Note I said a fun gun on small game, not a fine gun! Seriously, though, a .410 with the right load and reasonable shots is perfectly adequate.

I shoot the vasty majority of my squirrels on the ground. Rarely past 25 yards. Some further, but not too many. Mostly with a .22, sometimes with a 12 gauge, but lately with a 20 gauge. I only take open shots. What I don't do is blast into shaking foliage or shoot into brush and hope a squirrel falls out. And #6 is the smallest shot I will use on squirrels. #7.5 and finer just makes a damned mess. #8 IMO is worthless unless you are just using it for pest control. I prefer high velocity #4 or, 2nd choice #5. #4 will shoot completely through a squirrel at reasonable ranges, will break leg bones, and won't bloodshot meat. I have no doubt Federal Hi Brass 3" #4 .410 at over 1,100 fps would be an authoritative squirrel killer at reasonable ranges.

[Linked Image from i.imgur.com]
https://vcdl.org/