Every barrel is a little different and the hotter the ammo the faster the wear but that sounds reasonable . Most 308's go 8 - 10 K but the Palma guy's , running fast ammo are around 4 K .

What method are you using to measure the throat ? With the Stoney Point / Hornady gauge you need to use the same bullet start to finish not just the same kind because of manufacturing tolerances giving different ogive lengths bullet to bullet .