Precision
From BattleTechWiki
Precision ammunition can be used in standard autocannons and their lighter cousins. At the expense of mass, scientists from the Federated Suns managed to modify autocannon ammunition to accommodate a gyrojet shell that can correct its course in mid-flight. Debuting in 3062, precision ammo soon found its way into the armies of other nations.
In CBT play, precision ammunition grants the firer a -1 to hit modifier. This comes at a cost, however, that of only being able to carry half as many rounds per ton (round down).
