what kind of underbored barrel?
i found with some barrels when underboring on twist lock guns, that the bolt tip would enter the undebored section and that sticking, would cause eradic velocity. never observed that with the cocker threaded bodies.
if not that, or an issue with the regulator, look at over oiling. some say you can't over oil a mag, but oil in the barrel from being fired through the whole gun, can really mess with both accuracy and consistency.
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