Sounds like you used a hammer and a piece of leather. Hard belt leather I assume? That should work well. I use a small hammer with a hard rawhide head, originally made for leather tooling I think. You'll never mar a barrel with leather or smooth plastic as long as you make sure there are no steel chips in your leather piece or the leather hammer head. The little brass handled hammer with screw-on brass/plastic/steel hammer face that Lyman markets works very well also. It's easy to spot small steel chips in the white plastic and you can reface it if necessary.
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