If I assume garden variety merchant (low carbon) steel, then the yield strength of the material is going to be a minimum of 32,000 psi (221 MPa). You would want to keep your design stress no more than about half that value (16,000 psi/110,500,000 Pa) -- assuming that your "baby" this beastie.
The value you need to figure out is what is the maximum pressure being generated. For real, propellant-powered rifles, that value lies somewhere in the 28,000 psi (194 MPa) range. This is an area where I do a fair amount of work, but I know virtually nothing about air rifles.
If I have a tube of ID/2 of r, and a wall thickness of t being pressurized to P, then, assuming the units are all proper, the stress in the tube from pressurization will be: S = Pr/t. Remember that if you are working in Pascals, then your dimensions are in metres! Your Pr/t value should be less than your design stress -- and you would reduce it from the 16,000 psi/110,500,000 Pa value if you do anything other than put pressurized gas (behind the projectile) through your barrel!