I have a 270 win.in a bdl. I would like to put on a 6mm barrel and neck down a 25-06 case to 6mm. What are the pros and cons.

Not a big deal to do. Basically a 240Wby with a normal shoulder and no belt. You will burn a lot of powder, raise the potential to burn out a barrel and get a 100FPS gain in velocity. (Comparing loads listed in my Nosler book for a 6mm and 240.) I would use a long barrel to help burn the extra powder.

Lot of people swear by the 6-284 which has pretty much the same capacity. I built a wildcat years ago that had about 11% greater capacity than the 06 case. I pulled the barrel. I found I was just burning a lot more powder for small gains. Rebarrel with a 6mm. A 6mm or 6mmAI would be all the powder I would put behind a 6mm.

So sell the long action and buy a short action

Ever think about the 6.5-06 or the 6.5-270 (6.5 Brenneke)? Flat shooting and better bullet selection. Should fit right in your action.

Before I would give advice, What are you trying to achieve? It would make it easier to help you.
Butch