From Speer ... "Where IMR 4831 is recommended in these tables, H-4831 may be substituted. However, in no case should IMR 4831 be substituted in equal charge weights for H-4831 ..."
For the 264 and 140gr bullets use Ramshot Magnum!!!!!!!!!!!!!!!!
It is the powder for that combo!!!!!!! out of my 27.5'' gun im getting very good velocity ( the kind that im not going to publish) but in a 26'' gun you should be able to get 3200-3250 with very good brass life..
Im shooting 140XLC's and 140 Bergers and it shoots them both to the same ( basically) point of aim.