Applied Ballistics 'Shoot Thru Target' Challenge

[Canadian Bushman]

Canadian,
The scope I used in the 375 CheyTac test posted on page 3 was also a NightForce 56mm objective, but it was a 5-25 ATACR. I wonder if it's related to the large objective and/or 30mm tube...?

I know i shot consistently smaller 100 yd groups when i used the nightforce benchrest scope. I felt that scope was much easier on my eyes, and also had a finer focus adjustment. However it was also a 30mm tube and 56mm objective.

 

[KYpatriot]

KYpatriot,

On page 1 I posted results of a 338/300 grain solid bullet in a 10 twist. It was a marginal stability example, nothing anyone should be using for real but I was trying to magnify the effect to make it visible.

Then on page 3 I posted some targets shot with the 375 CheyTac and 352 grain solids.

I've also tried standard loads with 300 grain Bergers; no dice.

Thanks Brian. I was hoping that some shooters who consistently see this phenomenon would replicate your test, and thanks again to Canadian Bushman for doing so. To me the whole thing is like too much emphasis on the validity of a 3 shot group. Sometimes what we think we know....isn't. The results so far confirm my understanding of aerodynamics as I was taught them, so none of this is a surprise but I am totally open to a result otherwise, as long as it is a good data driven approach like this test. This is fun...

Regardless, I'm still interested in what causes shooters to get non linear groups at range, even if multiple iterations of this test stack up evidence that it isn't the bullet or external ballistics. That leaves the scope or the shooter.

Personally, I think Newton still implies that once a bullet clears the muzzle enough to escape muzzle blast that it will forever continue with whatever vector it has away from the point of aim and therefore dispersion will be proportional to time of flight UNLESS an external force alters the path. We all know aerodynamic jump, Coriolis, spin drift all alter the flight path of a bullet, but are usually dwarfed by the wind effect even on low wind long shots. It is possible for a one moa grouping of shots to be "corrected" to half moa at 1000 if it just so happens that the bullet's random vector away from the point of aim, causing the 1moa dispersion at 100 yards, were opposite of one of these forces like the wind which then has time to "steer" them back in at long range. Clearly this would be a rare event, and a random one that isn't repeatable on demand, but could account for some instances of non linear dispersion. It's common sense that some of these forces can cancel, like using a left twist barrel to counteract coriolis. But these known forces are constant while dispersion in a group is mostly random, so even "accidental" steering of bullets back towards the aim point will never happen consistently and cannot account for rifles in which this problem is persistent.


I like threads like this...let's me pretend to get a little something out of that aero engineering degree I worked so hard for and never really use, lol.

I still have a natural curiosity in why these things happen, and if the answers make me a better shooter that is just icing on the cake as I'll take all the help I can get in that department. Thanks Bryan for your efforts in that. I enjoy reading your works and those before you like Mr Harold Vaughn.

In fact I volunteer some teaching time at a local homeschool cooperative and next semester I am teaching a science research/technical writing course for high school level kids next year. I was thinking of buying a pressure trace system and trying to get data to answer some kind of question related to chamber pressure, doing some statical analysis on it, and writing a technical paper as a class on our methods and results. So if anyone has any ideas on what to test or investigate I'm all ears...it has been quite a while since I have done this sort of thing as a student myself. Please send me a pm if y'all have a pressure related question we could try to answer for this project!

Hope everyone keeps testing their non linear dispersion on this thread...it could very well get figured out right here.

 

[zfastmalibu]

Bryan, I feel like you did not read the link I posted. It has nothing to do with bullet yaw. It is strictly vertical dispersion. Its called positive compensation and is well known in the rimfire br circles. It is why very stiff rifles can be hard to tune, have narrow load windows and rely on es strictly.

As far as what Jack is talking about, it does not just occur at low velocity. It can happen by just changing seating depth. A well tuned load will shoot in the wind better.

1k br shooters shoot a lot of rounds at 1k. Its what we focus on and we see all these little things time and time again. Not everything can be proven and you must go with the data you see over and over.

 

[HARPERC]

Very educational, thanks to all participating.

