I've been working with firearms all my life but I'm kind of new to air rifles. I'm trying to reassemble my Winchester 1000x rifle but I cant seem to get the piston spring back in the housing. Its impossible to push down and I'm having some problems. Anybody wanna help me out?

This air rifle has a very long mainspring that projects too far away from the houseing to simply compress the spring in the houseing useing youre hands only and putting the pin in its place as done with weaker airguns. So youll have to get a mainspring compressor or make one,search the web on mainspring compressors theres a lot of information on how they are constructed