A "splat" is an
ordinary lead slug with a
special package on the inside. In each
bullet is a
glass capsule containing
ground glass and
picric acid. When the bullet is fired, the capsule
shatters, allowing the
acid to mix with the surrounding
lead. This results in
lead picric, which is highly
explosive.
Thus, when you shoot someone, they go "splat."
Kudos to Jon Land