As a noun, machine gun is two words, not hyphenated. As a compound adjective, machine-gun is hyphenated. If you are discussing fuel, use fuel , not gas . To readers who are not American, gas means matter in a gaseous state, not the fuel for an internal combustion engine. Please use airship , not dirigible .
Airship is unambiguous.
We prefer engine power denoted as 150-hp engine, but will accept 150hp engine. Please be consistent throughout the text. For gun sizes, we prefer 20mm or 20-mm to 20 mm. Please choose a style for aircraft designations and stick with it. If there is only one correct form, please use it.