I have quit using my .410 for squirrels for several reasons. First, it is very difficult to find any ammo around here except #4 shot which provides very few pellets for a pattern and the darn things are expensive.
MO used to prohibit the use of .410 slugs for deer hunting but that changed 2-3 years ago. Fortunately, MO has allowed the use of centerfire rifles (.243 and up) forever so I never really cared about slug hunting.
If I have to shotgun hunt for the squirrels (certain conservation areas and when the trees are full of green leaves) I use the 20 guage.