This one floored me when I found it, because if feels so contemporary:
Bust a cap: fire a bullet, also bust caps. Engage in battle. There are two references, and both appear to be from the Civil War era, so it probably came from that period, and is something a Civil War soldier is most likely to use. Also, snap a cap.
Filed under: Civil War Tagged: | Slang, victorian slang, Victorian Slang of the Week

Definitely sounds like soldier slang.