I've only had bullets keyhole once on me at 50 yards. I was loading for a .204 Ruger in a 14" barreled Contender pistol. I was trying some 40 grain V-Max and at 50 yards 3 out of 5 bullets keyholed. Dropped down to 32 grain problem solved never had a keyhole again.
I'm not saying that "keyholing" can't happen.....it does. However, it is usually due to the base of the bullet being deformed before, or as, it leaves the barrel.
I had borrwered a .270 about 2 years ago and had a helluva time zeroing because the bullets were keyholing. Aftere consiberable expense and aggravation I discovered that ballistic tips on the bullets were too long for the chamber and when I switched to a win. failsafe HP (no tips) the rifle shot like it should.
Tumbling is not always caused by deformed bases but can be evident when there is any form of instability.
