I got bored this weekend and decided to try to hash out HESH. Fire, Fusion & Steel doesn't really have a HESH equivalent; explosive rounds are either straight HE or Munroe effect HEAP/HEAT rounds.
HESH rounds have Concussion 1.3x the Concussion for an equivalent HE round (same diameter, same TL). Their Blast is the same as an equivalent HEAT round. Pen is 1/3 (0.33x) that of an equivalent HEAT round, but at Long range or longer they attack against the top armor of the target (usually equal to its rear armor).
Example:
A modern (TL9) 120mm gun has the following characteristics:
HE C:29 B:38, Pen 11C
HEAT C:19 B:31, Pen 137C
HESH would therefore be C:37, B:31, Pen 46C
The extra concussion damage is intended to interact with
these rules for damaging structures.
Edit: forgot to include that HESH is terrible against composite armor or spaced armor, so its Pen should be halved against those before taking into account their normal effects on explosive ammunition.