Rather than a single pet, I have opened a zoo in my storage cupboard. If limited to one that the others probably think is my pet, it would be IMR4350.
I started out a long time ago with IMR4350. It's worked wonderfully for dang near any cartridge I've loaded for. It's not as versatile as one of my favorites like RL15, but has always proved to be a performer and something I always have on the shelf. I'd love to make a switch to H4350 since it's not as temp sensitive, but it HAS NOT been as readily available as IMR in recent years. Hence the reason I'll always have and use IMR4350... When I look at load data and targets from the 270, 30-06, 300wm, 22-250 and 338wm, I always see IMR4350 in that data for most of my pet loads. I don't think it's the "best" powder, but something I can always count on, kind of like the old 30-06 springfield: Not the best, but always works...
