No, I meant the x51-a mag full of fat cased 147 gr pistol bullets would be heavier than the same number of much more powerful 5.56x45 with 62 gr bullets, as the heaviest part of a cartridge is the bullet ...... you'd get fewer of the .357SIG cartridges in a mag of the same size, compared to the 5.56, with less range, less power, less penetration, more weight, at additional cost ...... where's the up side? ..... Other than you like the cartidge?
They did away with the 7.62x51 NATO/.308 WIN to get more capacity, trading the power and range that they felt was unnecessary (at the time) for that increased capacity ...... you are advocating trading some of that capacity for even less energy and range?
Where is the up side?
TheGolden Rule of Tool Use: "If you don't know what you are doing, DON'T."