Is the rim of the .455 thinner than that of the original 11mm? If so, the conversion may have created an excess headspace condition. With a round like that, it wouldn't be a safety issue, but the misfiring you are seeing could be the result.
the cartridges sit noticeably lower also I notice there is a machined line in the cylinder well beyond the edge of the cartridge that shells may have been intended to rest against?
It may have been reamed for .45 ACP. Try one to see if it seats correctly in all six chambers. If it does, then you just need to work up some mild loads for it using soft lead bullets.
I'm sure you know this already, but do NOT shoot factory .45 ACP out of it.



























