I totally agree with Treefork, .40 cal is way too small to be taking game with reliably and humanely. Personally I would not use anything less than .50 cal and always prefer .625 over everything.
A friend of mine down in Mexico and I were shooting at a piece of 3/8 inch plywood for lack of something better to use. He had just got his brand new Target Zone .50 cal blow gun and wanted me to come over and do a little shooting with him.
When he shot those .50 cal target darts they hit the plywood with a little "tick" sound and he could pull them out of the ply wood with his fingers. When I shot the .625 of mine they hit the plywood with a "THACK" sound and I needed pliers to pull them out of the plywood as the darts were pushing out the other side of the plywood with target tips. The broadheads hit harder and do more damage going into the animal.
When it comes to hunting calibers.........go big or stay home. grin