Yep, you need to bell (flare) the case mouth a bit…
They make neck expander dies and you can purchase whatever size expanding plug you need. They put a bell on the case mouth and expand the neck to a certain diameter…
Or, if you have a steel punch of some type and it is tapered, you can put it in the case mouth and if it touches the case mouth before bottoming out in the case, a light tap on it will put a minimal bell…
Or, if you have a set of needle-nosed pliers, you can put them in the case and turn them. That will also put a slight bell on the case mouth…
Both are crude and don’t size the neck for the boolit diameter, but it will let you seat the boolit without shaving lead…
Unless you have several different diameter boolits and cases you are working with, it is not cost-effective to by the expanding die and the plugs…
Good-luck…BCB