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The older 500 Whisper is made from a Weatherby magnum case. The SSK 510 Whisper is made from the 338 Lapua case. Both shoot 510 diameter bullets (includng 50 BMG fodder). The Lapua case is a little larger diameter than the Weatherby case at the base and rim. They are not interchangeable.
I haven't made the 500 Whisper cartridge, but generally the process is to cut any of the big weatherby magnums (xxx-378, 378, 416, 460) a little over the finish length, and push it (with a good sizing lube like imperial wax) into a full length sizing die with expander. Then trim the case to finish length. Check the neck outside diameter with a bullet inserted for clearance in your chamber. Neck turn if necessary. I wouldn't expect it to be difficult as the diameter reduction isn't great and the shoulder angle isn't steep. I've made 338 Whisper out of 308 Win which is essentially the same process.
The advantage of the 510 Whisper is the availability of 338 Lapua brass. Besides Weatherby, Norma brass is available for the 500 Whisper in the form of 30-378.
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