0,6 and 0,5 mm - it's big differences and might be seen as uneven surface and spots inside bores using light-and-shadow method, but in barrel before burst. Now it's very hard to say what was the reason of such irregular mesurements, but I vote for burst and steel tension as result of the burst.
Any way I have to say, that 0.020" - it's d*mn not enough for the shhoting with modern ammo such as S&B for example, I know this ammo very well and this ammo is pretty hot and very often the pressure is higher, than CIP standart recomends.
