You are at the bottom of the Hodgdon loading range (41.5 being the starting load) so you
could have a light load high pressure deal going on. With the added heat the load preformed better and now you are getting more of an explosion instead of a burn. Speaking just for myself I would try a single round loaded 1/2 grain heavier.
http://www.hodgdonreloading.com/data/rifle
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