The Lott chamber has a gentle radius at the end of the neck part of the chamber, not a sharp shoudler as with normal chambers, ie: the very much older .458 Watt's which the Lott almost duplicated. This gentle radius is why factory .458 WM can be fired in the Lott chamber and shouldn't be fired in the Watt's chamber. With a Watt's chamber, the sharp edge will cut or shave jacket material from the bullet and promote inaccuracy. Too, after several .458's through a Watt's, the buildup would prevent full length ctg.s from chambering.
: This - 'use .458WM in the Lott' is only a good idea if you cannot get full length brass - which is as simple as blowing out any .375 case or merely buying Lott brass. There really should be no reason to have to use the shorter case. Shoot enough of them and they're all you'll be able to shoot, btw - same as shooting too many .38's in a .357 mag.