Take a ruler and hold it across the muzzle to measure the inside diameter. If its around 3/4 " =/- 1/16 its probably a 12 ga. If its closer to 5/8" its probably a 16 ga. If you have more sophisticated measuring devices, the muzzle diameter of a full choke 12 ga. is usually around .700 and a 16 ga. around .635, ususally within 10 thousandths either way. FWIW
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