Actually, the expander plug for the expander dies for those type straight wall cartridges by both Lyman and Redding are very similar. The sizer die can only have a decapping pin for
true straight wall cases. I just remove the decapping pin on all sizers and use a universal decapper die. That may have been confusing in the first post with the pic- my error. They are two different dies... the sizer only sizes and the expander die has larger ID so it doesn't size- only holds the expander plug. Here's another pic showing 4 slightly different 45 cal rifle expander plugs that will fit both the Redding Exp and the Lyman M die. The loaded 450/458 cartridge in the previous pic was sized with the Redding sizer to a neck inside diameter of .450". The neck expander die with .454" expander opened the neck ID to about 453-454"... depending upon springback.
Pic from left to right 4 different expanders that will fit either the Redding Exp die or the Lyman M.
The main sizer diameter and minor flare diameter of each is:
1) .454 .460 2) .454 .459 3) .457 .460 4) .449 .460
I use the # 1) .454 .460 and end up with a neck ID of about .453-.454" again depending on brass springback because of annealing, age, thickness or brand of brass. I adjust the expander in the die to only use the .454" portion not the .460" portion. I do not get the "bulge effect" after seating a .458" bullet as was the OP's concern. If the sizing die is near
minimum and sizes that
minimum very far down the case neck and into the body area. And if there is a large difference in diameter between that neck area after sizing/expanding and the diameter after seating the bullet... a bulge will show. Again largely cosmetic and odd looking in addition to being likely off axis. It's the off axis part that would negatively affect accuracy. I've shot a bunch like that and doesn't seem to cause much issue except a very small loss of accuracy. BTW, straight wall handgun ammo many times shows the bulge effect for the same reasons.