I was starting with the most obvious cause Grasshopper
I still think you have a lack of lube issue, I've heard the complaint of stuck cases from others using spray lubes.
I use Imperial sizing die wax and have never stuck a case. I too load the 300 RUM. Maybe try spreading the lube with your fingers to make sure you have complete coverage. There's alot less surface area to cause friction on a 300 saum.
As for your other problem was this brass fired in your chamber? If it came out it should go back in with no issues, try chambering an unsized, unloaded piece of brass.