John Whiscombe used the Harmonic Optimization Tuning System (HOTS) on his latest Mk2 guns. The HOTS is an adjustable weight at the end of the barrel that screws in and out as required, it works, have a look at this thread on a US forum:
Also have a look at this article on HOTS:
I like the Dave Welham system as it is more adjustable and the weights can be placed anywhere on the barrel, as placing them at the end of the barrel limits your adjustment as the weight is at the end of the ‘wavelength’ having kind of a pendulum effect.
Mate of mine in Poland was after adding extra weight to his Rapid and has put weights on the barrel, have a look at this picture:
He has found that it has improved accuracy and made the gun less pellet fussy!
There is an article in this month’s Airgun World about harmonics, it was written by a University Professor that developed the HOTS system for John Whiscombe.