You just have the bulge because the sizing die makes the sides of the case straight where it has a taper as it comes from the factory and also because the cast bullet is a little wider than the jacketed ones. That taper makes the brass wider near the bullet base, avoiding a bulge with jacketed diameter bullets (.355"). If you want that taper, you'll have to use a steel sizing die with lubed cases or you could try the Redding two-ring die. Personally, I would ignore it. The only thing you buy with keeping the taper is a bit less brass working from one load to the next. 9 mm brass is cheap and easy find at the range, so I wouldn't personally worry about any life shortening that causes.
If it feeds, your COL is not wrong. The shorter you seat it the higher the pressure, so there's nothing to be gained by going shorter unless you have a feed problem. I find the most accurate cast loads are seated out to touch the lands rather than shorter, provided cartridges that long fit in your magazine and feed OK.
Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Rifle Instructor
NRA Benefactor Member