It is true that an M1 or M14 barrel that measures over 5 or 6 can still produce target accuracy. I shot a DCM M1 until I wore out 3 barrels and I was on my 4th barrel on the rifle. I had a Springfield unfired barrel on my rifle that was mfg in 1966. It was over the 9 mark and still shot well at 600 yards. One day I was at the 600 yard line at the SGC's Amite, LA Range and the shots started to go wild. Could not get better than 6's. The next barrel was a new LMR mfg in 1952. I used it until the throat gauge read 9.5. It was shooting well and it suddenly went wild, like the previous barrel. I replace it with a SA 66 new barrel. It shot well until #8 on the throat gauge. That barrel also went wild suddenly. The next barrel was a Barnett/Douglas Supreme with heavy barrel shank. It was the most accurate of all the barrels I used on the rifle.
I had a 7.62 NATO Garand with a Kreiger barrel and the .30-06 Barnett barreled rifle always shot tighter groups than the rifle with the Kreiger barrel. I adjusted the bedding on the 7.62 NATO rifle several times to no avail.
I would still not buy a Garand at the CMP store that measured 6 or more in the throat.