A 280 Improved should use regular 7mm bullets. No you cannot make cases from 30-06's or 270's for that matter as the shoulder is about .040" longer on your 280.

Use 280 Remington brass or loaded ammo in that chamber.

Also use a feeler gage to make sure there is no sign of insipiant head separations. Not every improved chamber is made right.

I am getting a 280 AI in a trade.It has a Douglas 24 inch barrel I believe and it is a large ring Mauser.I don't know much about this round.Do I use 7mm bullets in it?Also can I form brass from 30-06 cases.Any info would be apreciated.Thanks in advance.