J E Custom
Well-Known Member
I saw this video and had to share it with the membership. It has some interesting video of sonic and sub sonic bullets, Enjoy.
J E CUSTOM
J E CUSTOM
Bullet Shock wave
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NEMTHunter
Well-Known Member
Very neat!! I liked it. Thanks!
J E Custom
Well-Known Member
Yes he does. It realy helps to understand how a muzzle brake works.
I wish I had his equipment but if I did I probably wouldn't get much else done.
We used slow motion cameras to develop our muzzle brakes and understand many of the effects of changes, It is a wonderful tool if used correctly.
One thing I did notice was the sub sonic bullets appearance to yaw.Something was not right (Maybe the twist rate must be different for sub sonic bullets). I don't have a sub sonic rifle cartridge so maybe someone can enlighten me on twist rate requirements for them.
Glad you enjoyed the video.
J E CUSTOM
NEMTHunter
Well-Known Member
That is one thin I noticed to. It seemed to tip forward or down maybe is a better term.
My understanding is the slower you shoot the bullet the more twist you need to stabilize it. I think in subsonic rounds you need a faster twist then normal. Or that I feel it needs to be to get stability.
My understanding is the slower you shoot the bullet the more twist you need to stabilize it. I think in subsonic rounds you need a faster twist then normal. Or that I feel it needs to be to get stability.
For a given bullet, a certain amount of gyroscopic stability is required to stabilize its flight. When the bullet emerges from the barrel slowly, at a given twist rate, the spin is also slowed. For example, a 1:12 twist spins 1,000 r/s for every 1,000 f/s of velocity. If the bullet requires 1,500 r/s to stabilize at 1,000 f/s velocity, then it will be unstable out of a 1:12 twist; the twist would need to go to 1:8 (2/3rds of a foot) to accomplish that.
The force (spin angular momentum) required to resist destabilizing torques from aerodynamic forces and mass asymmetries decreases as the aerodynamic forces decrease at lower velocities, but in the transonic to subsonic transition realm, there's a big spike in these forces that causes the bullet to momentarily destabilize and yaw, then restabilize if the remaining spin is sufficient.
The situation is different down range for a bullet that has dropped to 1,000 f/s from an initial muzzle velocity of 2,000 f/s, because spin decays much more slowly than linear velocity. Initially at 2,000 r/s out of a 1:12 twist, the spin might have decayed to only 1,800 r/s downrange - more than enough to stabilize the bullet at that lower 1,000 f/s velocity. Subsonic it may be, but the destabilizing forces are still there.
I noticed the yaw as well - in the higher-speed pass, in fact. There's nothing like an empirical visualization to make what is happening clear.
eklarsen
Well-Known Member
Does that mean that even subsonic if you are close to 1100 FPS you will still get some sonic "boom" and you need to slow it down more to avoid the crack? In reality I don't notice that supersonic crack when I am below 1100 but his professor friend says it is so...
