The differences really are not all that great, that is why they are uttered in the same breath. You don't find 7.62 and .308 listed as separate callibers anywhere do you? I never have... They are listed as the same, because for all practical purposes, they are the same... OOOH, that will probably start a food fight. have shot a lot of factory 308's, a lot of milspec 7.62's and mixed and matched a LOT of ammo thru all these firearms with no noticeable performance issues in any of them. Does some ammo run more accurately in some firearms? Of course it does, but that just shows the varyances in production parameters, and the ultimate flexibility of brass to fireform to whatever chamber(within limits of course) it is fired in...
At any rate, with my RFB, this isn't a length issue anyway. It is a diameter issue. The case diameter at the base of the neck to be specific. I think my RFB chamber is a little oversized in this particular area... Havn't bothered to actually spec it and compare it to SAMI specs, because from a practical standpoint it dosn't really matter. Now that I know about it, I just don't work the brass in that area as there is no need...