From what I've seen it's more that the effect is only pronounced when you use slugs or buckshot. A few youtubers have documented their own tests, and the results are pretty similar.
This guy did the most extensive testing, and found that for buckshot an 11" barrel has ~75% as much energy as an 18" barrel, ~90% for birdshot, and ~70% for slugs.
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u/OkBurner777 Mar 27 '26
Glad it intimidated the others into domestication
Also case in point that shotgun barrel length has little to no effect on projectile velocity, unlike rifle and pistol calibres.