IMO, 3 shots does not make a group. Shoot at least 5 or more to see the true potential of the barrel/load combo. Just getting lucky that two are touching in the 3 shot group.
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Mauser9mm; If the bore is thinner at any point inside the barrel, then it would heat up faster at the thinnest part, which would shift the barrel slightly in the other direction, even mid-way.. Have that same problem throughout the barrel and it could significantly change the harmonics, not to mention the direction of the muzzle slightly. Yes, it is splitting hairs but a very possible explanation I would think.
