I tried to size some .243 Win. once fired cases last night. I have an RCBS full length two die set. I wanted to neck size only. I smoked the end of a case and continued to adjust the die until the neck was sized. The case was sized nearly full length before the die contacted the neck. What am I doing wrong?

Nothing--thats the way it is. The die isn't a neck die and isn't going to perform like one. What you are doing is usually called "partial sizing". In reality you are doing about a 95% resize except you are trying to leave the headspace the way it came out of the gun. And many times this doesn't work because the shoulder will move forward because the sides of the case have been squeezed and it will chamber harder.
Neck sizing isn't all it's cracked up to be anyhow. Alot of times runnout is worse with a neck die because the case isn't held during sizing the way your full length die can hold and straighten it. If you really want to neck size try a lee collet die. It will give you excellent runnout and will only work the neck area. Plus no lube is needed so you avoid the step of lubing and cleaning up the lube after the resize. I have several neck sizers and rarely ever use them anymore--usually I used them on new brass to just run the sizing ball into the case mouth for a "ding straightener".

Partial sizeing or neck sizeing with a FL die works better with some cartridges then others. The 308 based and other straight walled cases really need a true neck size die to do the job. The more tapered cases 300 HH ect can be done with the FL sizer.
Personally I prefer useing a true NS over a FL. The FL dies seem to induce run-out for me, although others have claimed otherwise.

I ran into trouble with 270 wsm. Backed the FL die off and a few of my rounds would not chamber. I had to take them apart, fire the primer, then do a true FL sizing. For some reason, it only happened on my reloads with brass (winchester)from factory loads, my loadings on purchased unfired brass (winchester) performed fine with the PFL sizing. I wish Lee would put out a 270wsm collet die and a factory crimp. They say they are not in the works, and I am not sending for a custom die. They will come around, eventually, and I am patient.

Bill--the reason it didn't work with once fired brass is that brass had "grown" to match the size of your chamber when you fired it. When it was resized it had to work the brass more and the shoulder most likely moved forward a little (maybe .001-.003") and it was now "longer" than your chamber. With the virgin brass it was already to "sized" dimension so the die actually did very little work to the brass and the shoulder didn't move.

I would recomend that anyone that thinks they like reloading should get a stoney point headspace measuring kit. It comes with 5 bushings and a holder that will work with virtually any caliber. It's one of those tools that you don't think you need but once you own it you won't go without it. Christmas is coming------put it on your list and make shopping a little easier for your loved ones!!!!!!!!!!!!!!!!!

the two different groups were both fired, then PFL sized - the chambering issued showed up for the second firing of brass from factory loads (this was the first time I dumped powder in these cases) and did not show up for the second loading in brass that I bought virgin.

I do have stoney point HS pushing, the troublesome brass was longer in that dimension, I dont have the data with me now.