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a 20-gauge shell is placed in a 12-gauge shotgun. what is the danger?

Placing a 20‑gauge shell in a 12‑gauge shotgun is extremely dangerous because it can turn the gun into a barrel obstruction bomb pointed at the shooter’s face.

What actually happens

  • A 20‑gauge shell is smaller than a 12‑gauge bore, so instead of seating correctly in the chamber, it can slide forward into the barrel and get stuck where you cannot easily see it from the breech.
  • Because the obstruction is hidden down the barrel, a 12‑gauge shell can then be loaded normally behind it, making the gun look “ready to fire” even though the bore is blocked.

The main danger

  • When the 12‑gauge round is fired, high‑pressure gas slams into the stuck 20‑gauge shell, creating a severe barrel obstruction.
  • Instead of the shot and gases exiting the muzzle, the pressure can cause the barrel to bulge, split open like a peeled tube, or even fragment and damage the receiver and stock right next to the shooter’s hands, face, and eyes.

Possible consequences to the shooter

  • Serious injury risks include shrapnel‑like metal fragments, burns, loss of fingers, severe facial trauma, and permanent eye damage if the barrel or receiver ruptures.
  • Even if the gun does not catastrophically explode on the first obstruction shot, it can be structurally weakened in ways that make future failures more likely and more violent.

Why this mistake is well‑known

  • This specific 12‑ vs 20‑gauge mix‑up is so notorious that manufacturers standardized 20‑gauge shells as yellow to warn shooters and reduce accidental loading into 12‑gauge guns.
  • Firearm safety training and hunter‑ed courses frequently highlight this exact scenario as a classic example of why you must never carry mixed‑gauge shells together and must always physically and visually check ammunition before loading.

Safe practice takeaway

  • Never attempt to fire a 20‑gauge shell from a 12‑gauge shotgun and never mix 12‑ and 20‑gauge ammunition in pockets, bags, or on the bench; keep each gauge stored and carried separately.
  • If there is ever any chance a wrong‑gauge shell has been in the gun or could have dropped into the barrel, unload immediately and have the firearm inspected and cleared by a competent gunsmith before further use.