With 3/4 oz loads you were essentially shooting a 28 gauge. When done out of a 12 gauge barrel, it does not pattern as well. Consider celebrating your accomplishment by getting a tube set.
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FWIW mercury in a tube is better than dead weight. Commonly used by clay target shooters in the stock and clamped under the barrel. Reduces recoil, smooths out the swing, but harder to start. YMMV.
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