 

[KYpatriot]

Zfast I read the link and find it interesting. The findings would imply that when I screw on my 17 oz suppressor tightly to a 26in barrel that my group size would change. It does not. The poi changes slightly (1/2 moa) but the dispersion does not, which should not be possible if velocity variation is present, which of course it is. I can't reconcile that fact with their theory. It could be mere chance except many suppressor owners don't see appreciable changes in group size with suppressors mounted. I have used ocw techniques with success and that seems similar to the theory in the link but it has worked for me at all ranges, while the weighted barrel technique if it is real would only work at one range.

In the end, I have never had a rifle group better at range than close consistently, and have never had a load with good ES that grouped well at 100 but poorly at range. In other words, for me any load that shot well at 100 shot well at range too, commensurate with the ES. I haven't shot anything in a long range rifle larger than 250 grains so as has been pointed out by some here perhaps that is why I haven't seen what some are seeing.

 

[dkhunt14]

I had a Dasher that shot 2 and 3 ES on 10 shot strings but wouldn't ever shoot under 15 inches at 1000 and it was vertical. The same gun with a different powder charge shot around 7 to 8 ES and would shoot 4 inches in good conditions at 1000. I have seen a few other guns do it also. I think it happens in big long heavy for caliber bullets more often. I have seen the same things Zfast is seeing or Tom. I don't know the answer but have seen it happen. I also believe the powder charge like on a ladder just shows the nodes and to get real small you need to move the bullet. I know if you don't get your gun shooting 10 shot groups in 1000 Yard BR in the 4 inch range in good conditions you might as well stay home. Matt

 

[KYpatriot]

Matt there is another theory I'm sure you have heard of called OBT ( optimal Barrel Timing). It is along the same lines as the link zfast posted with respect to timing the bullet exit, but they suppose a mechanism much different in that instead of the vibration changing the direction of the muzzle they suggest there is an annular shock wave that bounces back and forth between chamber and muzzle that changes the bore diameter slightly, and that for best accuracy you time the bullet exit so that this disturbance is far from the muzzle. It seems plausible as well. I wonder if there isn't several of these things happening at once.

http://www.the-long-family.com/OBT_paper.htm

At any rate, to stay on track, I'm not sure how barrel vibration has anything to do with non linear dispersion, if such dispersion exists.

 

[BryanLitz]

zfast,
I addressed the article and positive compensation in my post #49 on page 7. As for your statement:

A well tuned load will shoot in the wind better.

Since it is off the topic of this discussion, I'll simply say that I respectfully disagree with that statement. If you'd like to start another thread on this topic, I'll be happy to join it.

1k br shooters shoot a lot of rounds at 1k. Its what we focus on and we see all these little things time and time again. Not everything can be proven and you must go with the data you see over and over.

I respect your experience in the 1k yard BR shooting discipline. And if you're happy with your understanding of converging groups, then that's great. However, there are some others including myself who are more interested in the question of why, and we don't find the current explanations to be completely satisfactory. It's OK if our discussion doesn't interest you, and you don't have to agree. No harm no foul, and I look forward to discussing other topics with you because you're clearly someone who's got a lot of knowledge and experience.

Matt,
Did you mean to say SD, in relation to your chrono numbers?

I had a Dasher that shot 2 and 3 ES on 10 shot strings

I've worked with a lot of chronographs, and can tell you that it's very rare that a chronograph is capable of resolving velocity measurements within 2 or 3 fps. 2 or 3 fps SD (which would be more like 8 or 12 fps ES) is more possible, and actually within the ability of some of the better chronographs to measure.

As to your observation of tuning loads for 1000 yards, I understand what you're saying. I understand positive compensation, and accept it as an explanation for how you can tune a load for 1000 yard groups. However, that mechanism doesn't explain how your 0.4 MOA 1k yard BR rifle would shoot 1" at 100 yards with loads having SD under 10 fps. Likewise, tuning loads the way you describe can only act to diminish vertical spread, not horizontal. The effect we're searching for is one that would cause a rifle to shoot 1 MOA on a 100 yard target, and 1/2 MOA on a 200 yard target. It's not that we don't accept your accounts, but they just don't answer the question we have.
I have the same message for you that I had for zFast; if you're happy with your understanding of how this all works, then that's good. I thank you for your contribution and respectfully ask that you allow those of us who wish to pursuit the matter more deeply to do so.

