Quoted:
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?

From AR15barrels, no matter what you may think of him:

From the barrel torque shoulder where the barrel extension torques against, it's 6.75" for carbine, 8.75" for mid-length and 12.125" for rifle
to the step at the rear of the gas block seat.
Breach face is 3/4" back from the torque shoulder.
Gas port is usually 0.275" to 0.295" forward of the rear step, depending on if there is a handguard cap in the design or not.
You should be able to figure everything else out from that...



2. What size hole do i drill for the gas port? Given a .257 wildcat based on a 223 or the like will be at a  lower pressure at the gas port than a .224 bore, I would expect a slightly larger gas port size than for a 5.56 build.  I would start at a proper 5.56 gas port and open up IF you have short stroking problems in cold temps or lightly loaded ammo.

Again, from ar15barrels,

Some collected sizes FWTW,

11.5" barrel, .625: dia., 3.850" from muzzle;
Port sizes from .081" to .089"
11.5" barrel, .750" dia., 3.850" from muzzle
Port sizes from .086" to .094"

14" barrel, .625" dia., 8.375" from muzzle;
Port sizes from .063" to .078"
14" barrel, .750 dia., 8.375" from muzzle;
Port sizes from .070" to .086"

16" barrel, .625" dia., 8.375" from muzzle;
Port sizes from .063" to 078"
16" barrel, .750" dia., 8.375" from muzzle;
Port sizes from .070" to .086"

20" barrel, .625" dia., 8.375" from muzzle;
Port sizes from .086" to .093"
20" barrel, .750" dia., 6.875" from muzzle;
Port sizes from .093" to .096"
24" barrel, .825" dia.,
Port size .089"

I have measured some of Colts 16" barrels in both diameters, and Colt uses two diameters of gas ports, .063 or .070" for the .625" barrel and .075" for the .750" barrel.

Barrel Length Gas Port
20" .092
16" .062
14.5" M4 .062
11.5" commercial .074
11.5" XM177 .067
11.5" carbine .072
LMG .104
M16A2 20" .092+ .001
14" .062+ .001
Commando 10.5" .081+ .001
LMG .106+ .001
R6511 (222) 20" .0995+ .001
7.62x39 20" .125+.001


3. Whats the proper way to headspace an ar?

There are a few different methods, some easier than others.  

One way is to face off the rear of the barrel, cut the external threads but leave the barrel extension shoulder shallow.  Chamber the barrel, leaving a small amount of space (say 0.010") between the bolt nose and the barrel rear face using a GO gage to set headspace.  Carefully cut back the barrel extension shoulder until you just take all the wiggle out of the bolt when the barrel extension is threaded on with the go gage in place.

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?

Does not matter.  I think Derrick Martin advertises he drills on the groove, but at bottom, it does not seem to matter.  

Thanks for the help!