From looking at several thousand targets shot by M16s, the vast majority of holes in them are round. A tumbling round leaves a larger oval hole. It's really obvious when an M16 round has hit something to cause it to tumble before striking a target.

Once the round strikes something it may tend to tumble, as do most hot small caliber rifle rounds, but not always.

Personally, I own a .223 and a .22. I use the .22 for plinking, general tuning up my marksmanship, and general usage, as it's freaking cheap to shoot a .22.