My experience is dated and thus refers to different weapons and ammunition (but in the same calibers other than the 120mm for 4.2" [107mm] switch) but Mortars rarely got cranky with good crews. Mountains were no problem and they generally were far more accurate there than Artillery...
Possibly interesting aside, the Chinese in Korea could put a Mortar round in your hip pocket but were lousy rifle shots; the Viet Namese were not nearly as good with their mortars but were good rifle shots...
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