I just got finished reaming a NEF .357 Mag to MAX. I rented the reamer from 4D. Fred sent the reamer quick, your rental time doesn't start until you receive the reamer and ends when you send it back. The reamer is similar to a tap, as in tap & die. You need a tap handle that will handle a 3/8" square reamer/tap. That was the hardest part of doing the job, finding the handle (some how mine seems to have disappeared and the kids don't have a clue where).
Check to make sure your rifle is unloaded and remove the barrel. You need to remove the extractor if so equipped. If the barrel you have is the second generation extractor (see FAQ) which is what I got on my new .357 Mag barrel, you only have to remove one pin. Punch out the pin which is closest to the chamber end of the barrel and then pull the extractor out. There is a flat spring sitting on top of the extractor, notice which direction it is sitting, with curved side up. Now you can ream the chamber. I put a couple of patches in the barrel just forward of the chamber to prevent metal shavings and cutting oil from running down the barrel. I applied cutting oil to the chamber and then inserted the reamer (the reamer Fred sent had a solid pilot) into the chamber. I didn't feel any slop, it was a solid fit. I turned the reamer until the shoulder on the reamer met the shoulder for the cartridge rim in the chamber, stopping a couple of times to clean the cuttings off of the reamer. Then I cleaned the chamber and barrel and placed a dummy cartridge .357 MAX (no powder or primer, just brass and bullet) into the chamber to check that it fit. Removed the dummy load and put the extractor back together. Before replacing the extractor, I did polish the rough surface and edges on it and applied a little moly lube.
Overall a pretty easy experience. The first time I removed the extractor I punched all 3 pins out. Then I discovered that only the one pin closest to the chamber end of the barrel held the extractor in place.
If you were to order a floating pilot reamer you need to know what diameter to order. Slugging the barrel would be the best way to determine this.
FYI: The seater plug for RCBS .38 spc, 357 mag, 357 max dies with rifle type bullets is part number 90018.
HINT: Connect a 3/8" drive socket extension to a 1/4" drive reducer. The reamer fits in the 3/8" end and the 1/4" end goes into the tap handle!