andym79 wrote:Surely 38-55 brass sized and trimmed should be fine so long as you keep the loads at 38-55 levels, until 375 brass can be found?
Be careful with this. Seems to be a bit of misunderstanding here about what's being done to the brass.
To clarify when sizing up or with within the same family of brass, all you're doing is changing the diameter of the neck. The rest of the brass is the same (in some cases a neck may be a mm or two longer or shorter, but the rest will be the same)
e.g.
.243 win and .308 win brass are all the same except for the neck diameter. It's not like the walls of the .243 are thinner because it's a smaller calibre, or the .308 is made stronger because it's a larger one.
Same for .270 and 30-06. No difference except for the neck.
Same for .375 and 38-55... Once necked down you're using .375 win bullet weights and should be using .375 win load data.
Forget 38-55 once the resizing is done.