200 METER TEST

  • 455 Views
  • Last Post 20 February 2019
joeb33050 posted this 17 February 2019

 

While trying to find out if jacketed bullets shoot more accurately/easier than cast bullets, at cast bullet velocities, I reduced powder charges and velocities until either the bullets shot wildly or a bullet stuck in the barrel.

 

Sometimes tipping bullets are accurate, sometimes bullets shoot wildly without tipping, and sometimes bullets shoot sideways but form reasonable groups.

 

All records are about 5-shot, 100 yard groups.

 

Here’s an example:

 

 

It is clear that something happens as the powder charge is reduced below 6 grains of Titegroup, MV falls below 1628 fps. The wild shooting is below 4.5 grains of Titegroup/1188 fps MV.I can think of three possible causes for group sizes getting larger:

 

The bullet becomes unstable, or

 

Very low charges ignite/burn erratically, causing large velocity swings, or

 

Velocity approaches/crosses the speed of sound, causing ???

 

The summary of testing to date:

 

Then, at 200 meters:

 

MV is measured, 100 yard v and 200 meter v are calculated, using the bullet manufacturers BC values. These bullet started going wild at 200 meter v < 1261 fps and 6 grains of Titegroup, but were not shooting wildly as 100 yards.

 

This suggests that the wild shooting is NOT caused by “very low charges igniting/burning erratically causing large velocity swings”.

 

Either bullet instability, or bullets slowing to the speed of sound cause wild inaccuracy.

 

This testing also suggests that bullet performance at any range is modeled by shooting at short range (100 yards) at the long range velocity. Group size at 1000 yards varies as group size at 100 yards at 1000 yard velocities varies.

 

Attached Files

  • Liked by
  • James Nicholson
  • M3 Mitch
Order By: Standard | Newest | Votes
frnkeore posted this 18 February 2019

Jeff, This is a quote from a LR shooter/experimenter:

“Going transonic” is generally not a good thing for bullets. The bullet can lose stability as it enters the transonic zone. It can also become less slippery, losing BC as a consequence of dynamic instability. In this video, Bryan Litz of Applied Ballistics analyzes what happens to bullet stability (and BC) as projectiles approach the speed of sound. Transonic effects come into play starting about Mach 1.2, as the bullet drops below 1340 fps.

Frank

Attached Files

  • Liked by
  • RicinYakima
  • M3 Mitch
frnkeore posted this 20 February 2019

Let me try to help the understanding of the difference between subsonic and supersonic and what happens, in between.

Subsonic:

The air flow is stable and laminar (meaning it's basically attached follows the shape of the object). In this range, a radius leading edge helps keep it that way.

Supersonic:

Is completely different, it's like throwing a switch. Your up against a malleable wall, in that the wall will bend and the best way to make it bend, is with a very sharp point or edge, as in a airfoil. That "wall" takes a large amount of force to push it, impeding the forward moment. The sharper the bend, the better the flow.

I'm not talking about drag, you also have that, it's in addition to drag and the shock wave  takes more of the bullets energy than drag does.

That is why a subsonic bullet loses much less velocity than a supersonic bullet, all other factors being the same.

Transonic:

Is kinda like having chucks of that wall, thrown at the bullet, until it fully forms. Any portion of the bullet, that not straight, i. e. the ogive, no matter it's shape, will become supersonic, before the rest of the bullet, causing uneven forces in the front of the bullet, compared to the rear. So, you have uneven forces on the bullet and uneven forces while the sound barrier is building.

In that case, your trying to overcome the increasing resistance (shock wave) and the bullet is being pushed around (yaw), decreasing the bullets BC at the same time. If the bullet is pushed around to much, it looses it gyroscopic stability.and tumbles. If not, it can regain that stability at subsonic speed and continue on, point on.

If your wondering why the point become SS first, it's because the air has to travel farther in that area to catch up with the straight flowing air, increasing it's speed at the nose and causing those shock waves to form first, in that area.

Frank

Attached Files

  • Liked by
  • M3 Mitch
  • RicinYakima
RicinYakima posted this 17 February 2019

"Either bullet instability, or bullets slowing to the speed of sound cause wild inaccuracy."

Bullet makers BC number is usually the best number at a speed they determine. BC decreases as velocity drops.

