COL will depend on whether your rifle has the early 2.6" magazine or the later .308 length magazine(2.78) both rotary carrier style, the 99C removable magazine was only '308 length'. My 99's all have longer throats, with cartridges that work through the magazine restricting length rather than having bullets bump the rifling, your may be different. I haven't used 3031, mostly 4064 and now Varget for 150's and Reloder 15 for 165's and 180's. Nothing wrong with 3031, just haven't tried it in the .300S. I have shot antelope with the 150 InterLokt and the 150 Speer BT. Both worked fine but antelope are not big. Many 300S have generous headspace, use Redding Competition shellholders to set die headspace or carefully adjust dies to avoid excessive resizing to extend case life.