Is a moderator worth the trouble on an air rifle, especially an old Heanol from the 60s recently rebuilt.
Most of the noise seems to be the spring slamming forward rather than the pellet exiting.
I shortened the spring as it was seemingly "excessively powerful" after the rebuild. No difference in noise.
However, my son recently bought a springer break barrel brand new with a fitted silencer, its almost totally silent. Pleased with it he bought similar one for his daughter without the silencer, so loud it frightens the gun dog.