I think there is a big difference in tumbling before impact and tumbling after impact. Certainly we do not want a bullet tumbling before impact. I was watching a utube yesterday that I should search for this morning. A fellow sent a box of 9mm to utuber for testing. They were very pointed full metal designed to feed perfectly, shoot straight, but tumble on impact. In ballistics gel they traveled straight for about six inches then tumbled. The wound channel when tumbling was comparable to a good hollow point. I remember as a child the old WWII guys talking about one of our war opponents that used a bullet that was long tumbled. They said fellow soldiers got messed up badly when hit by tumbling bullets. I prefer mine fly straight to a small desired point of impact, but after that I want it to kill any way it can. When deep penetration is desired on dangerouse game I think tumbling would be a very bad thing.
Edit: found it.
https://youtu.be/GlHkjHX360s
Found this one too.
https://youtu.be/eSAasX-_l6c
I like accuracy so will not choose a rifle bullet that may have any in flight instability. Back to the subject of brush guns I think any confusion comes from the differentiation between brush gun and brush bullet. I have a Ruger Predator with an 18 inch barrel in 308 Win. It's a nice brush gun, but a 6.5 would be a better brush bullet.
Have you ever experimented with a gyroscope? The faster it spins the harder it is to turn 90 degrees in your hand. Much like a spinning top. A slow spinning top if touched tumbles around on the table and stops. A fast spinning top if touched with your finger simply moves aside slightly. Fast spinning bullets have this same gyroscope stability when flight is disrupted.