Your bore depth is a little much for a mortar, at least a traditional Coehorn pattern. What you are describing would be more of a howitzer, and should be mounted accordingly.
For a mortar, you want the bore to be about two calibers. In other words, two golfballs stacked in the bore should be about flush with the muzzle.
And your estimated chamber size will be excessive for a GB mortar. With a 1" x 2" chamber in a mortar-length tube, your'e never going to be able to load it anywhere near full capacity without blowing a lot of unburned powder out the muzzle. The chamber in my GB Coehorn is 1" by 1-1/4" and that's pushing the envelope. Any more and I'd go beyond the point of diminished return, where more powder is just wasted and does nothing to increase range. With a longer bore, you can burn more powder, but then (as I stated before) you are getting out of the realm of mortars and into howitzer territory.
If and when I build another GB Coehorn, I will back off on the chamber capacity. Probably to about 7/8" by 1-1/8" max.
As far as your 3" OD (I'm assuming were talking steel here), that will be fine as long as the chamber diameter is kept to 1" or less. You will also want a minimum wall thickness behind the chamber equal to or greater than the chamber diameter. The walls of the GB bore will be plenty thick at 3" since this is the low pressure section of the tube. With just a straight walled barrel and a suitable trunnion, you cab build a relatively accurate Confederate pattern Coehorn.
So there you have it. That's just the minimum basics, from which you could design a safe and enjoyable scale mortar.
For more info and some actual plans check the stickie threads at the top of the page.
