Here are the pics. You could make the length anything you need.
This is it all rolled up...a big tame duck egg, or maybe goose egg sized bundle that easily goes in a pocket or pouch:
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The barrel end is just a slit in the leather so you can make a loop.
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I slide this down the barrel over a thimble and pull it tight. It won't interfere with the ramrod, and one can still see through all but the very lowest of rear sights.
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The leather is quite thin:
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The butt end is just another slit, but longer, with a loop of leather sewn across it. The slit is reinforced with stitching at the bottom end of it.
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To install, simply slip over the butt. Keep in mind, this is meant to be a quick on/off sling, so only the weight/pressure of the gun is keeping this in place.
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I actually purchased this one someplace, but the design is simple and easy for a DIYer. Lightweight and use-as-needed, then pocket it. Works on any buttstock.
Hope this helps.