Ordnance of the Week: M21 AT Mine
The Pentagon recently released an updated fact sheet on its military aid packages to Ukraine. The latest packages include M21 anti-tank mines, a mine variant which has not been actively procured by the US military since the early 1960s, but which has been maintained in stockpiles for decades.
Image from US Department of Defense Iraq Ordnance Identification Guide, January 2004 (5)
The M21 AT mine is 230mm in diameter and 206mm high. It weighs a total of 7.6kg, including a main charge of 4.95kg of Composition H6. The mine is used with the M607 fuze and can be set to be activated by one of two configurations, either with an extended tilt-rod or by direct pressure. Without the tilt-rod, the mine is activated by 130.5kg of direct pressure on the M607 fuze and will immobilize a vehicle. With the tilt-rod in place, 1.7kg of pressure against the rod (1) will achieve 7 degrees of tilt (2) and activate the mine. (Although FM 20-32 describes a 20 degree deflection required).
Figure A-29 from FM 20-32 outlines the basic dimensions and components of the M21 AT Mine.
It is important to note that the M607 fuze incorporates a belleville spring-loaded firing pin actuated by either direct downward pressure on the pressure ring or from bearing pressure from the tilt-rod assembly. The pressure breaks a frangible plastic collar and allows for the belleville spring to deflect and drive the firing pin into the detonator. The presence of plastic components in this assembly should be noted by EOD technicians and other operators as the effects of long-term storage on the resilience of this plastic collar are unknown.
The M607 cutaway as found in TM 9-1385-51
The M21 mine has a cylindrical steel body and a concave steel plate above the main charge which produces a penetrating slug via Misznay-Schardin effect. When the M607 fuze functions, it activates a black-powder expelling charge which clears away the fuze and earth above the mine. Simultaneously, the charge presses down on the firing pin within the M21 mine itself and activates a 0.15 second delay which allows for optimum target positioning and for earth above the mine to be adequately cleared by the expelling charge. After the delay, the M120 booster at the base of the mine functions and detonates the Composition H6 main charge. Given adequate clearance, the concave steel plate in the M21 is projected by the main charge and forms into a penetrator via the Misznay-Schardin effect capable of defeating 3 inches of armor at a range of 21 inches. (3)
Cutaway figure of M21 mine from TM 43-0001-36
The main charge of the M21 mine is 4.95 kilograms of Composition H6. However, for EOD technicians it must be recognized that this is older, American-manufactured Composition H6, which differs slightly in composition from modern Australian-made composition H6. There are possible slight variations in RDX and TNT quantities, as well as differences in the manufacturing of the RDX itself. (4) While this may not introduce any noticeable differences in disposing of M21 mines by demolition, the author thought this was interesting and worth noting.
As always, a special thanks goes out to Bulletpicker LLC for providing easy access to ordnance references and for hosting our Basic Ammunition Identification Guide for Ukraine.
Sources:
(1) FM 20-32: Mine/Countermine Operations, Headquarters Department of the Army, April 2005
(2) TM 9-1385-51: Identification of Ammunition (Conventional) for Explosive Ordnance Disposal, Headquarters Department of the Army, February 1967
(3) TM 43-0001-36: Army Ammunition Data Sheets for Land Mines, Headquarters Department of the Army, April 2000
(4) R.J. Swinton, T. Bussell and L. McVay, “A Critical Diameter Study of the Australian Manufactured Underwater Explosive Composition H6,” DSTO Aeronautical and Maritime Research Laboratory, August 1996
(5) M21 Photograph, Iraq Ordnance Identification Guide, Department of Defense, January 2004