Apologies for the late interjection.
Mrs W had a quick look at this thread and reminded me of a device she once saw me applying a match to. It was constructed in the mid 80s at Finningley with the specific target of student navigators. It was the 'tapering' design where the combustion chamber was of a greater diameter than the barrel. This has the obvious advantage of reducing the length of the weapon, but also seems to increase the muzzle velocity due to the venturi effect. It was particularly effective as I remember, with some fairly major damage done to the Mess. It does however require copious re-enforcement at the taper.
Good luck Seymour.