Here is a air soft modification that is simple and will increase the muzzle velocity. The gains are moderate but if you have to disassemble your CYMA or similar air soft pistol any way, this Instructable will help make your gun a little better.
I bought this pistol for under 10 bucks and was surprised at it's accuracy at short distances. Surprised a few squirrels and other vermin with it. I being cheap also put some mini paint ball ammo through this CYMA P.638. Bad idea! The paint ball ruptured in the breech gummed it all up. Now I have to take it apart and clean it, so here we go..
Step 1: Disassembly
First we must take it apart. The arrows in the photo show where the screws are that hold it together. The orange outer barrel cover is glued on so be careful don't try to force it off or apart.
This modification will use one flat washer 1/2"diameter or 13mm., and one 7/16" nut. Utlilizing the original spring but increasing it's compressed energy so it will release more energy when it expands. See photos below.