I've reloaded a lot of ammunition in my day.  Most of the handgun ammo has been on a Dillon RL550B and an XL650, which is the current machine.  Most of the rifle ammo has always been on a single stage and for bolt-actions.

I am now trying to reload .223 Remington for the first time.  I have RCBS does that were given to me, so I'm trying them first before buying Dillon dies if I need to.  They are the SB, which I'm assuming mean small base.

I have a Dillon case gauge.

I am starting with a bunch of recent LC once-fired brass.  In the un-resized condition, it fits the case gauge perfectly.  I resized one and now it will not fit the case gauge, with part of the rim still not sinking in.

What do I need to do first?