more questions

[dg]

ever noticed that regardless of the range, caliber etc (but assuming adequate twist rates and stabilisation) that the meplat pin holes in targets appear to have random offset orientation from the centre of the bullet hole ?

from my limited observations, it does not seem to matter whether the shot locations are close together ( more than satisfactory accuracy) or scattered ( shotgun groups) when these examinations are made, the pin holes are rarely centred.

at long range, due to the dropping flight path of the projectiles, it would be safe to assume that if anything the pin holes would be below centre but this does not seem to be the case, their location continues to appear to be random.

note that,as best that can be measured, if an adequate twist is used, regardless of pin hole location, the bullet holes on the target seem to be round.

most would have noticed that if the twist is marginal or insufficient or if the projectile has approached or gone subsonic, that target holes are obviously elongated or egg shaped.

could it be, that even our most accurate loads are oscillating during flight and if so, what is causing it and would we obtain finer accuracy if this wobble was reduced or eliminated ?

cheers
dave g

[Tony Z]

Dave the theory says that gyroscopic effect has a bullet fly with the bullet spinning about its axis at the angle of launch from zero to impact. Wind has the nose lead into it very slightly. The angle of attack of the bullet passing through the paper at 1K has it belly down, point up. So the pin hole should be high center of the hole. Wind from the left would have the pin hole high left and the hole being oblong because the angle of attack is now belly down and across the target to the right.
If it is random, i suspect other things are at play. It would support the argument by Henry Childs and Charles Ellertson where they claim VLD bullets never "go to sleep" and wobble down range from muzzle to impact. It is this exact scenario that led to the development of the 187 BIB. Joel Pendegraft using the BIB went on to silence the naysayers and claim a few world records along the way with his stretcher tubed heavy gun. One of which has only just recently been bettered.
I believe that bullet design is still evolving where BCs sell bullets and small groups prove performance. The two rarely combine. The super slick monocore bullets are never seen anywhere outside of forum posts and some of the groups i have seen are atrocious. I firmly believe rebated boat tail bullets are in need of a closer look. They share the BC of the VLD, and the launch characteristic of a flat base. The problem is from a manufacturing viewpoint, there are more dies, more stages and more cost.

[dg]

g'day tony

thanks for the detailed reply.

I agree that advertised BC has a great influence on sales and popularity, but as you said, consistent small groups on paper is what really matters.

in relation to the bib's , are you able to comment on the pin hole locations ?

just by coincidence , yesterday I was out at the range sorting targets (600 and 1000 from the SCC) in preparation for our 600BR next sunday.

probably motivated by recent discussions, before patching, I had a close look at the bullet holes on a few targets.

on both the 600 and 1000 targets, regardless of caliber, the pin holes were rarely centred and are noticeably offset.(never thought to take any photos).

but shown below is an old target from a fly match about 20 years ago shot with a 30 cal. (sorry for the quality, just took it in artificial light)

but hopefully the off centre pin holes are noticeable.

not that any conclusions or meaning can be drawn from one example, but just by coincidence, for the two middle touching holes, the pin holes are near central while as can be seen the upper and lower shots have off centre pinholes ??????






it would be interesting to see if others are able to comment or add photos of their target holes.


only my opinion, but I reckon if projectiles are wobbling in flight and are inconsistent in their wobbles (as maybe evidenced by the pin hole randomness) then this situation would effect drag, trajectory, time of flight, wind drift and achievable accuracy.

but then, I have been wrong on many occasions in what I think will or should happen and what really happens in LRBR.

cheers
dave

[B4408]

Has anyone done high speed camera pics of projectiles at the target, especially at long range? May show something.

Bruce

[macca]

Sometimes I think ignorance is bliss.

[MISSED]

I always thought of projectiles as spinning tops
The faster the spin the less the wobble but as it slows the tip start to wonder but not the body
Until spin slows to much which lets gravity take over.

[Rabbitz]

Hi All,

The test engineering in my background prompts me to ask if we are not looking at these imprints and adding 2+2 and getting 6.02x10²³? In other words, could there be an alternate cause?

Could the offsets be due to the flex/reaction of the target material on impact rather than the entry angle of the projectile?

If it were the target, then centred entry holes on touching shot could possibly explained by a change in the tension as the paper tears.

I am not trying to be a smart-arse, I am trying offer some lateral thinking to the problem.

