HI All. I am new to the forum. I found this thread while researching information on .300 WSM accuracy. I had the very same problem with my .300 WSM, a Winchester Model 70. Factory loads chambered fine but my reloads would not even though they were resized and properly trimmed. Even though the once fired and resized cases would fit perfectly in a Wilson .300 WSM Case Gauge. I reload all of the ammunition for my firearms & this is the only caliber I have had that problem with. I use Redding dies. Full length resizing dies & a Redding Competition Seating Die. I finally called Redding explaining the problem to them. They have great customer service. They asked me to send them my dies along with some brass fired in my rifle. A few weeks later they sent my brass and dies back to me with no explanation of what they had done. I called again & was told by one of the Techs (I don't remember who) that my resizing die was not properly manufactured causing the problem. He was famaliar with my case and said they had sent me a new resizing die. I still don't know what the exact problem was but I have not experienced it since they replaced the die. All of the cases fit perfectly now. I hope this helps someone else.