[MadMax]

I dont' think your muzzle velocity would necessarily be all over the place. The Hicapa operates on discrete gas pressure phase where a drop in breech pressure puts the gun into recoil mode. Since the pellet has to leave the barrel for the breech pressure to drop, the shorter stroke (which occurs after the pellet is gone) shouldn't affect the muzzle velocity.

I would guess that a spring guide could be machined to fit down the centre of the spring (like a scaled down AEG spring guide) so that the shoulder sits inside the recoil block. I would prefer to have the spring guide bottom out instead of crunching a spring to pull the nozzle back because I find that it fatigues springs to bottom them out repeatedly. I think the coils rub against each other which causes surface wear. I think the spring guide should be adequately captured between the nozzle, slide, and block since the spring is already adequately guided in that groove.

Still a stiff spring is the simplest solution that could be found. If you have a good supply of these bits, then who cares about spring fatigue.