Apparently we get this from importer,
can somebody explain what this is?
Not to clued up on FAC
FAC stands for Fire Arms Certificate. This, in th UK, refers to air rifles with higher than 12flbs energy. So the spring will increaes your power. But not by that much if I am not mistaken.
I suggest you contact Louis (Importer) for statistics etc... His contact details are listed in his portfolio.