The ONLY difference between Mk7z and Mk8Z was that the 8z had a boat tail bullet for better long range performance out of Vickers MGs. Vickers MGs were issued with two range plates on the sights - one for 8z and one for 7/7z.

The old tale that 8z is bad for rifle barrels no doubt came from Commonwealth soldiers being told not to shoot it in their rifles cause its trajectory wouldn't match the range graduations on the rifle sights.

A similar thing happened to NZ soldiers on Gallipolli - we used MLE Long Tom rifles still with Mk 6 ball, and Kiwi soldiers were told that the Aussie and English Mk 7 ball would not work in their rifles....... it certainly would work, but the trajectory would not match the sights at long range.... How many Kiwis died scrounging for Mk 6 ball when there were ample stocks of Mk 7 ball around? (Info from Gallipolli: the New Zealandicon Story by Chris Pugsley)