I don’t understand why a standard action can handle 300 RUM or 300 Nosler but not 300 NMI?
I will leave it to someone smarter than me (Jim Borden?) to explain the technical limitations of a 1-1/16" tenon diamater action when used with .590 bolt face cartridges, but here is some real world experience.
I've owned five different Norma/Lapua magnums:
I've had two 300 Norma magnums on Defiance actions with their .750 bolts. When staying within sane pressure limits (e.g. CIP max pressure spec, factory Berger ammo, etc.), these actions will 100% handle the .590 bolt face with no problem. I shot out several barrels and the action handled the the Norma cartridge just fine. The fact that highly respected gunsmiths like Terry Cross use this action for their Lapua builds should tell you everything you need to know.
I've also owned one Norma and one Lapua magnum on the Surgeon XL, and one Norma magnum on a Borden Bigfoot. These actions have beefier bolt lugs attributable to a larger 1-1/8" tenon diameter. I can say pretty definitively that with my small sample size of barrels, the 1-1/8" actions will allow you to use higher powder charges before seeing pressure signs. With an identical chamber and Bartlein barrel of the same contour, the same load that extracts like butter in a Surgeon XL would require a mallet to open the bolt on a Defiance.
So, the apparent "problem" with 1-1/16" tenon actions and .590 bolt face cartridges only seems to arise when handloaders start stepping on the gas and chasing velocity. We can certainly debate the merits/safety of pushing the pressure limits, but my experience would suggest that a bigger tenon diameter gives more safety margin when it comes to pressure.
FWIW, I no longer have the Defiance actions, but I still have the two Surgeon XLs and the Borden Bigfoot. I won't do another .590 bolt face build on a standard R700 action.
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