[Jack]

Larry, Ken Waters did a nice write up of the 256 Newton cartridge in Pet Loads, and he used an original Newton rifle to do the work up.
One thing Waters pointed out in the write up was that there is some very, very suspect loading data around for the 256 Newton. He reported that some published loads were several grains more powder than would fit in a case, even compressed.
Going from Waters work on the 256, the Newton holds less powder than a 6.5-06 (the Newton case is shorter), so clearly the Newton won't produce 6.5-06 velocities.
I've seen in print a number of times that a 256 Newton is a 270 or 30-06 necked down to 6.5- not so. The Newton case is definitely shorter. Ballistically, according to Waters, the Newton is a tad faster than a 6.5x55, but not a match for a 6.5-06.