The only cheap spring option is going to be a shotgun which is limited to CQB or woodland if you put a lot of work in. Even in CQB you'd have to play right.
Have you considered The re-enactment route? Get your parents used to you playing with deactivated guns etc and dressing up. WW2 airsoft is an easy progression from there. Local group would be https://www.facebook.com/groups/SomersetLI/