The MO-120 RT-61 (factory designator) or MO-120-RT is a heavy mortar of French origin. It was designed by Thomson-Brandt as the successor to the MO-120 AM-50. The RT in the designator stands for rayé, tracté, which means rifled, towed.
The RT-61 is currently used by the French Army (where it is known as RT F1 or Mortier de 120mm Rayé Tracté Modèle F1—"120 mm rifled towed mortar, model F1"), and is produced under licence by Germany, Italy, Belgium, the Netherlands, Brazil, Japan and Turkey (as HY-12 "Tosam"). The MO-120 RT-61 is issued to artillery units, where it augments the 155 mm towed artillery.
In French service, this weapon is normally towed by the VTM 120 (Véhicule Tracteur Mortier de 120 mm), a wheeled armored vehicle that is a derivative of the VAB 4x4 series of armored personnel carrier. Towing is accomplished by a towing hitch that is screwed onto the muzzle of the weapon. The VTM 120 also carries 70 rounds for the mortar and offers basic ballistic protection from small arms fire and shrapnel for the crew. The RT-61 can also be towed by the AMX-10 TM (Tracteur de Mortier), which is a version of the AMX-10P tracked APC. The Japan Ground Self-Defense Force have a self-propelled version of the RT-61 with 50 rounds, the Type 96 SPM.
The mortars, which originally equipped infantry regiments, have now all been transferred to the artillery regiments, where they augment the 155 mm towed artillery. Currently, there are eight 120 mm mortars assigned to each French Régiment d'Artillerie - although they are not deployed during peacetime.
The RT-61 uses standard NATO rounds as well as the specially designed PR-14 (HE) and the PRPA (HERA). The weapon can be fired either by dropping the round down the tube (after aligning of the rifling bands) resulting in an automatic firing once the bomb hits the tube base, or by a controlled firing by dropping the bomb down the tube and pulling on a lanyard that will in turn set off the triggering mechanism in the base of the tube.
Manufacturers other than Thomson-Brandt as mentioned above include Thomson-CSF/Daimler Benz Aerospace (France/Germany(?)), Hotchkiss Brandt (Netherlands) as the "HB Rayé" and under license by MKEK in Turkey.