I also own and shoot a similar boxlock double on a Webley action. Mine though was retailed by Isaac Hollis of London and spent its previous life in India. The engraving on both side depicts a Leopard - I think it would be a very confident, or possibly foolhardy, hunter who went after Leopard with this cartridge !
The barrels are marked 360 EXPRESS BLACK POWDER 2 1/4 CASE on the top, and 360 EX 55-190 below [presumably the regulation charge and bullet weight]
Being an oppressed UK shooter I have limited access to the range of powders and bullets available in the US, I also only use the rifle to punch holes in paper at 50yds, so my reloads err on the lower velocity side.
I use a Rapine mold 366195 bought many years ago - sadly no longer trading. I used to load with Trail Boss, 11gns giving 1300fps, but the EU banned its import a while back. I have also had good success with Unique 8.5gns giving 1200fps.
However I now use Lovex DO60 a czech powder, which if you read their web page they say is similar to Accurate 5744 [which I presume is available in the US] I find 13.5gns gives me 1100fps and regulates well about 3in high. This load does leave unburnt kernels up the barrel.
I find Bertram brass can be a touch thick at the neck for my right side chamber stopping the action closing. I size the bullets to 365 and use allox lube. For the right chamber I have a number of cases I keep separate, these I have put in a bench drill chuck and thinned down the neck using a strip of fine abrasive paper.
Nick
