Is your FNX-t in 45acp? Cause if it is i would doubt the threading on that barrel will be the same as on your 22lr gun. That means you will need a different piston to use the octane on the 22
A lot of 9mm barrels are threaded 1/2x28. The same as most 22lr barrels so it should work with a easy to get piston.
Im still trying to figure out if the gun will function with that big ol can hanging off the end of a 22. It might turn it into a single shot. I dont know.
Ive got a 1911 in 22lr and a 1/2x28 piston...i might have to put that together and see what happens. Here i thought i had put all the combos to the test