How much to reduce load from Norma brass to Federal

[Ironworker]

300 Winmag 195gr Sierra TMK in Norma brass was 74grs of N560 with zero pressure signs. I picked up some once-fired Federal brass, which weighs 20grs more. I wonder how much I should reduce the charge with the same bullet, powder, and primer.

 

[358WCF]

With near max loads around 5% was always a good rule of thumb until that time it wasn't. After I got the bolt open & the brass shavings out of the ejector hole around 8% less was at the same velocity (& apparent pressure) as the original load.

Maybe that's why 10% was suggested as a safe reduction by the manuals & writers I was reading way back when, even though 5% is usually safe enough. A quick series starting low & going up even a grain at a time uses very few resources. Or you could dick around with QL or Gordon's tool. The Powley slide rule has also been very useful for me in the past. Whichever you choose, you will still likely have to work up, in your rifle, with the actual components used.