Sounds like progress.
I am a little curious about"I'm not using a crimping tool"
When you set up your seating die,do you screw it in till it bumps on the shellholder?
Im sure your RCBS seating die has a crimp function built in.If you are setting the die on the shellholder,you are crimping!!Probably way too much.
Take a piece of your sized,belled brass,put it in the shell holder,ram up.
Screw your seating die body down till you just feel it contact the case.
Lock the die body.
You will have to re-adjust your bullet seating depth to correct LOA.
Load and seat all your bullets.You will still have some bell.
Now,back off your seating stem,and gradually lower the die body of your seating die as you cycle a round through.You will see your bell dis appear.Use your calipers to measure the dia at the case mouth as you crimp,With jacketed bullets,it may be just a touch,but with cast bullets,there will be a spec...it may be .470 but look it up,to crimp down to.
Gross over crimping,you may have done,and it would cause your problem.