I've got a Ballisti-Cast Mark VI http://www.ballisti-cast.com/Mark%20VI%20graphic.htm They said that it will not put on the gas checks. Do the Lyman non-crimp on gas checks need to be put on before sizing?

Sean,
I always size the bullet after I put on the gas check,regardless of the make of the GC.
The sizing process seats the gas check firmly and evenly.
Frank

Sean it seems to me if it will size the bullet, it will also work with or swage on any gascheck in the process. But as has been said, YES, put the gascheck on first.