before you go the chamber route, if you make a powder chamber in it, and it is not a mortar (pointed 45 degrees up), ask yourself (or others) how are you going to get the powder into the powder chamber?
A howitzer has some angle to it, which will help keep the charge in place, but if you are thinking powder chamber to keep the charge down to keep the range down, a howitzer angled upwards might be counter productive to distance control.
Of course, putting all that aside, I guess if you plan it out from the beginning with the possibility in mind, if it doesn't work out you can always open the chamber out to full bore.
My 2.25 inch mortar does not have a powder chamber. I sort of wish it did for short range target practice. I can back it down to about 1/3 of an ounce of Cannon Grade before I have issues with irratic ignition. At 1/3 of an ounce, a 2# lead ball goes about 60 yards. At 1/2 ounce it goes about 100 yards. With charges under about 3 ounces in a 2.25 inch bore there are "techniques" needed to make the charge go in straight.
My firewood cutting pattern is "wierd" this year "for some reason". For some reason, I am cutting in a long, narrow path, and am at about the 60 yard point right now. A couple more days and I may hit the 100 yard mark. :-)