Conventional wisdom of defensive shotgunning says that larger projectiles penetrate more. So loading birdshot guarantees absence of overpentration, while slugs will sail through the foe and possibly hit bystanders. But shotguns are sometimes unpredictable, and this theory only holds “all other things being equal” which they seldom do.
This particular 20ga shotgun was loaded with the assumption that minimum penetration would be ideal for home defense. Should birdshot prove inadequate, #3 buck (the largest commonly available in 20ga) would come up, and then slugs. The gun was test fired earlier at 25 yards and penetration was found as expected: minimal for birdshot, moderate for buck and considerable for slug. This time, test firing was done at 7 yards picked as the more likely in-house distance and the results were quite surprising.
Slug sailed right through the tabletop used as the test material: veneer-covered particle board doesn’t stop much. The worry that slugs would also overpentrate on humans was laid to rest by firing them into gelatin: only about 7 inches of penetration were achieved, with massive fragmentation of the projectile. That shouldn’t have come as a surprise, as the rifled slug is thimble-shaped and made of soft lead. For deeper penetration, either Brenneke hard cast lead, sabot copper or DDupleks steel slugs would be required. But slugs with deeper penetration are probably not the default home defense munition.
Buckshot was next. Spread was as expected, roughly XX inches. The pellets penetrated the tabletop but had little energy left on the other end. #3 buck pellets are about quarter inch in diameter, weigh just under 24 grains and have initial velocity of about 1200fps. That’s similar in energy to .22 Short bullets but with lower sectional density and no stabilization through rotation. The load makes up for the low individual pellet energy by launching about twenty projectiles at once.
The surprise of the day was delivered by birdshot. Plain, soft bulk pack birdshot produced a smaller pattern than buckshot and greater penetration as well. That became possible because the wad cup stayed with the pellets at the point of impact and the entire assembly hit as a solid mass. By 25 yard mark, the wad would have separated fully, but up close birtshot worked as a large pre-fragmented slug. Penetration beyond the barrier was low, but the initial punch was much out of proportion to the expectations. The shots were fired from cylinder bore and they may have contributed to the tight pattern. Although choke tightens patterns of loose shot, it could have separated the wad from the shot quicker than the unconstricted bore. Given the unpredictable nature of barrel and load interactions with shotguns, the only way you would know for sure if your own weapon duplicates this result would be by conducting a test firing of your own.