What KRP is saying - it can't be done.
The case head diameters are 2 different sizes.
0.588" for the .338LM. The parent case is the .416 Rigby.
0.532" for the 7mm Rem Mag. This was a propriatary case from Remington.
Also the .338LM is a beltless case.
Proud member of WTFDTSG Club.
Nice try means you suck spelled different.