Getting back to the topic, tomorrow I plan to do more shooting with the .375 CheyTac on the shoot thru target to try and repeat my results from page 3. Canadian saw the same thing (when aiming at a more distant aim point, the rifle grouped better at 100 yards than it does when aiming at 100 yards). If this can be repeated, then I'll start exploring different sized aim points, and if the effect can be observed using the same scope on a less punishing rifle (thinking maybe the high recoil may be causing some scope related optical effect).

Looking forward to any other live fire shoot thru testing.

And the offer still stands for anyone wishing to travel to the AB Lab in Michigan.

Take care,
-Bryan

 

[bruce_ventura]

Bryan,

Very interesting thread. I'm trying to imagine the conditions under which the diameter of the corkscrew trajectory would be large at short range, and yet the overall dispersion would be low at long range. Ballistics is not my main thing, so I have to go back to the textbooks.

The physics seems to dictate a large diameter, light weight bullet, large dynamic pressure and a high lift coefficient. The coning angle should be large at the muzzle and decay quickly, and the slow precession frequency should be low, both of which imply a moderately high gyroscopic stability.

I've observed corkscrew trajectory diameters of a few bullet calibers for .50 cal brass VLD bullets at 100 yds. The dispersion at longer range for these loads and this gun was not measured at that time. So, maybe you should look at monolithic brass, VLD bullets in .416-.50 caliber. GS factor of about 1.5-2.

On the other hand, Vaughn's book says the corkscrew diameter can get really large at excessively low gyroscopic stability (GS<1.1). That doesn't seem like a condition that most shooters will encounter often.

Another issue may be location of your lab. If the nonlinear dispersion effect exists at all, you may not be able to see it in Michigan where the air is denser than at high altitude. Maybe you should wait and do the ballistics experiments in the summer when the temperature is higher.

 

[cowboy]

I have personally never shot a see through - short and long range target. One heck of an idea by the way. But.... I have shot better MOA at distance than I have at 100 yds on the same day, same rifle, same load, near identical conditions more than once over the years. Being an engineer this drove me nuts for a long time. Started playing with different type/sizes/colors/shapes of targets along with paying more attention to size of cross hairs on various scopes along with lots of playing with parralax.

Bottom line is I now believe it is an optical/nut behind the bolt situation and not a bullet/load getting more accurate with distance. Aim small and I'll be surprised an eat crow if it's bullet/barrel related.

Keep in mind my comments come from many rifles, loads, scopes etc. with the largest bullets being the 230 gr. so if there is something going on with the really big stuff it will defy everything I found in the smaller stuff.

BRYAN: I thank you for going through this process and think this is one of the most interesting posts I have ever come across on a shooting forum since the inception of the Internet.

Carry on - I'm all ears.

 

[zfastmalibu]

No problem Bryan. If you dont find what you need try asking for some targets at accurateshooter.com Lots of long range shooters there. Good luck
Alex

 

[DocUSMCRetired]

This could be done twice. The second time with a scaled down target to simulate 300 yards. I am going to try this with a .22LR, using a target at 25 and 100 yards, the 25 yard target scaled to 300.

I think this is shooter related, so I am going to see what happens when you are given a small target at a reflexive shooting distance.

 

[Canadian Bushman]

This could be done twice. The second time with a scaled down target to simulate 300 yards. I am going to try this with a .22LR, using a target at 25 and 100 yards, the 25 yard target scaled to 300.

I think this is shooter related, so I am going to see what happens when you are given a small target at a reflexive shooting distance.

I think it may have more to do with the large optics at close range. I typically use 1/4" dots to check zero at 100 and the results are the same.

 

[DocUSMCRetired]

I think it may have more to do with the large optics at close range. I typically use 1/4" dots to check zero at 100 and the results are the same.

Thats why I am going to use a larger optic and run at 25 and 100 yards.