Quoted:
How about the MG-42 scaled up for the .338 Lapua?
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Some things just aren't meant to scale. The MG42 mechanism is already running on the ragged edge, with 7.92X57 cartridge. I would wager that a conversion/upscaling to .338 Lapua without taking that into account for the basic mechanism would create problems. Some mechanisms just aren't suited for particular cartridges--Look at the issues the Spanish had making roller-locked 5.56mm weapons with the Modelo "L" and Ameli.
Dunno what mechanism would be best suited to the masses and pressure curves involved with the .338 Lapua, but I'd be really surprised if you could just scale up an MG42 mechanism to work with it.
A lot of folks just assume that because a weapon works well in one caliber, it will work well in others. That may be true for some designs, like the Madsen light machine gun, but it's not true of all of them.