So I purchased a new set of dies, from Lee this time. I resized the brass I had already run through the rcbs die (which was causing the issue) I could feel an odd resistance as the expander ball ran though the start of the case mouth. it was only at the start. The cases now chamber easily.
What I would look at with the symptoms you have mentioned, along with your mention of using a non-digital caliper, would lead me to look at a one place on the shell.
If you have even slightly longer than spec brass, or even brass at the upper end of the okay range, as you seat the bullet with a die that wants to crimp, it is common you will cause a very slight bulge in the brass at the mouth. This is often impossible to see, and without a very light touch on the calipers, very hard to measure. However, with a tighter chamber it will not allow the "loaded" round to enter far enough to close the bolt. This only takes a couple of thousands of an inch to create the problem with some firearms.
Even brass flow towards the mouth, can cause a feeding problem, even with no changes to the die set. This is because of thickening of the brass at the mouth, one reason for inside or outside neck trimming of the brass. This is more common in the higher pressure rounds, something the .300 certainly falls into. The thickening of the brass in the neck, along with any crimping can set up the situation you were having.
The reason I suspect this is your statement of feeling an odd sensation as you entered the brass, when resizing with the Lee die. I have had the same experience when changing bullets, or misreading the information for setting up the particular round from my records. Good looking round, but no way to get it to chamber. Checked and found the bulge, extremely small, but enough to mean it would not work.
If you reset the RCBS die it most likely will work correctly, once you have the brass in spec, along with using only the neck tension to hold the bullet in place.