I know exactly what is going on,I just explained it on another thread.
The mauser magazine is designed for a different case.The 243 is both shorter in length and larger in diameter at the shoulder.
Rounds should stack in the magazine forming equilateral triangles of contact,limited by the box,the follower,and the rails.
What occurs,the rounds become tight at the shoulder hile they are still flopping around at the rear of the mag.They do not come up to the rails,so the bolt rides ove,till it digs in the side of a case.Then it pushes the case forward and jams things up.
Now,I learned all this building a .308,and it is still mediocre.I can't say I have solved the fix.As we cut metal to fix,we risk taking too much,and making scrap.
If I were to pursue it,I would just define the difference in geometry and modify the inside of the magazine box on a vertical mill.I might also modify the follower step on a surface grinder.The magazine might need to be blocked to place the cartridges farther forward.Then,I might study the rails with marker and a light touch with a die grinder.
I think all of this is just crazy.It is crazy because a 6mm Rem is a 7x57 mauser case necked down to .243,and a Mauser was made to work with it.
I cannot understand why Parker-Hale or anyone else would choose the .243 over the 6mm if it is going into a 98 mauser.