If you have ever been caught in an automatic weapons crossfire at night you will have seen tracer bullets collide and ricochet off at crazy angles. Those are full metal jacket bullets.
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Bullets can be made from many materials including wood, steel brass, copper, lead, tungsten, bronze, plastic, etc.
It depends on several factors, not just the size. Velocity and muzzle energy are also factors.
A 7.62mm is the same as a .300