Anybody got a reamer based on 284 dia bullet rather than the original 288?

I think that 275 H & H is, as original, is a .284" bullet unlike Ross's .280 Ross or Jeffrey's offering in that nominal calibre.

Give H & H an email. You will get the correct answer to this question.

Far better idea to make them with standard 284 groove; barrel and bullet availability is ten times better. Unless you just want to make things hard.