Does seating the base of the bullet past the base of the cartridge neck adversly affect accuracy?

Is this because you have a very long bullet? I've always set mine up to the lands. Never thought about the backside. Tony

Pretty long bullet yes. Truth is I just finished putting together the 284 Win that I built then relized I hadn't throated it for the 140s. Guess i'm suffering from CRS.
Before I pull the barrel & throat it I wanted to get some opinions.

I don't know how far it sticks inside the case, but I think a consistent crimp would be the key. Work up a load and see what happens. Tony

My feeling is that if the chamber is true, and the throat concentric there shouldn't be a loss of accuracy. If anything the greater space taken within the case while slightly raising pressures, will also help load density...

No.