[masraum]

I think a lot depends upon the wood and the bullets. If the bullets are going really fast or relatively slow (in bullet terms) would make a pretty big difference. If the wood is really soft or really hard that may make a difference (lead passes through or embeds in... In some circumstances both bullets will impart the same energy and either neither, or both would knock the thing over (depending upon the mass and stability of the firewood.

Rubber would be more likely to knock the thing over. The bullet would either pass through or get stuck in the wood. If the initial
The (big, as Thom suggested) rubber bullet would hit, deform, and then reform which would impart more energy. Assuming that this is some sort of magic rubber bullet that doesn't disintegrate when it's fired that fast.

I wonder how the MythBusters test with firing into water relates. When they fired a relatively low powered bullet into water it went much, much farther than a really high powered bullet. The really high powered bullets, especially the 50 cal disintegrated in the first 12 or 18 inches.

I'm terribly curious to hear the answer that Lube has for us.