The only logical reason would be that the barrels don't shoot to the same poi at the distance the birds are being broken at. I've seen shotguns shoot diferently but never a Beretta, Browning, or other high quality shotgun. I'd suspect that you're being pysched out by the fact you're using a different barrel and are subconciously doing something different. This can easily be checked by shooting the gun at a given distance (I'd suggest 25yds) at a patterning board or target and seeing if the barrels are regulated.
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