The HEAT round. The Challenger 2 does not fire a HEAT round from it's L30 gun. In fact, no MBT for the British Army has fired a HEAT round since the Centurion with it's L7. Both the Chieftain and the Challenger 1 shared the same main armament which, once again, did not fire HEAT.
The reasons being sabot rounds are more effective against armour and HEAT being less effective due to the Soviet's increased use of ERA armour.
Can someone chime in as to why we have Sabot and WP but no HESH? And why HEAT is being used at all?
I'm not completely clued in as to how HEAT is coded in the game to be less effective against tanks compared to sabot rounds in PR (to which I'll go and have a look in the files now).

As you can see, above, sabot and HESH but no HEAT.
Finally to quote an official source, the MOD:
Challenger 2 main battle tank - British Army Website

The main reason I am pointing this out is because HEAT and HESH work in two very different ways. HEAT is commonly considered a standard high explosive round when it is, infact, a shaped charge in which it's explosive force is concentrated at pushing copper through the enemy's armour. Compared to HESH, HE or frag rounds it is no way near as effective for anti-personnel purposes.
HESH on the other hand works by physically warping armour due to the power of the explosion from contact with the warhead. With these two facts in mind, HESH is far more effective against infantry than HEAT is. To sum up:
1. British Army does not use a HEAT round
2. HESH should be in game to replace it


