I had a 15/16"octagonal .36 cal Sharon barrel back in 1975 that I bought straight from Hall in Kalispel on my trip to visit Les Bauska and buy some of his barrels. It was one of Hall's deep groove buttoned barrels. I was in the shop when he was dragging the button through the tube. The whole shop shook (vibrated) from the enormous pressure inside the bore- stop-go-stop-go-stop-go in rapid succession. This barrel didn't split, like some did.
He said it would be a 'real' shooter. I didn't know any better, so I bought it - $50.00 iirc - HA! - put in powder and a ball and it would shoot - it was OK, I guess, but I could not get over that weird internal copndition of the barrel, like it had cross-ribs inside form metal flowing back into the bore behind the button.
The bore was impossible to look at, if a 60 watt bulb was at the other end. It was VERY smooth, mirror-like, but those ribs throwing and magnifying light - OUCH! Hard on the eyes. The loose and tight spots were about 1/4" apart- it looked like a fiddle-back violin on the inside - that is, the lines of loose and tight spots showed like fiddle-back wood grain. Vdddddddddddddddddddddddddddddddddddd- is what a tight patch felt like as it was pushed through the bore. Maybe you had to be there - HA!
I put on a huge sloping crown and used to shoot .375" balls with a denim .22 patch. The barrel's 36" twist seemed to shoot them well enough as they turned into elongated little flat nosed, round butted slugs. I used a steel 5/16" rod and won a match with it. A 3 minute timed even at 25 yards.
I shot naked .375's and loaded 4 or 5 at time with 30gr. of powder. I had 27 holes in the scoring rings and over double the second place score. This is not a recommended load, btw.
Yeah - deep hole buttoning does not work - I threw it away and replaced it with a 38" twist, cut rifled .50 Bauska barrel that REALLY shot beautifully.