I'm having a .40" put together with an ETA of late fall. Spring 2008 will see me using it mostly for control of the ground squirrel population on our pastures. I think the 40-caliber is a very practical bore for this task. It is a far more appropriate caliber than the 54 that I'm using at the moment. (I never intend to, but shooting at a target when ground squirrels are running all over the place demands resolve that I don't have.)
With ground squirrels, you typically don't have to look around for more than a few seconds after a shot to find the next target. At least, that's the case in our pastures. The meticulous cleaning requirements of the smaller caliber rifles would slow things down. (Of course, skinning and dressing after a successful shot take me an excruciatingly long time, but I'm getting quicker.)
The extra mass should yield more accuracy over the smaller calibers which can be traded for longer range. With ground squirrels, that's useful. With tree squirrels, not so much.
Lastly, it's nice if the bullet gives a ground squirrel enough of a jolt to knock it away from the hole that it will almost definitely be standing in. Ground squirrels can somehow crawl out of reach down a hole even after a clean head shot. It's annoying. A round ball from a 40-caliber rifle will be more likely to knock them back enough to prevent this annoyance.
Of course, now that my dog will eat ground squirrel meat (cooked and only if she's hungry and there are no other options) and since I've been tanning the hides, I try harder to leave a clean carcass. Using the carcass in these ways leaves me feeling a lot better than letting it rot in the field.
