Ice is pretty low density, 1 versus 11 for lead. It is also pretty brittle and will tend to fragment on impact so wound channels will be short. It will also tend to melt under the heat of the combusting powder. Finally, a normal barrel will spin it at a very rapid rate. (37,000+ rpm for .45 ACP) Which will cause it to shatter on barrel exit.
On the other hand, get something going fast enough and it gets dangerous.
So suppose we go for something like a smooth bore musket using compressed gas instead of combusting powder. It looks like modern air rifles can go 1,500 FPS. We could probably go better than that with a really high pressure supply and fancy ultrafast open valves. Maybe go 25mm bore, a length of 50mm and thus a projectile weight of 22 grams or so. Kinetic energy looks to be north of 400 Joules. This should be enough kinetic energy to do some real damage even if the projectile doesn't penetrate very well.