I made the adaptor for using a portable 12 Volt air compressor out of the end of a pneumatic speargun hand pump, the part that fits directly into the rear inlet port of the gun. On the Mares guns this part screws off the end of the hand pump body tube or you can buy the ends separately. My old Mares guns use a "quarter twist and lock" bayonet fitting on the end of the hand pump, later ones simply screw in. The front end of this adaptor is a standard rubber tubeless tyre valve insert with the valve core removed, you need the threaded brass end for the air compressor's inflator attachment to clip over it like when you blow up a passenger vehicle or bicycle tyre. The two end pieces in my adaptor are joined by an old spark plug body using only the metal part, but you can make it out of anything tubular provided you have access to some machining facilities. Basically you need something to screw into the gun, you could cut the end off an old pump or use the whole pump body, but you need to block off the air breather hole in the pump if you do. The assembly has to be air-tight so pumped air transfers into the gun without external losses.
Most pneumatic guns come with a pumping table for the number of pump strokes from empty to reach a certain pressure in the gun expressed in Bar or Atm or kg/cm2. As the volumetric capacity of both the hand pump and the gun's internal air reservoir (including the volume of the inner barrel) dictate the pressure level achieved with a certain number of pump strokes there is no "universal" pumping table. My advice is pressurise the gun to the maximum and see if you can still load it. If not then bleed some air out carefully until you can. A good idea is to have the power regulator set to "low power" when doing this, it limits the air that can escape if you overdo it, only a momentary push on the valve stem lets more air out than you would think. After letting any air out flip the power regulator lever back to "full power" and then try the gun for loading effort again as this action allows air pressure in all parts of the gun to equalise.