[Andy_ONeill]

BTW. There is an official abbreviation of pdr rather than lbr. 6pdr (eg).

I think the 3pdr was much more likely to be found on ships than tanks by ww2.

British ATG used a much hotter load then US and more again than Soviet. Shells could be expected to be of higher quality than soviets, significantly so. Hence solid shells were less likely to shatter on impact and HE shells had more HE content for their calibre and probably more effective content as well. Soviet shells had to have thicker casings than most other countrues like german or US because they couldn't guarantee the tolerances. Iirc 15% less generally.
As if that wasn't enough, the fuses were less reliable.