Only hints I can give are as follows:
1. Look at the on-line tune guides, esp. at the "smoothing" parts. Roughness/burrs in the tube where the hammer moves can make for a variable hammer fall. Should be smooth and slightly lubed with a good long term lubricant (I prefer a rubbed in dab of molly paste) on both the inside of the tube and the hammer..as well as the spring and spring guide).
2. TEMPERATURE! Co2's pressure changes with temperature, so the vel. changes. Rifles running on 12gr. cartridges are the worst about this, not only is there a tiny volume of co2, but the metal 12grs. act like ice cubes in a drink...they have little contact with the main tube (where warmth from the outside comes from). Need to pace your shooting for a more level gun temp. It gets WORSE as the 12gr. start to run down. BTW: the more gas used per shot, as would happen in guns tuned to be fast, the more cooler with each shot.
3. No matter what the reputation of the pellets you are using, if they don't fit the bore or if they have weight/diameter variations, have to expect vel. variations.
Even on the rifles running on 12gr. cartridges, if I take my time and keep shooting at whatever pace needed to maintain vel. levels, will expect variations in the 1.2-1.5% range. Use percentages as a gun tuned to 550fps giving 6-7fps fps varaitions would be the same as a gun tuned to 750 giving 9fps variation.
