Using the head of a steel carriage bolt and a round grinder is perhaps the worse tools you can use.
The steel head does not imbed the grinding paste and would be ground by the paste imbedded in the muzzle end.
The only way to use a polishing haed is to use a brass ball. The ball has to be softer than the barrel steel. Hence lapping a barrel with a lead lap.
You are better off to file the muzzle square and flat, that is a bit tricky with a tapered barrel but I have done it often. Then use a big Robertson R.H brass screw head and some 600 grit paste and and lightly spin it with a drill to break the edges.
I used a chop saw to cut off the end of a sporterized Enfield and then a cone shaped abrasive Dremmel bit going straight in to polish the crown. The gun shot well under an inch at 100 yards when finished.
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