Thanks for the reply
The test wasnt done on my rifle by harry but by steyr in there factory. I was told the new port fitted was 3mm?
It would seem that the the answer to my question is more shots from the black and noone has really noticed any difference in accuracy.
I did wonder if this was a way of reducing the maximum power achievable as with the green port my rifle wont turn up .... beyond 12flb which is obviusly a good thing where as with the black on the chrono it can be turned to over 12..... makes no odds to me though as i usually set it at around 11.2 which begs another question which is what do other people set theres to...