I've used mine (44 cal knock-off with approx 7" barrel) a few times for head dinking snowshoe hare. I had to do a bunch of filing to lower the sight notch on the hammer to sight it in. Now it's right on the money at 20 yards. Does a pretty good job when I do my part. I only use it in years of high hare abundance with lots of shooting, so I haven't done it justice in terms of really working it out year after year.
Never contemplated deer with it, as much for its 25-30 grain powder charge as for the fact I have so many long guns to use too and only need so many deer. As accurate as it is, I'd have no qualms out to at least 25 yards, but I'm a little hesitant about further. It would do the job I'm sure with the right hit, but with little margin for error. And the older my eyes get, the more margin for error I need!
Bottom line, if you want to use it for deer first make sure it's legal in your state. Then get really good with it. Then figure out your own sensible range limits and have the intestinal fortitude to honor those limits. Deer aren't cast iron and hard to kill- if you put the ball in the right spot. Things get nasty quick when you don't, so it's all on your shoulders and not the gun.