Re: bullet tumbling???????????????
The main reason is a bullet flaw. Common flaws include cavity in the core, a core that's put in crooked or a very thin part of the jacket. Any one of these will unbalance the bullet quite a bit. When it leaves the muzzle, it immediately tilts sideways and will yaw or cone many degrees. They'll strike the target at most any angle up to 90 degrees depending on how much they've tumbled.
Providing the bullet's don't bounce off something down range, the only other reason they'll tumble and keyhole is when their velocity drops down through the sound barrier and they were only marginally stabilized by the rifling twist when fired. This problem happened with Lake City Arsenal's M852 7.62mm NATO match ammo with a Sierra 168-gr. HPMK bullet fired in a 22-inch, 1:12 twist M14 barrels. Muzzle velocity was barely over 2500 fps and they would go subsonic at about 800 to 900 yards. It was common to see the keyholes they left in 1000-yard targets at altitudes below 2000 feet in cooler temperatures. And accuracy was pretty bad, too. M118 match ammo using 173-gr. FMJBT bullets shot at the same speed remained supersonic through 1000 yards with good accuracy.