I am happy for this theory to be shot down (as it were) to eliminate it as a possibility.

[Tony Z]

Let's remove pitch and yaw and precession from border line stabilization from the equation. That leaves only the angle the bullet settles on as a possible and probable cause. Where and how this disruption comes from is the reason flat base bullets like the BIB made their way into long range BR. There are theories galore about this stuff but pictures say a trillion words.

https://youtu.be/ZDUL_1vrABI

[dg]

hi rabbitz

similar to your thoughts, I also wondered about whether at the point of impact onto the target paper sitting on or backed by coreflute, if this sudden shock could deflect the axis of projectiles ?

so last night I loaded the most un VLD 6mm projectiles I have (Hornady 105 HPBT - very blunt little devils, but from previous use I know they will get to 600 or 1000 without drama).

I assume that if any 6mm projectile was going to be stabilized in a 1in 8 barrel and get to 600 when doing approx. 2900fps , the Hornady should do the job.

today, I had a quick trip to the range and wacked 1 sighter at the hill and then 9 at the 600 target.

BUT as can be seen the target was positioned on a cardboard box, so only the paper would be penetrated by the projectile.



somewhat surprisingly, given the circumstances and rush, the dumpy hornadies did not go as bad as expected.



this the shot out the left past the 5 ring



those in the centre



close up of those in the ten ring



as luck would have it, those that hit the patches don't show the pin holes that well, but you make up your minds on the offset of the pin holes




as tony mentioned photos, are worth quite a few words.

these photos tend to confirm that upon hitting only paper, the majority of the pinholes continue to be randomly offset.

would passing through tissue paper produce the same or different results ? I have no idea, and if anyone wants to conduct their own similar test, go for it, and then report and display your findings.

for whatever reason, the fact is that for either paper backed by coreflute or unsupported paper, the pinholes are off centre and random in their location.

given the time of flight for 600, it should not be unreasonable to expect projectiles to achieve consistent stability.

maybe the axis of the projectiles are being deflected at the point of impact ??? if not could the off set pinholes be a sign of partial instability ??, if none of the above, what other forces could be contributing to this situation ?????

this is the computer simulation for pitch/yaw to 200 yards by litz.

https://www.youtube.com/watch?v=KH9SCbCBHaY

only wonder what it would look like for 600 or 1000yds ?

cheers
dave

[Tony Z]

Dave a while before i gave it away i built and trialled this devise because the issue of off center pin holes was really bugging me. I built it and fitted it to the old 30 cal HG. Shot it once and had very good results then sold the gun and use this as a paper weight now.

It has a parabolic cone to split the blast from the muzzle as the bullet slips though the bore which has around ten thou clearance on a 30 cal bullet. The cone sat clear of the muzzle by around a 100 thou and this in itself was to be a subject of trial and error to find working parameters. The cone diverts the blast through the vent holes and outward at 5 degrees and away from the bullet while it looks to depart its second muzzle. The vent holes are also angularly set at five degrees to minimize torque. The only gas, in theory, that is near the bullet is the gas trailing it. Vaughn talked about this theory of a bullet in free flight being accelerated without bore friction and also being free of disruption. He did it by boring the barrel and venting the gas sideways. That constitutes a brake where at the time of this experiment brakes were not legal for heavy guns in IBS rules.
Mine is not a brake but allows the bullet to be free of the gas envelope. It worked with 200 SMKs very well. Unfortunately i never got around to trialling it with a true slippery VLD. I know the concept works but unfortunately i no longer have the targets to show the result. Never got used in competition.
The other benefit was around a 100 fps velocity increase similar to Vaughn's experiment. That in itself was well worth the effort, but unlike his attempt i did see an accuracy gain and even looking bullet holes.

[dg]

apologies tony, I was interrupted before I commented on the amazing u tube clip you provided.

unlike theory or computer simulations it demonstrates visually the difference between flat bases and boat tails upon exiting the muzzle. little wonder why BIB's have achieved the results they have.

it is easy to see the concept of flat bases being validated in regard to initial stability.

I also appreciate the thought behind the muzzle gas envelope eliminator. very ingenious.

if I understand your comments correctly , accuracy was improved and did it eliminate or reduce the pin hole offsetting for the 30 cal bullet holes?

no drama either way, but with boat tail projectiles (vld or non vld) I am still intrigued with their thought provoking behaviour.

cheers
dave

[justjeff]

Tony,

We could always screw it on my 13 twist and shoot BIBs vs 200 SMKs. See if it makes a difference.

