Loading for the 350 RM in a Remington 673, OAL is restricted to 2.800 inches. However, potential OAL in the chamber is considerably longer. Is anyone aware of the reason Remington has such a long jump to the lands in this particular calibre? Would there be a significant pressure spike were the magazine lengthened and bullets loaded to within, say, 10/1000s of the lands? I realise that the older 600 and 660 models also did not permit loading out to the lands. Certainly would appreciate your thoughts on this.

The "freebore" you see in your rifle will do a couple things. It will allow higher velocities at safe working pressures or it will reduce the peak chamber pressure. It likely also impairs accuracy.