I bought a #4 some time ago and with a stroke of "wisdom", used .300WinMag reamer to cut the chamber for a .350Rem. Mag. Case necked to hold a .312" bullet.
The barrel on the rifle had a .315" groove to groove "new" barrel with only 2 grooves. I was told it was a machine gun barrel. That I do not know for sure, but with a load of Re#19 and 174gr. .312" rn HORNADY, it makes 2,960fps and put 5 into 1 1/2" at 100 meters.
I've not hunted with it, but did install a higher front sight as the front "battle" sight puts the impact 14" high at 100 yds.
I have not shot it since changing the front sight. No alteration of the extractor was needed.
I fiddled with the mag. lips a bit, but it will only feed 2 rounds properly. No big deal as it has a 5 round short mag.
I thought I had made a new wildcat but found Ken Waters had already done this a year before I did. Ken called it a .312 Express.