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.

Step 2: Modification

Remove the cocking barrel cover and you will see a spring over the barrel and the orange cap on the end of the barrel, this spring slides off the barrel when you separate the two upper halves of the barrel. You will also see a movable piece with a spring behind it. This is the part we are interested in. Pull back on this part and lift up to remove. Hold on tight as you have to compress the shorter spring to remove it.
The photo shows the three pieces that make up the pneumatic part of the pistol. The spring, piston , and compression tube/ breech loader. careful with the compression tube and piston as a tear in the rubber seal on the piston ,or scratch on the inside of the tube will effect muzzle velocity.
The stock length of these three parts together is 94mm. By adding the nut and washer to the inside of the piston we increase the length by 6mm, so are parts together now are 100 mm.
The 6mm is all I could add without making more mods.
Now put the 7/16" nut in the piston and the washer on top of nut so the spring has a flat surface to rest on. Put the piston in the tube, you could give it a few drops of pellet gun oil too. Put the spring in the piston and WHALA! It's done.
What we are doing here is using the nut and washer to effectively shorten the compressed spring, making it release more energy as it expands when you pull the trigger.

Step 3: Cleaning Barrel

Before we put it back together it would be a good time to clean the barrel and breech areas. As I mentioned before I had some mini paint balls explode in the breech. To remove the barrel it is necessary to separate the two gun halves. Remove all screws except the one nearest the compression spring support, just loosen that one. Spread the gun apart enough to slide the barrel assembly off of the pin that holds it in place. Take note of all the parts that stay in the halves as you open it, if the trigger should fall out or something else becomes dislodged you will know where they go.
The barrel assembly consists of an outer and inner ( the real barrel). On the end of the real barrel is a rubber seal, it serves two purposes 1. to seal the compression tube to the barrel and 2. to hold the 6mm airsoft pellet in place until you pull the trigger.
One screw holds the breech end of the barrel together, and that bloody glued on orange cap on the other. Don't try to remove it, just take out the screw and spread it apart, the real barrel will come out with a little work. Take the rubber off and clean the inside of the barrel with alcohol and cotton swab. Look down the barrel to make sure it is all clean. You might notice some bumps in there, don't try to remove those just any build up inside barrel. Those bumps are to encourage the pellet to back spin when leaving the muzzle. Not all barrels have this.
As an extra insurance against any compressed air escaping from the seal and barrel I put a SMALL amount of RTV silicone around the outside of the barrel where the seal will set. Make sure there is no RTV inside the barrel and put the seal back on. Run your finger around the seal where it meets the barrel and remove excess RTV.
Put the barrel back in the outer barrel, make sure the inner barrel is all the way in before replacing screw.

Step 4: Reassembly

Spread the gun halves apart and slide barrel assembly back on pin. Put all the screws back that hold the pistol together.
We will also add two new screws to strengthen the barrel assembly in the lower part of the gun. This is not necessary but I did it because of the added spring pressure and energy release that the pin will endure with this mod.
Drill a small hole through the lower part of the gun into the barrel assembly, not much room so be careful. Screw into these holes screws a little larger than the holes. they should be a wood or sheet metal threaded screw with a point.
Put the smaller spring , the piston and compression tube back in the pistol. The nut and washer are inside the piston, with the spring against the washer.
At this point lets make sure this will work, pull back on the compression tube until the piston with its latching boss engages the trigger latch mechanism, the piston will stay back and the compression tube is free to move forward against the barrel assembly. IF it does not latch then the added nut and bolt are too thick. 6MM( nut and washer) was the most I was able to get it there and have it latch correctly.
Now hold the compression tube TIGHTLY, using a downward force and pull the trigger. Hold on to it and let it come to rest against barrel assembly. IT is a strong spring and its under more pressure now with this mod. BE CAREFUL.
Slide the bigger cocking spring over the barrel ass. and replace the upper cocking part of the pistol halves and your completely done.
Have fun and wear appropriate protection in what ever you do.
I am working on vids before and after, so you can see the improvements.