In the mid-80s, the Infantry School was involved in the trial of a couple 120 mm mortar systems.
One was a four barrel arrangement that was mounted to the ramp of an M-113 APC. When the ramp was lowered it became the baseplate and the muzzles could be reached for loading by a soldier standing on the top deck. It could fire the four rounds (electrically fired, not drop fired) in a four round salvo or individually.
Also tested were a lightweight Israeli Tampela towed mortar and the Thomsom Brandt 120 mm rifled mortar. The T-B 120 mm rifled mortar is the one that advertises a 13,000 metre range with rocket assisted ammo, will fire smooth bore or rifled ammo and takes up to a seven-man crew to operate.
The Statement of Requirement for the 'competition' had been written to see the T-B rifled mortar win, the essential and desirable requirements matched that weapon system like they were cut and pasted and no others. After becoming the SME Mortars at the School I reviewed some of the very few documents that were still available, and compared the requirements to every mortar system listed in Jane's to determine that.
Although some folks involved had been convinced we were going to a 120 mm system (one of the reasons for the abortive two-tier advanced mortar course 1985-89) none of the weapon systems were found to meet the requirements for battalion fire support at the time.
I believe one of the principal reasons for the project's cancellation is that there had been no doctrinal requirement established. Although the 120 mm mortar was the representative battalion fire support weapon in the Corps '86 establishments, that was only ever supposed to be a training model for Staff Colleges, not an acquisition plan.
From a reality perspective, one of the greatest limitations of the 120 as a battalion weapon was the significant increase in logistic support required (based on comparative ammo weights) to maintain a similar weight of suppressive fire. If you can equally keep a soldier's head down with an 81 mm or a 120 mm mortar bomb, why use the one that takes four times the logistic effort to deliver it. Some of the promises for new and interesting types of ammo sounded good, but you can only carry a limited total, and how much HE and WP do you trade for sexy single-purpose rounds?
The various 120 mm systems available certainly have their niches, and whether they are manned by infantry or artillery troops is immaterial in the long run, but the case wasn't made at the time to continue pursuing the test program for the Canadian Army.
We haven't had a heavy mortar in service since we withdrew the 4.2 in mortar. I'm not sure when we saw its last bomb go downrange, perhaps one of our more experienced gunners remembers.
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