Jim,
ALL powders gain some pressure, and hence velocity, as temperatures rise above 70 degrees, but temp-resistant powders gain less. Others can vary from results like 1680's, which are obviously OK, to some that will blow primers or lock up bolts on a hot summer afternoon on a gopher or prairie dog town. And if course barrel temperature affect everything too.
On the same hot day I tested the .17 with 1680, I also tested my .204 with its usual prairie dog load, the 40-grain V-Max and a max charge of Ramshot TAC. It only gained 24 fps from 70 to 115 degrees.
