The .223 and .308 are both high intensity cartridges generating up to 60,000 psi. These pressures mean you have to use properly heat treated medium carbon steel like 4140. Heat treat to about 28 - 30 C Rockwell.
Rule of Thumb here - Chamber walls need to be at least as thick as the cartridge diameter. You also have to use quality barrels for this.
I have the plans, but have not built anything yet, and I am a school trained gunsmith with some years benchwork.
Once I have built my first one, I plan to upscale it for .38 Special.