The only reason folks say to use a shotgun when the leaves are still on is because it's harder to see the squirrels and to shoot them. I personally only hunt squirrels with a .22 or a pellet rifle with iron sights. If you want to become a better deer hunter use a .22. You will become a much better shot be able to notice much more minute movements and your eye will catch any small change in the woods quicker. It basically sharpens your hunting skills. A added bonus is you don't have to pick shot out of the squirrels when you clean them. You can get a nice Marlin .22 for under 400 dollars, which will be more accurate enough to shoot squirrels.

I don't know that it's so much about leaves. IMO you just don't have to pass up as many shots if you use a shotgun.

Clearly what to use is disputed!

You can get additional screw in chokes for that gun. Modified is OK, but I use full choke.

I use a Mossberg 500 12ga Modified choke, or NEF pardner for all my small game hunting needs. My dad uses a 20 ga H&R single shot.

I use #6, #4, #8 and 7-1/2shot all are good all for squirrel,
Rabbit # 4 #6

My dad uses #6 and #4

It's up to you.

As far as leaves on off tree it doesn't matter what you use.

Shot travels 100 yards
22 travels 5 miles

You have to be more accurate and chose shots more carefully using a 22. i.e. shots are taken closer to tree trunks where the bullet is likely to kill the squirrel but lodge into the tree.

either one will do.

I think by this point you get the idea. Both weapons have there uses. I recommend if your on a budget getting the shot gun however for the simple fact that it has many more possible uses and you can branch out if you decide to hunt other animals later on. A Remington 870 is a wonderful and reliable shotgun and I use them often but for my personal use I bought a mossberg 500 12 gauge for about $200. It will fire the 12 or 20 gauge shells and can use different chokes in the barrel for different uses. I will admit the reason I chose this model was I taught my wife to shoot and she had a problem with the 870's safety, I liked the thumb safety on the mossberg for her use.

I got an 870 20 ga with modified choke and it works great. The reason for shotguns before leaves fall is because they are harder to see. If you wanted, you could get a 20/22 combo. my step dad has one and its great. Its a 22 on top, and a 20 ga on bottom. for squirrel and rabbit, I prefer #6. Ive used 7 1/5 and a lot of the time the squirrel would be alive when it hit the ground.

Make sure you heve a good ground shot with the 22. Wouldnt shoot into the trees. I use a 12 ga or my 410 for small game. #6 shot is good all around for small birds and game. I have some harrington /richards single shots. Nice and accurate quite inexpensive to buy too. 150 or so.

If all you are wanting to do is pest control get the shot gun. Personnaly I would get a full choke but that is your call. I had an 870 12 ga and it was great.

DO NOT USE NE THING MORE THAN A 20GA! you won't have any meat left. if you do use a 12 ga aim a little off the head so you don't end up with a gnarly piece of SH*T. it will be bad. 410 will work just fine.