I know the US Army is looking at a turreted mortar system with a range out to 20kms (currently 19.5 for the M119 with Ch8 RAP). I haven't heard of any system that fits the bill yet. What have you seen in that respect? As it stands, 81mm does about 6 km and 120mm about 11kms. There are some foreign ones doing 13.5kms.
The problem with even longer range is a bigger bang in the tube which means a more robust tube and base plate and before you know it you've got yourself an artillery piece stuck on high angle. Flat trajectory means shorter time of flight. ToF matters.
I sometimes wonder why one needs a longer range mortar. What type of fight do we envision where the mortars need to be a dozen kilometres away from the battalion it supports? Or drop rounds fifteen kilometers in front of the battalion's forward position?
I guess I'm back with the idea of a lighter mortar that can be handled by a small crew and can drop lots of dumb neutralizing rounds in close to their own battalion and also launch long range UCAV rounds to destroy point targets out beyond that.
I think its time to sit back and closely study the mortar, gun, rocket, UACV mix needed at the different echelons.