300 WSM is .535x2.100 :) take that case neck it down an you got a serious longrange rifle. better watch the pressures. Id look at the 6.5 rem short mag its pretty good if loaded a little lighter would be perfect for a PSL Im sure with some extractor work you can get a .032 smaller case head to extract.
biggun, didnt even get as far as looking at the case diameter on the x54 vs the others.
Rim diameter 14.40 mm (0.567 in)
Go to Wiki and do a search on the different rounds, most have that info on there if you don't have a reloading book.