As i ordered one last night ( Vulcan 3 ), one brand / acronym have been left out "AGT" AirGun Technology
I think the guess as to the meaning of FX is valid, FA would be a silly combination to use, i am pretty familiar with the language of our neighbor to the east,,,,, a country we over history have been at war with for over 3 decades,,,, but now are very friendly in general.
I cant think of any Swedish words starting with X that would be valid in this airgun context,,,, actually only a few come to my Danish mind.
The Swedish word FAen,,,, means Damn,,,,, they use it a lot like some use the "F" word