Plisskin Shift in region 43

Lucy D

The Plisskin Shift is currently my top rifle, supposedly covering region 42 - 43. I'm pretty sure that in the past such dual-region rifles have got me all the way through both of the two relevant regions. But I'm now on the third trophy hunt in region 43 (kill Mudd with a heart shot) and I can't seem to continue unless a buy the Clayton Testament 2, which is a region 43 specific rifle.
My Plisskin Shift is sufficiently upgraded to meet all the power etc requirements for the Mudd hunt.
So what's going on; should dual-region rifles cover both regions in their entirety, or are they actually only "half-way" weapons?