Seamless Carbon Steel Tubing, 0.38 Inch OD, 0.07 Inch Thick, 0.25 Inch ID

For sizing, the outside diameter is 0.375 inch (3/8), with a 0.065 inch wall. That puts the inside diameter at about 0.245 inch (roughly 1/4), so it is a 3/8 OD by 0.065 wall tube.

Your instinct is correct. That 3178 psi working pressure is based on a 6:1 safety factor, which is the standard applied when hydraulic shock and mechanical strain are considered considerable. If conditions in your system are more severe than that, an 8:1 safety factor should be used instead, which brings the working pressure down further. On the flip side, if shock and strain in your application are minimal, a 4:1 factor can be carefully considered, which allows for higher working pressures. The burst pressure on this tubing is 19,067 psi, and the working pressure figures are derived from that using the Barlow formula.

Made to SAE J524, the tube is annealed specifically for bending and flaring, so it is a good match for typical 3/8 inch OD bending setups. The outside diameter is 0.375 inch with a 0.065 inch wall and about a 0.245 inch inside diameter.