If you do try this and would like a suggestion for a real good cutter / scraper read on ?
Get a good name brand Ie;( Morse, Greenfield Hanson etc.)High speed steel Tap a Four Flute.(High speed-cobaltt is sharp and not too brittle carbon tap is ok wont stay sharp but sharp enough for one barrel)
Now you will have to decide by observation and measurement what size buttt??/ 5/16-18 , 1/4-20 , 3/8-16, 5, 6, 8,mm. something in those sizes i suspect.
I dont know how to draw on hear so I will describe??? Grind the Tap leaving one straight Flute, and totally getting rid of the other three (assuming your using a 4 Flute Tap) with a real steady hand and being careful this can be done on a pedastool grinder and bench stone. Ya it Takes time and careful measurement.
The reason for a Four Flute hand Tap is The width of the "flutes" they will be closer to youe groove width.
Try to never hit or touch the cutting edge you are saving and you will have a real nice and very sharp Groove cutter.
If you have or Know someone with a Surface grinder you got it made, Go and have them grind it for you.
One last thing if the bore is rusty try and get the rust out before using your nice new cutter RUST is hard on cutting Tools. use steel wool, stainlees Bore brush, chemicals whatever??