the outer pipe of the airram will be the spot the gun should be mounted on.
i didnt include bumbers and spring/rubber band in the drawing, i assume one could make those on a few different ways, depending on the aesthetic needs...
so, as soon the pressure behind the projectile begins to build up (aka firing the gun) it starts pushing the complete gun backwards... simulating the effect of acceleration of a greater mass projectile...
i really cant think of a different method, wich would still be easy to tweak, AND sturdy...
for example. put in a small balvalve between the barrel and the airram, and you can tweak the amount of pressure wich would bleed into the ram.
give the sliding assembly stronger/weakers springs to adjust the relative travel distance of the recoil action....
please DO post some pics on youre final design tough

edit:
oooh i have and extra design idea, to make it a bit more realistic maybe..
i was thinking how to replicate the SLOW return of the assembly into firing state.. like the real big guns do...
if you add a high flow check valve between the barrel and airram, in a way that if you fire it lets air pass quickly into the airam simulating the recoil, but blocks it after the shot essentially keeping the gun in the "recoiled" state..
then make a small hole in the valve, or make a small bypass.
you can let the air bleed back into the barrel/air to let it slowly return by the springs into the neutral state again...
i assume this would be a rather easy way to make the recoil as realistic as possible...
edit2:
made a sketch again


the way stuff would look like greatly depends on the materials you use and skills and tools one would got.. but i hope the principle is clear now...
edit3:
just occured to me that the bleeding hole is rather useless unless the airram is airtight..
the bleeding effect so it returns slowly would also occur if you simply make the airram not-so-airtaight
