Just to shed a little light for the OP on why that worked. The RCBS seating die has a "lip" for lack of a better word machined into it that's there to crimp the bullet into place at the top of the seating stroke. It's a poor feature and needs to go, they've been making them that way for a gazillion years and for some reason won't change them. Some other die makers put them in their standard dies too, the better match grade dies won't have it. Hardly any bullets come with cannelures any more which is required for that crimping lip to be able to work. If there's not a cannelure on the bullet or the cannelure's in the wrong place then the die will try to crimp anyway but there's no place to crimp the brass at the top of the case into. Since there's no place for the brass to move, the force is transmitted to the weakest part of the case which is the shoulder area. This will cause the shoulder to buckle slightly, it's so slight that you generally won't see it with your naked eye but you can feel it though if you run your fingernail over it. It's just enough to keep the case from chambering however. When you screw the die out of the press a bit more you keep the top of case from touching that crimping lip and buckling the shoulder.
It took me YEARS to figure it out on my own why some of my reloads for my 7mm rem mag wouldn't chamber using my standard Lee dies (this was before the internet). It was all because I had the seating die screwed down too far and this was happening. I wanted to ring the neck of some engineer at Lee when I finally figured it out!
