Yep, longer barrel, even bull barrel, means more potential for barrel whip (just think sheer length:thickness ratio), and accuracy goes down.
Some velocity advantage should be gained from the longer barrel, as well as there being less pressure exiting behind the bullet as it emerges at the muzzle and thus upsetting it less. I suspect they would eventually have gained their desired accuracy by dropping pressure and velocity a bit. Hitting your target makes you feel much better than chronometer numbers do.
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