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Is there any difference using a 6"x2" reducer on top of my 15.5 gal boiler? Whether it is a cap, concentric or bowl?

Can you provide more details?

Heat?
Spirit type?
Packing?

Etc

I would say for most purposes, no. The cap style will get you the most packed colum height for a reflux rig, while the concentric or bowl might behave a little like an onion, but to a lesser extent, for pot stillin'.

Yeah, elevation wise the end cap reducer can help you save some valuable height if maximum plate count (theoretical or actual) is needed.

Beyond that, at the hobby scale the concentric reducer is probably not bringing a bunch of value to the table. Especially if the cap on your kettle is not a dome (or the like).

6x2 or 2x6, lol? It's a keg, so is that 2" to go from the stock keg neck to a 6" column/riser? Why you got a 6" anything on a tiny little keg?! Or do you have a 6" ferrule on your keg top so you can get your hand in it from time to time but are reducing down to 2" for the business? Inquiring minds want to know.

6 x 2.

6" is more optimal to provide accessibility for cleaning / inspections on a keg.
Unless you're a little fella, 4" is just a bit small.
And 2" is completely inaccessible.