Well I found 50 pcs of 338 brass locally. I did bum 3 pcs of 300 brass and sized them down. I didnt use imperial sizing wax but if I do anymore I will be sure and use it. I just used RCBS sizing lube. It worked ok got some small dimpling at the neck shoulder junction. Trimng to length was the worst part of it. I use a forester trimmer and ultra mag brass is too long to fit with the neck turning attachment installed. Had to remove the collet each time to insert brass. At least it gives me options to use when brass availability gets bad.
i made 3 up this morning. here is a pic. i'm gonna shoot 'em this afternoon.
it looks as though the shoulder doesn't have as sharp of an angle as factory made cases, but i'm sure after one firing you could never tell. i got a little dent on once case. neck thickness seems very close to the 338 rum.
left, factory 300 rum. middle, 300 rum sized to 338 rum, not trimmed yet. right, original 338 rum.