Seating dies do not cause runout. That happens in the sizing operation.
Seating stems that do not properly match the profile of the projectile can cause significant run-out, by 'grabbing' the bullets and seating them crooked.
I know alot of people are biased towards x brand or y brand just because......But has anyone compared Lee Seating dies to other top die brands to compare runout between them? Using same prepped brass,method and proper set up........
My own experience with Lee dies has shown their tolerances to be all over the map. Whether or not you get a tight, concentric seating die with a properly fitting seating stem... is just a roll of the dice.
Use what you like. If it doesn't give you the result you were looking for, check to see if it's a problem with the tool. If not... try something else.
My biggest aversion to Lee seating dies, in particular, is the aluminum seating stem. It is far too easy to damage, and the alloy is incredibly soft (easily deformed with use / threads easily stripped). That alloy is so soft, that every machinist I have ever met in the firearms/reloading industry has referred to Lee's aluminum alloys as "Blue Bonnet" (as in the brand of butter).
It's easy and affordable to have a machine shop turn a steel replacement stem. But, if you're going to do that; you might as well just have them make the die, too.
Use what works for you, and don't worry about changing it.... unless you find that the end product is not to your satisfaction (and can legitimately
be blamed on the tool).
As long as we're buying "mass market" dies... we get what we get. The tolerances are variable and somewhat loose (some brands more than others), to accommodate the myriad of different components and firearms on the market. If we want something with tight tolerances, it has to be custom made.