With a heavier bullet you may be about perfect with no valve spring....Bob
my WP tuned dragon claw does right at 394FPE with 320gr rnfp slugs.its been like that for 3 years. and its a hunting machine !!
The mods didn't look that hard if you have a drill press. http://www.africanairordnance.com/424820687/category/625603/sam-yang-airgunsVideos