Variant Rules for Use with the Grim-n-Gritty Hit Point and Combat Rules.
This document has lengthy rules for implementing firearms in The Sleeping Imperium campaign. However, the same rules could be used in nearly any D20 campaign. Firearms in this system are deadly. The supplement is unusual for providing a large amount of technically correct details about firearms, and their design, construction, and use. The document does not provide long lists of various firearms as character equipment, however--only a few samples are provided.