He means domestic made ammo is mild enough to use in the .318. It's that way because of an influx of old "J" bores many years ago and hasn't changed.
Not so. You don't want to shoot the higher pressure .323 loads in the older .318 JRS bores.
Conversion rifles will be marked with an "S", and are gtg......
Common "Wisdom" old wives tail...
PO Ackley proved that bullets size to the bore at first movement, long before pressures are high, and the US 8mm Mauser loadings gave deference to the early J bore guns by loading low pressures and velocities, just in case the "S" bore ammo found its way into "J" ("I") bores.
I realize people go nuts at the thought of a .323 going down a .318. They might study up on Ackley's experiments.
The main thing to keep in mind is the 88 action isn't as strong as the 98, and loading for it should be mild, even with .318 diameter bullets. I don't advocate using "S" bore bullets, but I also don't duck for the trenches.
This is a great rifle offered here. I wish I owned it, and the seller had a nicer one.