@chuckshooter , To get from BR to Dasher, he will need that false shoulder not just the neck. The Dasher neck-shoulder junction is a little longer than a straight BR.
@Benchrest Braxton , Not sure why you couldn't get that false shoulder down far enough? It may be that I don't follow your post?
You can also use a BR die to set that False Shoulder. Just crank the BR die out to set the shoulder with your bushing.
I assume you still opened up the neck with a 0.25-ish mandrel and then sized it down with the bushing?
You have to factor for the neck-should chamfer in the chamber, and also the chamber neck diameter too. Thats why you should still open the neck with a mandrel before you start the false neck.
Roughly speaking, going from 6 BR to 22 Dasher is very similar to the 6 BR to 6 Dasher problems.
You can also use a slight jam to help hold that case back in addition to the false neck/shoulder.
Good Luck and in for the range reports.