Ordnance of the Week: OF-843B 120mm Mortar
In an earlier post, we covered Iranian mortars which were manufactured copies of both Soviet and NATO munitions. This week, we’re taking a look at 120mm HE mortars of Soviet design currently found in the war in Ukraine, particularly the OF-843B as well as some copies.
The OF-843B is a high explosive-fragmentation (HE-Frag) mortar designed for 120mm mortar systems like the Soviet 2S12 and commonly used with the M-12 point-detonating (PD) fuze. (1) The round is generally painted either light or dark grey with black markings and features a fuze well adapter, 5 gas check bands (grooves) along the major diameter, and a stabilizing tail boom with 18 flash holes and 10 fins. (2)
Diagrams courtesy of Bulletpicker LLC, found in Iraq Ordnance Identification Guide, US Department of Defense, January 2004
Both light and dark grey paint schemes can be seen in this photo from the National Police of Ukraine, who found these abandoned OF-843B mortars near Chernihiv in April 2022. A key identification feature is the number of fins (10) and gas check bands (5) which differentiate the OF-843B from the OF-843 and OF-843A, which both have 12 fins and 6 gas check bands.
Image from the National Police of Ukraine, published via Twitter
The OF-843B is also similar in appearance to the D-5 120mm Bursting Smoke mortar, which has a white phosphorus filler. However, the D-5 can be easily differentiated through the markings, which include a black band and an indicator of both nomenclature and filler. If markings are not discernible, the ffuze adapter on the nose of the D-5 features 3 spanner holes.
Basic Identification of Ammunition in Ukraine v3.0, Bomb Techs Without Borders, 2023
The OF-843B generally uses an M-12 point detonating fuze which can be set to super-quick or delay (for the purposes of attacking either open or fortified positions). (3) At the bottom of the fuze well, underneath the detonator, cardboard spacers can be placed to take up any empty space. The main body of the OF-843B is a very thick cast iron which surrounds a main charge of TD-50 or similar explosive. The original Soviet TD-50 used in the OF-843B was a 50/50 mixture of TNT (trotyl) and DNN (dinitronapthalene), although Amatol (a mixture of TNT and Ammonium Nitrate) could also be used. (1) The entire round weighs 16 kilograms with the explosive filler approximately 1.4-1.6 kilograms depending on the manufacturer and filler. (1)(4)(5)
Original Diagram from Technical Description and Operating Instructions for 120mm Mobile Mortar Chapter 2: Ammunition, USSR Ministry of Defense, 1990
The OF-843B is also manufactured in Bulgaria, which can lead to slight variations such as the explosive filler used. This Bulgarian round from Arcon Partners Ltd has a TD-42 filler: an adjusted TNT/DNN mixture. (5)
The incredibly thick cast iron body on the OF-843B (and many similar mortars) can make them a challenging target for EOD operators seeking to do a low-order disposal with commercial thermite lances. EOD technicians should positively-identify their target if possible and select disposal techniques and tools best suited for thick-cased munitions.
Demonstration of thermite lance attack on thick-bodied munition. Image by Bomb Techs Without Borders in partnership with the National Police of Ukraine
As always, a special thanks goes out to Bulletpicker LLC. There you can find our Basic Ammunition Identification Guide for Ukraine (Ukrainian and English editions) as well as numerous technical and field manuals for conducting ordnance research!
Sources:
Technical Description and Operating Instructions for 120mm Mobile Mortar Chapter 2: Ammunition, USSR Ministry of Defense, 1990
Iraq Ordnance Identification Guide, US Department of Defense, January 2004
Management of Mortar Fuze M-12, USSR Ministry of Defense, 1959
“Main Technical Parameters: 120mm OF-843B High-Explosive Mortar Bomb (HE) for 120mm Smooth-bore Mortars,” Arcon Partners LTD, 2023
“Technical Specifications: 120mm Mortar Round VOF-843B,” Bulcomers KS Ltd, 2023