If I understand you correct, you have a false shoulder on the outside of the neck. not a donut inside the neck.

if that is the case you should be fine. If the false shoulder is caused by the lapua brass being sized to far from the factory( you say RP brass is just fine) then the false shoulder may help you with excessive headspace. I would wait on the barrel and make sure the resized lapua doesnt have too much headspace.

Steve