You really shouldn't need a musket cap, but some people feel they need need the confidence created by all that priming charge.
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One of these from Track of the Wolf should fit: CVA-MS (6-1 mm), RLP-MS (6-.75 mm), or RST-MS (1/4-28)It's a 50 cal, Is there a musket cap nipple made for the Lyman Deerstalker I wonder?
Search - Track of the Wolf