pull the breech plug and push a soft brass wire bore brush down the bore. maybe with a good shot of WD40,or light oil.
I wouldn't think any method , if done with reasonable care, will scratch the rifling enough to cause any issues. and with a wire brush, there is room for the debri to be trapped in the bristles instead of being pushed into the bore surface as it would with a patch. you may be a little over-concerned about damaging the bore. I would definitely pull the breech plug, though, for sure... it will eliminate the chance of built up debri being left in the breech. make sure you check the flash hole for blockage, as well.