I dont know where the myth of tumbling Mk 7 came from but that is all bull. The tip composed of composites add stability.
That is why a long 6.5 mm bullet at 160 gns is more efficient than a .3 bullet at any wt up to 190 grains or more. The space in the tip was originally filled with aluminium and was totally enclosed in the fmj. It was chosen as a lightweight filler that wouldnt affect stability. No milspec FMJ are designed to tumble even on impact. That is enshrined in the Hague convention and supported by th Geneva convention. During the 1st World War as Aluminium became important in the war effort, too important to be fired st the enemy a solution was sought which included balsa wood, compressed paper etc.
For a true ballistically stable bullet you need a cylinder which in length exceeds its diameter by a factor of something like 5. Then for ballistic efficiency in flight ie low drag, you make the end pointed and the base boat tailed. This was the basis of the famed .303 SL streamlined match bullet of the interwars period.
You may have to trust me on this but a .303 FMJ only has the gap, note gap not cavity, filled to ensure that all of the projectiles mass is concentrated in its true cylinder length and that a hollow tip would/could be seen as infringing the aforementioned conventions. Concentrating its mass here really does increase stability, somthing discovered when the inspector of the Dum Dum arsenal sharpened all of his round nose ammunition in an attempt to emulate the German Spitzer bullet! The original 215 gn round nose was more stable than the 190 gn locally produced pointed ammo. The effect of sharpening the ammo was to expose the lead core hence the name Dum Dum. This resulted in increased velocities but poor stability.
The ammo was phased out after the Battle of Omdurman as it was deemed unfair to use on Christians and other Europeans!
For my evidence I have drawn from material from an HBSA lecture in 1992. It was given by Major Geear late of the Royal Artillery!