FYI, at that angle (45 degrees) 150 grains of cannon grade throws a 2# ball out of my mortar about 100 yards. It does not have a sub-chamber, so charges a bit less than that I have issues with erratic ignition.
I did some computer simulations and found that, within reason, range at high angles (45 degrees) is pretty much independent of everything but muzzle velocity.
So, if my 2# balls go 100 yards with 150 grains, your snooker balls are gunna leave faster and go "way further".
I did fire a concrete filled piece of muffer pipe, weighing about 1#, and with the same 150 grains charge it went "way further".... guessing a couple hundred yards based on time to impact (I heard it touch down eventually). It sorta scared me how long it took before it "re-entered the earth's atmosphere" (it is 1.0 miles between my house and the next house in the direction of fire, but who nows what is wandering out in the woods where I can't see).
Also, my mortar barrel is only 10 inches long. Longer means possible more muzzle velocity. Yours is longer.
Good luck, start small, and may the force be with you :-)