I think you will have a problem getting the 300 hh length in the magazine for a 300 wi n however you can open up the mag and extend it plus the feed port can be opened up to accept it I have done 2 and I have limited tools if you pull your bolt aa the way to the stop look in side the reciever and you will see there is a lot of room to extend the loading port to the rear you calengthen the mag a bit and you have it long enough mine don't look pretty when you take the stock of but function fine A gunsmith should be able to do a nice job with the right tools avilable Fred