Heavy weapons skill
Heavy governs Heavy weapons. The Skills & Upgrades guide identifies the grenade launcher as the main reason to invest, describing it as powerful and saying its ammunition is plentiful.
How it works
The same source cautions that other weapons in the category are less worthwhile in its assessment and says level 1 or 2 is sufficient for its recommended use. This is a guide recommendation, not a required cap or a universal comparison across player builds.
Read the supporting references
Character attributes, technical skills, combat weapon skills, and operating-system upgrades supported by the supplied System Shock 2 reference windows. Guidance explains the referenced mechanics; it is not a tested damage ranking. Check the stated edition and source version before applying a patch-specific detail.