Don't know abut squirrels, but my Dad and I killed a ton of rabbits with the .410. Load was 3" shell and 7-1/2 shot. Always.
Biggest problem I see w/ .410, or rather its application, is use of shot size too large. Even a load of #6 shot is pretty sparse from a .410.
I found a small supply of the short 2-3/4 shell loaded with #8, and suspect that will be a good load as well, but have not hunted it.