Is this normal? bullet tumble

[Mr Spooky]

I was watching this ak74 vid and it looks like the bullet is wobbleing in flight. Is this normal? Does a bullet really wobble that much?


https://www.youtube.com/watch?v=xCgmfbAbP-Q

[Jamnanc]

It's cause it's not an ar10. That is a sweet video.

[Circuits]

Sure does appear to be some wiggle in those high speed shots, though not outright tumble. My own 5.45x39 firearms, not so very unlike those shown in the video, have been much more accurate than any tumbling bullet could achieve.

Wiggle is clearly not the same as tumble, so it may well be the wiggling is normal even in my rifles, and I just haven't seen it for not having shot high speed slomo video of them, but their accuracy tells me that if wiggle is normal, it is not terribly detrimental.

Unless you see keyholing in your targets, your bullets aren't actually tumbling.

[Rhino0427]

The video is very cool. The bullet wobble is amazing to me...and then again, its not. I've always considered the AK to be an area weapon past about 200m anyway and the slop built into the system and the inherent instability of the bullet would better explain why. Still one of the greatest infantry weapons ever. Perfect for a conscript army that needs a tough weapon they can be "trained" on in 5 minutes.

[68Charger]

I'm going to hazard a guess that the bullet wobble is caused by an uneven crown on the barrel.
As the bullet exits the barrel, the gases escape more on one side than another, causing a wobble in the bullet.

But it's just a guess.

[colorider]

Around 1:35 it looks like the barrell flexes a lot. Like a noodle.

[jerrymrc]

Around 1:35 it looks like the barrell flexes a lot. Like a noodle.

They all do. Some more than others. Lots goes into the harmonics but it does happen.[Beer]

[68Charger]

Around 1:35 it looks like the barrell flexes a lot. Like a noodle.

That's very normal with piston driven semi-autos... Seen it before in AK videos, but the same physics apply to piston-driven ARs- It has to do with the mass being pushed backwards by the gas pressure, related to the location on the barrel.
Watch the flex in the barrel at the beginning of this video:

https://www.youtube.com/watch?v=8j_jMVynheY


On gas-impingement designs, the pressure above the barrel line is pushed back inside the upper, there is very little surface area at the gas block. But when the action is piston driven, the pressure is pushed on the barrel out at the gas block, with a surface area equal to the piston. (for every action, there is an equal but opposite reaction). There is still flexing that occurs with gas-impingement systems, it's just not as pronounced.