When I close the endcap with my finger and pull the trigger, I can hear air leaking out of the system slowly and the pop is not very loud when I release the finger.
We designed the SSG-10 for players. It must be airtight so that the power of the spring transfers into the BB acceleration and not into an air leak.
However, the SSG-10 was never designed to hold the pressure for longer than needed. An airsoft BB leaves the gun in less than 100ms. That is a tenth of a second. Within this time the gun must have good compression to achieve the necessary efficiency to produce 5.0 Joules with a spring that is still manageable to pull back.
Measure the muzzle speed of your SSG-10. If it’s power sits where it should be according to the spring table, your gun is perfectly fine. If your Joule rating is way too low, please contact customer support.