I have purchased a unturned .257 barrel for a 25-223 project. This will be my first wildcard and my first custom AR barrel. I have several questions about finishing the barrel:

1. How do i measure where to drill the gas port for a rifle length gas system? Do i measure from the breach or from the muzzle? If from the breach do i measure from the end of the barrel extension? What would the measurement be for a rifle length gas system?

2. What size hole do i drill for the gas port?

3. Whats the proper way to headspace an ar?

4. For drilling the gas port, do i have to worry about if i drill between on the lands, on the groove, or in between?

Thanks for the help!

1. Depends on when you measure it.  Final distance is measured from the barrel extension rim, as installed.  This will depend on how you contour and thread the barrel for the barrel extension as to where it winds up on the raw barrel blank.
2. Depends on the gas system length and nominal bore pressures - post some numbers.
3. Thread for and install the barrel extension to proper torque, then use a chamber reamer to create the chamber such that the bolt achieves the correct headspace.
4. dunno - not sure it matters, but not sure it doesn't.

I answered your identical question on the BIY forum yesterday.  

Is there anything else you are looking for?

I hate to keep having to say this kind of thing on here, but if you have to ask these kind of questions on here then you probably dont have any business trying to build an AR barrel form a blank. its not a simple process and its not an ideal first project for an inexperienced machinist. there are some great websites, books and videos out there that will help you to gain a better understanding of how to perform a job like this. might try checking them out.

this article is quite informative on the subject, but doesnt get very in depth on the actual process, just the steps to take.

link