Hello fourbore,

maybe the rumor is reasonable for some of the german rifles manufactured before WW1 ?!

But for currently produced rifles I load american bullets (Nosler, Hornady, Sierra, Swift) and european bullets (Geco Target and RWS TIG and Doppelkern, Norma Oryx) in .284" diameter and receive good results (read precise groups)in
- 7x57 (98system with a Lothar Walther barrel)
- Mauser 66 in 7x66 SE vom Hofe
- a 98based rifle in 7x64

So I would recommend not to worry about this "difference"...

best regards
Rolf