Attached Files

  • Liked by
  • Bud Hyett
joeb33050 posted this 20 February 2019

There are 10 barrel/bullet combinations in the JB Test that began to shoot wildly as MV was reduced. 9 of the 10 went wild somewhere around the speed of sound, the 10th at <2340 fps. Forget that one.

Either wild shooting was caused by bullet instability, or the trans sonic thing. I can't come up with another cause.

So, what happens to bullets at and below the speed of sound, what do they do, how do they shoot. I find it hard to imagine that my wild shooters will get back on track < SOS. That's what I don't understand.

Attached Files

  • Liked by
  • M3 Mitch
frnkeore posted this 17 February 2019

If the 200M velocity's are close to accurate, I think you have found what shooting in the transonic range does to jacketed bullet accuracy.

To get on paper for your 4 - 4.5 load, you'll either have to get a angled scope base or post a target above your target fame, for a aiming point.

Frank

Attached Files

JeffinNZ posted this 18 February 2019

If the 200M velocity's are close to accurate, I think you have found what shooting in the transonic range does to jacketed bullet accuracy.

Frank

 

Usually I'd agree but Joe mentions the 6gr load was stable out to 100 but tipping by 200 with a velocity of over 1200fps so well above the transonic threshold. 

Jeff

Cheers from New Zealand

Attached Files

RicinYakima posted this 18 February 2019

And then BC increases again as it drops below 1040 f/s. Physics is fun!

Attached Files

joeb33050 posted this 18 February 2019

And then BC increases again as it drops below 1040 f/s. Physics is fun!

 

The trans sonic story has never been explained so that I could understand it. I've shot enough long range to know that bullets that become sub sonic an the way to the range still hit the target reasonably accurately. Shoot hi vel 22rf at 200 yards, the become subsonic on the way, with reasonable accuracy at 200. I suspect we get wound up in graphs of BC vs. VELOCITY, with that left end big squiggle. 

Attached Files

Ken Campbell Iowa posted this 19 February 2019

there have been repeated attempts to test long range effects by testing at shorter ranges by reducing muzzle velocity such as to give equivalent velocity at the shorter range.

always also, someone brings up the thought that this won't work because the spin rate is also reduced to less than it would be from a higher MV ..... thus the lower MV bullet will be closer to unstable spin at any testing distance .  or more exactly, time ...   than the higher MV bullet ...

speaking of time, it is interesting that a 1300 fps bullet is stable for 1/4 of a second, but in the next 1/3 second takes off for the nearest bar ... aggressive precession, i suppose ...

***************

more testing would be good ... i doubt that bullets care much about our guessing and opinions ..

ken

 

oh, thanks, joeb ...

Attached Files

JeffinNZ posted this 20 February 2019

Jeff, This is a quote from a LR shooter/experimenter:

“Going transonic” is generally not a good thing for bullets. The bullet can lose stability as it enters the transonic zone. It can also become less slippery, losing BC as a consequence of dynamic instability. In this video, Bryan Litz of Applied Ballistics analyzes what happens to bullet stability (and BC) as projectiles approach the speed of sound. Transonic effects come into play starting about Mach 1.2, as the bullet drops below 1340 fps.

Frank

 

Well that I did not know Frank.  Thanks.  I was always under the impression that 'transonic' was very close to the sound barrier.

Cheers from New Zealand

Attached Files

frnkeore posted this 20 February 2019

If the bullets still have enough gyroscopic stability, after getting back through the turbulence, they will recover. How long it will take to recover, I can't say but, they should recover, faster than they destabilized. The forces become less below the speed of sound and more above it.

LR, BP shooters prove that. Many of them start off in the transonic zone or just above it. Those bullets, probably recover before 300 yards, depending on their BC and rate of twist. For BP shooters, it may be the reason that in later years, they have gone to faster twist rates.

If you'll note the graphs I've posted about rate of twist, that program try's to calculate the amount of "extra" twist it takes to remain stable in that transition area. Gyro's, if disturbed stabilize themselves if, the spin rate remains high enough.

One other point. Bullets are a cylindrical flywheel. The larger that flywheel, the less RPM it looses. That's why 22's take such fast twist rates and 45's don't need much. I believe the math works on at least the square of the diameter.

Frank

Attached Files

Close