WAM,
I make my recommendation of RL-23 over RL-26 based on my own experience with 26 in one of my .270s with 150-grain Partitions. While according to Alliant 26 is supposed to be pretty temperature-resistant, it wasn't originally designed to be, as were RL-16 and RL-23.
When I ran my standard "cold" test with the 150 Partition load using RL-26, shooting a target and chronographing at both a "normal" and 0 degrees Fahrenheit, the point-of -mpact shifted 1.5" at 100 yards--equivalent to 6" at 400. This was apparently due to the muzzle velocity dropping around 60 fps.
Such impact shifts are the most common problem with temperature variations, but in some rifles they don't occur even with much larger drops in velocity. One example is Eileen's NULA .257 Roberts, which shoots the same handload using Ramshot Hunter with the 100-grain Barnes TTSX into the same POI at 100 yards at 70 degrees and 0 degrees, even though the load loses around 100 fps. This is probably due to the very stiff, full-bedded NULA stock.
But the .270 I used to test RL-26 and the 150 Partition is a Winchester Model 70 Featherweight with a slim, free-floated barrel, which did result in a significant POI shift.
Incidentally, I have found such temperature-induced POI shifts can occur in any direction, including sideways, depending on the rifle and load.
