Just 2 words here: "Bio One". Jim Muscala's "must have" 12 page rules set. That would be my base rules for any realistic gun fights. We used them for the majority of playtesting and also for Boot Hill games.
To me this TSR published rules book is the most under appreciated document that was ever published by TSR.
I certainly hope this document isn't lost to the Hasbro locked game vault. That would be a major loss.