You would not actually headspace the 338 #2 on the rim, but on the case mouth.
Headspacing rifle cases on the mouth just sucks.
The case is long enough that length variation from firing and sizing becomes an issue.
We don't see these problems with pistol catridges like 9mm or 45 or with the 30 carbine however because the cases are generally thicker walled and don't change lengths as much.
Also, many pistol extractors do in fact control headspace, even though they are not supposed to.
If you want a 338 in the AR-15 action, get one of Marty's 338 Spectres and call it done.
6.8 SPC magazines, 6.8 SPC bolt and 10mm mag cases are all easily obtainable.
Randall Rausch
www.ar15barrels.com