I hope this thread doesn't devolve and end up locked. I'm finding the insights useful.
In places where non-lead bullets are mandated, availability of such bullets is a consideration as well. There are only a few extant non-lead bullets for the x39. I guess your out of luck if none of them are sufficiently accurate in your rifle. There are, by contrast, a plethora of non lead .224 bullets available.
While this is far from a controlled, scientific test, the results are interesting.
If you don't want to or can't watch, in an informal, backyard ballistics test, a .223 firing a 62 grain Barnes TSX penetrated 20-24" of clear ballistics gel. While I agree there are far better rounds for a dedicated deer rifle, such performance is nothing to scoff at. Who would have thought 40 years ago the .223 would ever be able to do that.