Jim Maccari/ARHs HW50/99 spring: https://www.airrifleheadquarters.com...88/1979288.htm
With tight guides and some extra spacing it duplicates this kit: https://www.airrifleheadquarters.com...488/552850.htm
I get around 11fpe in good Vulcans with this setup, some more in the .22's, and some less in the .177's