RClare,
Congtaulations. That's great shooting.
I also recently purchased a Vortek NW Magnum and shot it for the first time this weekend. I had very similar group sizes to those you mentionend using 250g SST's and 300g XTP's over 110g of Triple Seven. However, I was higly dissapointed in that only about 1 out of every 3-4 musket caps would go off! I was using a new batch of RWS musket caps (No. 1081) with the wings. The firing pin was denting the cap, but no cap ignition. Did you experience this at all? Could I have bad caps? Could the hammer not be striking the pin hard enough?