www.riflebarrels.com/borescope_hi.asx DSL or hi speed
www.riflebarrels.com/borescope_lo.asx dial up or lo speed
All you have to do is look in the barrel with a borescope.
A custom build is a lot different than a custom match barrel. Anyone can slap different parts together and call it a custom build.
A custom match barrel is first a match grade blank, air-gauged and lapped to very tight tolerances, commonly used by benchrest competitors. Very very few ARs have real match grade barrels.
These companies make match grade blanks but must be finished by a smith.
Krieger (not Criteron)
After starting with a quality blank it is chambered to very tight concentricty tolerances, the chambers should be within .0005, that is 1/2 of 1/1000th.
some production chambers are out of concentricity by .020 or 40 times what a true benchrest grade barrel is.
Companies that I KNOW build custom AR barrels.
Frank White Compass Lake Eng.
White Oak Precision-a div of WOA using Shilen blanks
MSTN-may use PRI barrels
AR-Performance- edit- we no longer have time to machine custom barrels
There may be others but I have seen these in my hand and checked the chambers, I know they are capable of 1/2MOA