Back in the early days, the Texas Rangers and other Texas lawmen often had two pistols. One was a plain gun used for everyday work, their "work gun". When they dressed up for a court appearance, or to attend a social gettogether which were often big outdoor bar-b-ques, they put on their "special" gun which was often engraved, or nickel plated, and/or with pearl, stag or other special grips, and wore them. These came to be called "Bar-B-Que Guns". This term was mostly used in the southwest and not widely used until modern times when all fancy handguns are now sometimes called Bar-B-Que, BarBQ or BBQ Guns.
Here are some examples: