I kept my HD shotgun loaded with #6 for years. Heard all of this "birdshot is bad" stuff and decided to try it out.
Next time I found a dead cow, I did some tests. I shot a (already dead from disease, but fresh) cow in the neck (about 10" thick) from different distances. I used an 870 30" full and Rem shur shot (el cheapo) #6.
At 10 feet the shot penetrated completely thru, wad and all. Left a hole just larger than 12 ga.
At 15 feet it did the same, but the wad hung in the far side skin. Again, 12ga hole.
At 20 feet, there was very little penetration. 4" maximum (prolly less), and a nasty wound about the size of a golf ball.
At 25 feet there was no penetration, just an ugly pattern about the size of your hand. Sure, maybe a few pellets sunk in, but it wouldn't have dropped anyone.
Note that these distances are in feet, not yards. 15 feet is VERY close, way too close for comfort, close enough that if someone is coming for you, you aren't going to get another shot off before they're on you.
Also note that this is a 30" full, not an 18" cylinder. I don't want to be shot with ANYTHING at ANY RANGE, but I'm not a big believer in birdshot anymore.
Birdshot will do nasty things at extremely close range, but almost nothing at mid range or thru light cover. Buckshot on the other hand will blow right thru (both) car doors @ 25 yards.
Guess which one I'm loading now.