I think I read somewhere, at about the time I was loading for the Lazzeroni, that 'someone' had successfully made and fired Lazzeroni cases after turning the belt off the 378 Weatherby.
There is no reason I can see that it wouldn't work, but I hate to think of all the effort. At that time I purchased a mini lathe thinking that need might arise for me in the future for 'something'. I was think of using domestic brass as opposed to the RWS and other foreign stuff for which I had to pay $2 a case. That never came to be and I still have the lathe. It's been quite useful for some things.
From comparing dimensions, I also can't see any reason you couldn't use the Lapua case, unless sizing down will somehow distort the head, especially the primer pocket and flash hole. Those two things could be correct, with the primer pocket being the most difficult if it does distort as the brass is compressed. You could inexpensively try it with one case and see. The body of the brass compressing a bit shouldn't make that much difference. After all, you're just slightly returning it to one of the stages from where it came from a blob of brass when it was started to be formed. Doesn't hurt to try!
Why is the OP doing this? Because Lazzeroni brass is no longer available, and he has some expensive firearms which are just cheap pieces of metal without ammo. I suppose he could rebarrel to another cartridge but overcoming the challeges is part of the fun of all this.
