I only have 3 months experience, but had the same problem,iam only trying to help.
After noticing FC had one side alot higher on the mouth of the case,.751 on one side .746 or so on the other.
I trimmed the cases to .746 and opened the bell up more. After trimming i beveled the inside and out like a rifle cartridge to remove rough edges. Then took a little more time making sure the bullet was straight bofer seating.
My bulge went away after doing that, but i dont know which step fixed it. I think it was opening the bell up more than i had it set. Seems to me with the tight bell and uneven case mouth its was guiding the bullets in crooked bulging one side of the case.
Before the insults start flying, this is only how i fixed it on mine. But i dont have my 9mm bulging on one side anymore. Iam only useing lee dies no compitition or special dies. I didnt have any feeding problems on my glock but thought it would be bad having it like that accuracy wise. I posted the same question a while back and got laughed at and told by some i also didnt have the dies adjusted right. Havent change the adjustment of my dies but the bulge went away. Hope this helps…
PS. I had the same problem with a coal at 1.135,1.146 and 1.150, coal didnt help in my case . I run at 1.146 to 1.150 useing 124gr bullet. I had the problem not only with FC brass but mixed range p/u brass ,alsowith different bullets . I think belling more fixed it,or maybe i just started guiding the bullets straighter when seating, try belling more first,thats the problem with doing 3 things at once,not sure what fixed it.