FPS variation in an HW100

[krisko]

+1 yes some hw100 regulators even whistle.

it may take a few seconds for the regulated "chamber" volume pressure to stabilize, so dont rush the shots. especially when you are looking into the fps.
plus dont forget even JSBs have 5% weight distribution pass/fail criterium so pellets will contribute to the issue too.

i have seen the sinewave too, i think it is normal with the hw100. as long as the extremes are not wide apart - which are not, you are golden.
Looking at your good results, I dont think you will improve much by polishing to be honest. Worst case you will break your gun. So something for your consideration.