Vulcan 607 wrote:The chieftain was for a lot of years the most powerful tank in the world
T-64 was a thing already when Chieftain started to get mass produced so not really.
Vulcan 607 wrote:everything else had a 115mm or a 105mm
Heavy tanks were still around by 60s namely T-10M and M103 which also had 120+ mm guns. Surely their APDS round wasn't as powerful (M103 had none from what I remember) but they had pretty strong full bore AP.
Vulcan 607 wrote:it had heavy armour
measurements were done with ultrasound waves, http://imgur.com/a/F0TB6
which is more or less like on this scheme
TL;DR - it of course had better armor than T-55 but nothing exceptional, it was on par with M60A1 in terms of armor, really. Another thing is that it had a bad mobility for an MBT.