The ".357 load in a .38 casing" Skelton used did not use just any bullet. It was a cast SWC with TWO crimping grooves. Seat and crimp the bullet in the lower groove and the resulting, longer cartridge had about the same powder space as a .357 casing with the same bullet. Seat and crimp the bullet in the upper groove and the resulting shorter cartridge had about the same powder space as a .38 Special.

That particular bullet was designed that way because, back then, .357 brass was new, scarce and expensive while .38 Special was like white on rice.

People forget or never knew; Skelton and the others weren't doing this with just ANY bullet.