So I recreated the situation tonight with one piece each of 45-70, 44m, 45acp and 6.5cm. Exactly the same situation but with only 4 pieces of brass instead of around 120 so you could argue they would have heated up more this time (?), anyway, the hottest was the 45-70 by half a degree and that was 297.7 degrees. Looking at the info a couple of you guys have posted I reckon I'm safe. None of them are loaded full house and all in modern reproduction rifles (apart from the 45acp of course) and the 6.5cm is a bolt gun with triple locking lugs. I'm thinking the discoloration could have been enhanced by having just been sonic cleaned so the brass was as 'bare' as could be. Anyhow, thanks for all your input!