So I have an osprey 45.

Bought a 9mm piston for it.

The spring that was on the 45 does not take up the entire length of the new piston.

Am I good to use it or need a new spring?

The spring shouldn’t take up the entire length of the piston. A small amount of the threaded end on the piston sticks out of rear of the Osprey. You don’t need to change the spring on the Osprey 45 when shooting 9mm through it.

The full sized Osprey .45 is the quietest 9mm suppressor I have ever shot. I had a chance to visit the Silencerco range booth and shot the Osprey on my Sig MK25 while the Silencerco rep shot my Sig P226 with my TiRant 9M and later my Octane 9 and the Osprey
45 was quieter than both of my 9mm cans. Usually that's not the case shooting any other .45 can on my 9 pistols, must be the huge volume of the Osprey makes up for the larger bore channel?