You can't do it. 1) your 12bvss is a short action 2) your 12 bvss is a small shank and 3) the .338 Lapua can't/shouldn't be shoehorned into any action - the 110 Lapua action is heat treated differently to withstand the higher pressures of that cartridge. This conversion that you're talking about would be extremely dangerous.
I cannot stress this enough. Please do NOT attempt to do this.
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