I own an FX impact, and ordered a heavy slug liner (an airgun slug is basicly a lead bullet without a gascheck) and the first one they sent me must not have gotten oiled before delivery and rusted a bit in storage/transit. So i sent them a photo of the barrel liner and they sent me a replacement for free. the construction of the barrel to convert into a firearm barrel is simple, install a nipple at the end and reinforce the liner by covering it with a high pressure rated pipe with 5/16 ID, use an adjustable reamer to size, heat the pipe up to cherry hot and install the liner. the heat expands the pipe and as it cools it siezes onto the barrel liner. select the nipple you want to use and partially tap the breech. heat the nipple and breech end till cherry hot and slowly turn the nipple into it. the two will fuse very tightly.
as with all homemade barrels do some proof-testing before use inhand or near your face. I over charged the barrel by 50 grains and blasted from a safe distance away, multiple times. and measured if the nipple-breech-plug backed out at all. after testing you can also weld the nipple inplace for extra insurance. drill and tap the sleeve/highpressure pipe without entering the liner and make a screw to anchor it into the stock, normal glassing process.
FX barrels are actually top notch. the rifling is pressed into a polished barrel from the outside in an extruder-like die and are perfectly smooth inside.
all the other parts were made with a ginding wheel and angle grinder/ sandpaper. i made the stock from what i believe is cherry.
I may do some brass furniture for it as i have scrap brass stock but im not sure how much more time i want to put into this. it has been fun tho