Isn't SAAMI:
GO: 1.630 in.
NO-GO: 1.634 in.
FIELD: 1.638 in.
While the 7.62 NATO or 7.62mm x 51mm depends on who is making it:
FN FAL:
GO: 1.6325 in. (FN & Brit/commonwealth. Canadian is 1.6315 in.)
NO-GO: 1.638 in.
FIELD: 1.640 in.
7.62 NATO (M14 US MILSPEC):
GO: 1.6355 in.
NO-GO: 1.638 in.
FIELD: 1.6445 in.
Also it is important to keep in mind that the above numbers are chamber specifications. Cartridges are manufactured to fit the chamber. Looking at some of the 7.62 x 51 NATO ammunition I have actually measured the stuff generally comes in right around 1.628" to 1.632" as measured case head to the 0.400" datum on the case shoulder.
I noticed that Ruger has an AR 762 model that shoots .308 and 7.62 NATO. Is this the AK variety of 7.62 x 39? I looked on the Ruger website, but all it specifies is 7.62 NATO. I'm confused.
Joe
Joe, if you want a reasonably simple read on the subject with some Cartridge / Chamber drawings tossed in
this thread in the CMP (Civilian Marksmanship Program) forums is a pretty good read on the subject. Another reasonably good read on the subject can be found on Fulton Armory
What's the Difference between .308 Winchester & 7.62x51mm NATO?
There are reams of print on this subject so I suggest you read some of what is out there and understand what is involved.
Ron