
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