Consider the range you would shoot at rabbits and squirrels in your garden. Most of your shots will be under 75 feet, I would think, NO? If I can walk up to within 6 feet of a sitting rabbit in a lawn, I can't imagine you can't get to within 25 yards of a rabbit, even if they are getting gunshy because you are shooting a lot of them. A rabbits first defense is to freeze. It only runs when it is scared. As to squirrels, they defend themselves by going to the other side of the tree. 75 feet up in a tree is a long distance.
With those ideas in mind, I think you will do much better with using less shot, and not more. Will buffers help? Yes, but the only buffers that are small enough to settle down between your shot pellets would be something like PufLon, which is a super fine synthetic pellet, that is a little on the expensive side. I would not waste the money. At the short range you will be shooting, you probably don't even need to use 2 drams, and can use 1 1/2 drams( 41 grains) under an 3/4 oz. load. You could use 28 gauge plastic wads to tighten your groups, but I don't think its necessary at ranges of 25 yards and under. Most of your velocity will be lost in the first 20 yards. If you load more powder under a load, the velocity will reduce faster. Its a basic law of physics, and not something I made up. Whatever goes out faster, slows faster( in air.) Use light loads, but use #6 shot on rabbits and squirrels instead of #71/2 or #8 shot. Yes, the smaller shot size will give you more pellets, and if you were shooting pigeons or doves, I would recommend the smaller shot. But for rabbits and squirrels, you want a pellet that carries enough energy to penetrate the fur and hide. So, use #6, or #5 shot for these animals. At these distances, these pellets retain a lot of energy to penetrate the fur, hide, and muscles to get to the vital organs for a quick kill.
Having said all that, if you are planning to take shots out at 40 yards. ( 120 feet ), You need to get that barrel jug choked, just as Round ball did to his turkey barrel. And, you will have to be a better shot than I am and know exactly where that pattern is going to hit at that range. Then, of course, I would increase both the powder charge( to 2 drams) and maybe to 1 oz. of shot. I would also use the plastic shotcups, and see how that load works in the jug choked barrel at that long range.