Labgrade.....maby you should have slugged the bore. If it was at land diameter then it was a mistake and amongst other things would give you significantly higher than normal pressure. If it was at spec bore diameter then it may have been specially made for target work within 1.05° of the equator. Where the effects of right or left twist for compensating for coriolis effect are essentially nullified. Thereby favoring a non-spinning projectile. The foregoing applies only to ranges not exceeding the length of the barrel.
Mas
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