I liked that mythbusters vid. 18' is approx 6 metres, which is within Charge range and out of Point Blank Range.
No, I gather that neither of the 2 test-persons in the vid were expert quickdraw-shooters or very fast sprinters, but even so it was quite interesting.
Nor did it account for the possibility of the gunner ALREADY having their gun out and ready, which, unless the guy with the melee weapon is ambushing the gunner, is a very likely possibility.