Jeff

[Tony Z]

Dave i recall there being more uniformity to bullet hole shape. There is a caveat here though where it was not determined whether any accuracy improvement was due to the system or due to the addition of the weight as a damper. I do believe there was a gain from the system but it would be poor methodology to ignore other possible factors.
There is a shining light here in that the threads used on my current barrels and stretcher tubes are standardized and do mate up with the gas splitter. Looking at the 308 groups at 600 yards shows the pin holes made by the 175 SMKs are random and some bullet holes are clearly oval yet many of the groups are half MOA or better. It appears there is no real correlation between random pin holes and poor groups. So it looks like i will be doing further testing once done with the pimer tests.

[Tony Z]

Dave i have been thinking about this a bit more and it raises other questions.
It is generally accepted that bullet yaw smooths out fairly quickly. Others, as i have already said, claim it never does, or at least not in the ranges we play with. But let's say it does. Then the remaining angle the bullets nose is aligned at once the yaw smooths out could explain the off center pin holes. But this is where i have the problem. If the bullet pin hole is not high left on a left to right breeze, as the theory suggests it should be, wouldn't any other pin hole placement, question the theory and or suggest a different bullet flight profile that is less ballistically efficient in that breeze?
If the pin hole shows the opposite of what the theory says to that left breeze and has say a right side placement, surely the bullet angle is further sideways to the breeze and the effect on the bullet is greater so thus should have the bullet land further and lower right?

OK here is two scenarios.
Yaw smooths out and a bullet flight angle to the wind is set as the bullet heads off in a left crosswind. It flies downrange and the wind drifts it an amount according to bullet efficiency. If the yaw is gone or miniscule, random offset pin holes would suggest a bullet flight angle that would have varying bullet side profiles presented to the wind resulting in real time BC SDs. So the group is seen at the target with both vertical and horizontal dispersion as the random BC would mathematically suggest.

Second scenario. Yaw and pitch never smooth out and remain constant where the bullet presents its profile as a constant swirl through the air like a wobbly torpedo punt for those AFL followers. The amount of drift and elevation might be more predictable where variations are elsewhere in areas like velocity ES, frontal variations ie meplats. The pin holes are still random, as we see every match, as the pitch and yaw has not smoothed out and the nose keeps tracing its arc.

Let's carry it further. The claims are that you can run the faster twist rates to get a higher BC with the same bullet. A faster twist does what exactly to a bullet that is already stable at a slower twist rate?
Example. Hornady 7mm 180 grain ELD M at Mach 2.25 with an 8.75 twist shows a BC of 0.777. At 8 twist it is 0.796. At 7.5 twist it is 0.819.
All the ballistics calculators say this bullet will work in a 9 twist (some say up to 9.4) at around a 0.760 BC. So then how stable is stable and what is changing to get the higher BC apart from the bullet jacket tearing RPMs? There is only one way to get a BC increase with the same bullet and that is to reduce frontal area presented to the atmosphere. Since the bullet is unmodified, the only way i can see it done is to reduce the amount of pitch and yaw which reduces the frontal area presented to the air. Can we join the dots here and say pitch and yaw never smooth out contrary to what the 6DOF models suggest, or is my assumption flawed?

[The Raven]

I'm no expert but...

Projectiles don't follow some magical path that results in a perfectly tangential hit to the target face. Assuming a flat earth.
A projectile will act partially as a flying surface, so expect the 'nose' to be slightly upwards or downwards...depending on trajectory/distance/speed.
It's a spinning object, while it may have an axis expect some minor oscillation no matter what the rpm is.
Wind!!!
Gravity!!!
Target deflection. FFS it's paper!
Leading edge shockwave.
Left or right spin.
It ran out of gas!
It got a flat tire!
I didn’t have change for cab fare!
I lost my tux at the cleaners!
I locked my keys in the car!
An old friend came in from out of town!
Someone stole my car!
There was an earthquake!
A terrible flood! Locusts!
IT WASN’T MY FAULT, I SWEAR TO GOD!

https://www.youtube.com/watch?v=9TuLBa-rgBk