Way back, shortly after I bought the GK99, I also picked up a spring loaded rifle, in this case the L96 (by Well, Mb-01). 
As stock, these guns don’t work, as their precision versus their practicality is very off. Long clumsy rifle, that shoots one shot, then you need to reload the spring and get eyes on target again.
I started gathering parts and upgrade, read forum post and soon had a little bank of knowledge of what to look for and what to do to get it to act more like a sniper rifle as it should.
I did a whole lot of work on the gun, here listed not in any specific order:
I filled the body with silent coating material
This resulted in two things: The rifles weight distribution with a silencer on the tip got balanced out way better. Secondly the sound from shooting removed any echoing from the body itself and more like a “tumpf” – sound.
I had the whole body painted by a friend, the dull and basic Olive green did not suit me.
Internally I changed all the parts to Airsoft Pro:
- Steel cylinder, polished smooth and nicely with a large Sorbothane piece to take worst of the shock.
- Steel Cylinder head
- Aluminium piston.
- Steel spring guide with a ball bearing
- Reinforced CNC trigger set, tested to withstand a M190 spring.
With all the above, and the intention to use a heavier spring the charging handle is also recommended to have in steel.
The inner barrel is not the best there is, but I had a spare 509 mm Madbull Black Python 6,03 mm laying around. The Hop-up chamber on the other hand is probably the best you can get to this particular gun, that being the PDI chamber. With dual adjustment rods you can get it tuned in pretty exact with a little patience. I used the Maple leaf Monster rubber 80D in it, worked pretty well, but haven’t had the time nor the patience to tune it in spot on.
I also added barrel spacers to keep the inner barrel steady in the outer barrel. For visuals and comfort I got a check bag for it, looks pretty nice when done.
In time of writing, this gun is for sale @ kuulatori.fi. Now however, I got some feelings back and might take it out to the field for some testing and tuning again. The Gun is now sold!