I was thinking about that. I can use the fire forming loads for barrel brake in. Thank you for the response!you'll need to fireform it, the 338 NM is not simply a necked up 300 NM, the 338 has different (larger diameter) dimensions at the shoulder,
unless you order a reamer designed as necked up 300 NM brass, aka the 338-300 Norma Mag..... such as the 338-300 RUM aka 338 EdgeView attachment 208582View attachment 208583