You are shooting the exact same load I have shot in both .50 T/C's I have owned. I could not load a 3rd shot in either of them until I had thoroughly cleaned the barrel.
For me that meant running a patch wet with T/C#13 solvent down the bore. It will come up black with fouling because T/C's rifling is so shallow there's no place for the fouling to go. This is so that their 1:48 barrels will seal a conical bullet and shoot them fairly accurately.
Next I reverse the wet patch and run it down the bore again. It comes up with more fowling but not as much as the 1st time. Then I repeat the process with a dry patch - both sides. Now you're good for 2 more shots - takes a bit of the fun out of it, don't it?
For me that meant running a patch wet with T/C#13 solvent down the bore. It will come up black with fouling because T/C's rifling is so shallow there's no place for the fouling to go. This is so that their 1:48 barrels will seal a conical bullet and shoot them fairly accurately.
Next I reverse the wet patch and run it down the bore again. It comes up with more fowling but not as much as the 1st time. Then I repeat the process with a dry patch - both sides. Now you're good for 2 more shots - takes a bit of the fun out of it, don't it?