If the sizer does not reduce the neck area of the case small enough to hold a .426" or .427" bullet, the sizer is not tight enough for your rifle.

Lee will make a custom set of dies for your ctg. for something around $100.00 and so will Hornady. Their prices might have changed, I do not know.

The die with the primer punch, should be the sizer die- yes. It is the first die you use when reloading.

I take it the case is fairly straight, fast taper and without a shoulder?

Most bottle necked ctgs. have an integral expander button in the sizer die along with the primer punch beneath it. The button expands the case when the case is withdrawn from the sizer die.

Straight sided cases, ie: no bottle neck, have a separate expander die to expand the neck, after sizing, thus are sizer and de-capper, the expander die, then seating die = 3 dies.

This is tedius.

I am not familiar with the ctg. you are using - is it a virtually straight sided, tapered case, ie: 3 die set with expander different from the sizer die, or is it a 2 die set with expander and sizer in one die and the seater die separate?

All of the above information is detailed in normal reloading books.