Question over Shadow in the North plot ?

[GBlood]

I am confused over the, "Steam Gun" in this book. I can understand that the steam is used to operate the trigger of each machine gun, even the gravity cartridge feeds to each gun I understand. But if it is a standard size railway car, (allowing 20 to 30 feet) the 2,000 holes on each side, (the size of a rivet, or nail head), then the cartridge must be .22 cal, but even allowing for a popular cartridge size at the time of .45, (Colt invented it in 1873, and contains only 21 grains of Black Powder), no lucky single strike from a single bullet, (unless she is firing an elephant gun), should set off the chain of events in this plot. Or is it just a case of like, "In the movies" and just enjoy ?