Hmm. Well, a kiloton of TNT releases (AFAIK) approximately 4.184 x 1012 joules of energy. Furthermore, calculations performed to study asteroid impact indicate that an object travelling at approximately 3 km/sec will strike with an equivalent energy of its weight in TNT. Ergo, a 7-ton elephant moving at 50 km/sec would strike with the energy of approximately (7 ton) * (50km/sec / 3km/sec) tons of TNT. Worked out, I get something around 16.66*7 tons, or around 116 tons...leading me to state that said elephant would hit with a force equivalent to just about one-one-hundredth the energy release of the Hiroshima explosion (~11-12kt).

Nasty if you're under it, but not really an extinction-level impact. Plus, think of all the crispy heffalump you'd have scattered about. Now, for a tactical, conventional use this might have merit; said explosion would be perfect for whacking a bunker, or tank group, or large structure (dams, bridges, airfields, etc.) without concomitant release of radiation and thus fallout! This is at least a good idea as those morons in Washington, D.C. who seem to think you can make a 'small and safe nuclear bunker buster.' Unfortunately, the elephant is an endangered species, no? Thus, any attempts to weaponize pachyderms would probably result (at the very least) in one being branded an eco-terrorist.

For that matter, so would dropping them around the landscape. Ever smelled an elephant up close? Wheeeeee-yooo!

Update: Anark says Look at it this way: if your elephant's energy equals 7 tons of TNT when it's traveling at v = 3 km/s, then at 50 km/s = 16.7v its energy will be 7*(16.7)^2 = about 2 kilotons.

Anark is absolutely right and caught me with my physics pants down 'round me ankles whilst bending over for a fresh beer. Okay, so I had confused momentum and kinetic energy. The former is a vector; the latter a scalar, and the formula for KE is:

KE = 1/2 * mv2

...so, okay, as noted above, a 50 km/sec incoming ballistic heffalump contains within its fragrant hide KE equivalent to around 2 kilotons of TNT. That's still not enough to be a global problem; given that it doesn't release any ionizing radiation on impact unless you've force-fed the elephant in question some form of radioactive material in advance of the shot. This, however, would likely violate several strategic trunks reduction treaties as well as cause all your Mahouts to lose their hair and grow extra toes.

Heh. Never get a semi-pro nuclear targeter started on impact energies.