We’ve written about the primary ammunition for tank cannons, HEAT rounds and Sabot’s. But there was a brief period where it looked like squash would be the big tank killer.

The “squash” round is more technically known as a High Explosive Plastic or HEP round. Where a HEAT round uses the concentrated effect of the explosive warhead to burn through the target’s armor, and the Sabot uses kinetic energy to simply punch through armor, the HEP was designed to destroy tanks without ever even penetrating their armor.

HEP rounds consist of a very thin casing containing plastic explosives. When the round strikes the side of an enemy tank, it flattens against the outside of the tank’s armor. Imagine pressing a well chewed piece of gum against a wall with your thumb. A fuse in the base of the round causes the plastic explosives to detonate, forming a shock wave that travels toward the armor. This shock wave is designed to cause what is called “spalling.” As the shock wave travels through the armor, parts of the armor on the back side break off and fly through the inside of the enemy tank. This spall can kill or wound crewmen, rupture hydraulics, shatter electronics, and ideally ignite fuel and ammunition onboard.

During a period of time in the 50s, HEP rounds were very popular. HEAT rounds were limited in their penetration since penetration is a function of the diameter of the round. US tank guns at this time were only 90mm, giving an approximate penetration against armor of about 500mm.  The discarding sabot round hadn’t been widely adopted (kinetic penetrators don’t work very well from rifled barrels). So the potential of HEP rounds to defeat enemy armor made them very attractive. And as a bonus, they were very simple and cheap compared to other rounds. They also have the benefit of being very effective against bunkers and buildings.

But advances in metallurgy made the HEP round much less likely to successfully destroy enemy armor. By face hardening armor, and leaving the back side of armor less brittle, spalling was less likely to be sufficient to destroy the target. Other means of defeating spalling were also fairly easy to apply. For instance, on board the Bradley, the interior of the troop compartment is lined with panels of Kevlar to catch any spall. This is more to catch any spall from RPG rounds, but the principal is the same.

Currently, the M1 with its 120mm main gun doesn’t use a HEP round, but the Mobile Gun System variant with its 105mm main gun does have one available. This reflects the MGS role as primarily an infantry support weapon, as opposed to an anti-armor platform.