Just thinking out loud ... as I have a few small 1/8” to 3/8” (sold by the 1/16” sizes) diamond-tipped thin-wall “hole saws” for drilling holes in china and ceramics, for making lamps or other crafts. Whereas they cut to the diameter listed, if somehow you could fashion or weld a long extension, perhaps the bit/saw and extension would follow the ramrod groove OK and remove any excess pewter.
I bought a 3-bit set for $10-$15 shipped off eBay or Amazon, when I made some custom LED lampshades for overhead my kitchen counter (large over-sized white ‘coffee cups’, mounted upside-down). The bits certaintly weren’t expen$ive at all!