We've checked changing firing pin only as well as whole slide also. We've made so many tests that I don't remember all of them ;-) AFAIR the affected SP20 worked fine with parts from mine pistol... but I'm sure that the pistol of mine worked perfectly with any part from the affected one.
This is the reason that we haven't recognised any single part as the source of the problem.
I saw on this forum similar case described by randy1952 and jjgun, but as they've mentioned about "full auto" we hoped that it's not the same case as double-fires (they had to replace the barrels).
As english is not my mother tongue (and the "pistol language" is not commonnly used in work) I'm not sure what exactly have you had to raplace in your Hammerli.
Could you confirm whether the replaced part(s) were numbers 44, 48 from the manual? (like on page 3, could be found among others on http://stevespages.com/pdf/hammerli_sp20.pdf
) or point out the correct ones.
Keeping my fingers crossed that it's something smaller and easier to change that the main parts of the pistol.