bluedevil:
If the firing-pins are sticking in the primers, you should be able to see an obvious drag-mark in the lower part of the firing-pin indentation. If the indentations are clean, there may be other problems.

If your loads are hot enough to cause sticky extraction in a double, you are WAY overloaded and at immediate risk of damaging the gun, and perhaps yourself. I sure hope this is not the case! If you are using medium-slow to slow powders, and not exceeding published velocity for that bullet-weight, you should be in a safe pressure range. According to the 1926 ICI and 1925 Nobel's catalogues, the .333 Flanged factory-loads turned up 2150 fps for the 300-grainer and 2400 fps for the 250-grain bullet. Are you using a chronograph? Its by far the most important tool of the Brit double-rifle reloader.

BTW, found a couple of loads for the .333 Flanged buried in Wal Winfer's British Single Shot Rifles Vol.3, with the usual warnings, as follows:

275gr bullet, 50gr 4350, 2,200 fps.
300gr bullet, 52gr 4350, 2,200 fps.

Not the 250-grainer I know, but it all helps.