The idea that one could call out some hard and fast "Rule Of Thumb" about ricochets is DUMB.
Every shot that's ever been fired on this planet, and every shot that ever will,.....is by nature it's own UNIQUE balistic event,....regardless of how infintesimal the differences.
Bullets do really weird things,....that's my rule of thumb.
Some time working in Long Range Match Pits give one a feel for that fact. Watch a .30 cal "Berm Strike" go howling off into the impact area on a wet day,.......for starters.
GTC
