To my knowledge it hasn't been done by anyone here.

Several thoughts on my part:

You would be wasting a long action and asking for feed trouble using it for 300grn bullets. If you want to use the long action just do a 458 Winny and down load it. Lot less trouble than what you're asking for.

If you really want one though, I certainly understand that!

I would go with a short action though and build from there. Feeding issues would be much easier to fix.

To my knowledge no one builds unchambered barrels unless you spec it that way.....which some will do then. You need to use a reamer to chamber with.......It is possible to chamber like you're talking about but unless you are a VERY experienced machinist or something like that I WOULD NOT even try it. Even then you would need a throater after chambering. Not sure where you could find a reamer to rent.......you would have to special order it. Then you have to consider dies.......

You want to LIVE after you pull the trigger one time.

300 grn bullets do not require a fast twist rate but the cartridge you're wanting to build was used to shoot special 458BT bullets at subsonic speeds during the Vietnam war. It was developed by Barnes for that purpose. This info is on the 10th edition of COTW book. If it were me, I would use a fast twist rate such as what you're looking at for the purpose of launching those big heavy bullets subsonic, just like the round was designed for.

If I were you, I would start with something a bit easier to build so you can get some experience at doing this kind of stuff before doing something this specialized.

What ever you decide to do BE SAFE and have fun!