Nothing wrong with the sizing process. Problem is the seating die is not adjusted properly. Most die companies machine their dies so they have a taper or slight roll crimp. This is very obvious in dies for ammo shot in tubular magazine or in heavy recoiling loads. The crimp holds the bullet in place so it does not move under recoil within the magazine.
Adjusting a crimping die is very tedious and requires your brass to be trimmed to the same length. In a round like the 204 that has minimal, if any, recoil you don't have to do any crimping so set up is not near as critical. Get a Lee case trimmer and cutter and you will be set in the event you will need to trim, but likely will not get to use your new trimmer until about the 3rd or 4th firing when brass finally stretches enough to be at trim length.
As stated before, pull your bullets, and dump the powder. No need to pull the primer. Pull the decapping stem and pin out of your sizing die. Use a minute amount of imperial sizing die wax on the outside of your brass and run your brass up into your sizing die to remove that bulge at the shoulder. Wipe your brass off and get ready to reload.
Take your seating die, and untighten the lock ring. Spin it way up the die body and leave it there for now. Also take the seating stem and back it out most of the way. Take a sized, but uncharged brass and put it in your press. Screw your seating die down until you feel it come into contact with the brass. If the brass is properly sized, the very tip of the neck should be making the contact in the die. Back off the die 1/4 turn and try another brass. Turn the die back a 1/4 turn to see if the next brass makes contact. Should be close. What you are looking for is approximate level of the die when it makes contact with the brass. Since you don't have to do any crimping, find that spot where the die makes contact with your brass, unscrew the seating die 1/2 to 1 full turn, and screw your locking ring down. Tighten the locking ring and you should not bulge any more cases with the die. You should have sufficient room to let your brass grow a little without doing the bulging.
Now you need to seat your bullet. In my die boxes, I keep dummy rounds (no powder or primer) with each of the bullets that I shoot in my gun. I make up the dummy round by barely seating a bullet in a dummy case, and then smoking it with a birthday candle or covering it with ink from a sharpie or similar pen. I check the dummy round in my gun and see if the bullet touches into the rifling. If it does, it will leave scratches in the ashes or ink on the bullet. I check the length of the scratches and turn the seating stem down a little. Slowly adjust your die so your bullet is just shy of the rifling. Once you have the dummy round, if you have to re-adjust your die, you just put the dummy in the press and turn the seating stem down until it makes contact with the bullet.
Depending on the type of ammo you are loading, and the gun you shoot them in, you may have to adjust your bullet seating a little differently to make other accomodations. Some bullets in my 30-30 are not even close to the rifling. I have to seat them deeper in the case in order for the loaded round to cycle thru my magazine. Same with my -06. Bullets are not close to the rifling because I have to seat the bullets deeper in the case so they will fit in the mazagine so I can carry more ammo than a single shot.
In summary, bulging at the case shoulder is from the die trying to over-crimp a bullet. That shoulder, especially on the steeper shouldered brass, is the weakest link when seating a bullet. Screw your die up so no crimp is being attempted by the die and set your bullet seating depth. You should be golden until your brass is really stretched and by then, you should be trimming.
Good luck!
Steve
