I had the same thoughts after the STX guns first came out. I called a reliable FX dealer, even FX themselves, and both said it couldn't be done...at least in the traditional sense that you're speaking of. Of course you could always take an STX barrel liner and build it up to the right size with precision metal tubing or CF, do all of the other threading and drilling required but I'm not sure it's worth the $$ or trouble? My MK1 .25 Cat with the original smooth twist barrel is doing a wonderful job with 33.95's around 845fps at ranges where Impacts and Crowns normally live. Those guns are prettier and more adjustable, might even print groups that are a teeny tiny bit better but from where I sit...I don't see anything that's pushing me to upgrade barrels or guns right now
Glenn in Texas