I just loading my first 20 rounds of 357mag. One of these had a bulge in the case after seating the bullet. This is the first time I've seen this. What causes this? Thanks. Hopefully these pictures will show it clearly.

Off hand I would say a little too much crimp.  Back it off a quarter turn or so and see how it does.

I'll bet you're seating and crimping in the same step.  With the heavy crimp you used, doing both at the same time can be pretty challenging.

If you're using a single stage press, first seat all the bullets with the die body (and crimping part) adjusted high enough to not touch the cases.  Then readjust the die to move the seating stem all the way up and then lower the body to get the amount of crimp you want.

If you are running a progressive, you will need a separate crimping die.  Adjust your seater to get the LOA you want, then adjust the crimp die to crimp (without touching the bullet) appropriately.

I'm looking forward to hearing how this goes for you.

I'll try that on the next ones. Wonder why it only did it on just one?

Thought you stole some of mine.....

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I found that if the piece of brass I used to set the crimp was shorter then my longest piece that would happen.

It could be a lot of things.  Are your cases all the same headstamp?  Did every single case get belled exactly the same amount, guaranteed?  Are all of your bullets perfectly consistent in diameter?  Did you perhaps start that bullet maybe a tiny bit off from vertical?

Whatever the precise reason for that particular round, everything will work better with separate seating and crimping steps.  I wound up buying a new set of .38/.357 dies a while back.  I use the new seat/crimp die to ONLY seat, and the old one to ONLY crimp - I even took the seating stem out of it to reduce the confusion.

mine used to look like that, for the same reason as Mr Porter gave.

Trim 38 Spl & 357 Mag so they all same length for smooth crimp reason

Seating and crimping in same pass.  And, could be the seater was last used to seat 38 specials and now trying to seat 357 magnum rounds?