I am told that the backblast nozzles for those things were subject to gas-cutting wear, at least in the originals.
So they made the nozzles undersized from the factory so they had some recoil at first. As the gas cutting proceeded, there came a point where recoil was neutral. Then, as more wear occurred, they would start to pull forward. The implications for the Vespa's speed are obvious. :) :)
The nozzles were replaceable, so if they got to pull forward too much, they could replace them with factory nozzles. (The nozzles were also tilted slightly to counteract the angular recoil of the rifling.)
Source: "Hatcher's Notebook," pp 273-278, "Colonel Studler's idea," with illustrations.
I doubt this concept applied to the Messerschmitt guns, since one would want the gun itself to be ejected from the plane instead of staying in one place. However, that's just a guess. I'm having trouble finding out exactly how those Me 163 guns worked.
The "simplest" idea I could think of was a careful balance between the weight of the gun itself + the weight and MV of the projectile (MV need not be very high in this application) + some sort of friction device to hold the gun in place in flight. (Not to mention an unlinking system for the firing mechanism.)
But, knowing the Germans, it was probably more technical than that.
Terry
More:
http://images.info.com/cannon%20on%20vespa?qcat=images&r_cop=xxx&qkw=cannon+on+vespa(I love the red muzzle cap on some of those... like the red cap required on Airsoft guns! LOL Of course, it was probably just a warning device, like the red flag required on stuff hanging out too far from the back of your pickup.)