To be completely honest with you, unless you are like me, an experienced machinist who has done these things before, you have no chance of doing this safely. I am an experienced machinist, I have my own lathe and milling machine. I know how to weld, heat treat, and I am a smith (I forge knives, forge-weld wrought iron gating, etc.). It is a PITA getting all of the tolerances to match up to one another. Airguns are not meant to take nearly the amount of the stress that even the diminutive (yet lethal) .22LR, or even the .22 short can exert. Not to mention, it must have a barrel at least 16'' long, and have an overall length of 26'' or 28'' (I can't remember which...). Don't do it, it will only result in the loss of your eyesight, and possibly a couple fingers. A Proper .22 Barrel does not cost $300, unless you go for those fancy match grade barrels that everyone raves about (My 43 yr. old Marlin Model 60 keeps up with them...). You can get finely crafted barrel blanks for under $100. Better yet, if you look around used, you can find .22 rifles that are completely functional, completely safe, and are better than anything than one individual in your position could hope to craft (no offense) for $50 or so (really, really, used...). Why can't you just buy a little bolt action .22 and be done with it. 100% legal, 100% safe, and 100% reliable/functional. Unless you are a machinist, you are going to have one hell of a time trying to put that contraption together into a safe, functional, and legal firearm. Just buy one, they are not that expensive...
Source(s):
Avid shooter/Experienced Machinist.