I dunno, but Alliant doesn't list RL-26 data for any bullet lighter than 150 grains.
My guess, however, is that it wouldn't provide any more velocity with lighter bullets, at least in most rifles, because not enough powder could be crammed into the case. I went up to 62 grains in my rifle with 150's, which is close too all that will fit in a .270 case and still seat a 150 Partition.
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