I think it's possible, but I don't think it's the whole bullet. If you look at high velocity impacts, from meteors hitting the moon to (scaled down in terms of tensile strength) a drop of water falling into water, there is a central area in which the struck medium bounces back and would throw a column upward. As I understand it, this is what usually forms the central peak in many meteor craters.
The problem in "seeing" this is in realizing that the strength of the materials, be it of rock or steel plate, becomes a smaller and smaller part of the whole picture as velocity is increased. At, say, the 15 to 30 kilometers per second of a meteor strike, strength of materials effectively does not exist --as in the water drop hitting water analogy I posed above and the videos I found below.
Moreover, as I see it, the crater (in a steel plate, for example), may be formed just deep enough that the material of the impactor can "turn around" in the crater and can reflect straight back, or if the impact object is angled slightly some portion of the impactor can reflect back to the source of the impactor.
The parameters of both these incidents shown in the previous videos probably fell into the right combination for these things to happen.
Moral of the story is that unless your "backstop" is angled far enough, the possibility of fragments coming back is real. (You will notice that the armor plate backstops in indoor ranges is always at an acute angle to the firing points.)
I began my admittedly informal "study" of high velocity impacts when I started to wonder how come almost all craters on the moon are circular, when obviously, the impactors (meteors) do not necessarily come down perpendicular to the moon's surface.
I was quite surprised at the results. It turns out that unless the impactor strikes at a pretty acute angle (IIRC 20 degrees), the impact crater will be circular. There are a few known elliptical craters here on earth, one set of which are somewhere in Peru or Chile or someplace in South America.
Here are some examples of impacts where "tensile strength" is very low in the impact of water drops striking a surface:
Also, to help visualize the phenomenon, remember the movie clips we've seen of machine gun bullets hitting water at very acute angles? Note that even in these instances, a vertical water spout usually occurs.
High velocity impacts are interesting things, all in all. Our "boys" in those videos just happened to be in the intersections of probability, strength of materials, angle of impact, range from the target, ricochet angle, etc. I myself have had a couple of incidents similar to these. I once had a bullet come back and clip a little branch off a tree next to me.
Unlucky branch.
Lucky me.
Terry, 230RN