A reamer can be a factory chambering but still have a few differences, such as bullet seating. I know my smith said he went through a few reamer designs before he got what he wanted. I know I seat my bullets long so thats part of it. Some people told me there was no way I would get 180 bergers to 3050 out of my 7 mag but I've confirmed the drops out to 1072 and it is. I'm not a smith so Im not sure how to explain all the little things you can do to get everything out of the chambering, maybe someone else could do a better job. I know that my factory 7 mag was only getting 2900 with 168 bergers before I gave it to my smith and had it completely redone. So thats why I say factory or smith without a tricked up 7 mag reamer I would go 300 win mag, the factory 7 mag just didnt do it for me.