position of meplat pin hole in bullet holes

[goodi]

as can be seen in the attached photos, it is quite noticeable that the position of the meplat pinholes is random in dispersion and rarely central.

measured pinhole dispersion, can at times be up to 3/4 of the bullet hole diameter.

long term testing of vld projectiles in various calibres at distances from 300yards to 1000yards (using appropriate twists) produces similar random pin hole dispersion.

although the achieved accuracy from these projectiles is extremely satisfying, I am just inquiring if others have noticed this phenomenon and if so, if an explanation could be provided?

I have discussed these findings with much more knowledgeable australian LRBR competitors than myself, and they have reported similar experiences with vld projectiles.

could these results indicate, that vld projectiles are not “going to sleep” at long ranges and are continuing to exhibit a consistent pattern of slight instability ?

is so, could this partial instability be affecting trajectory, time of flight, wind drift and achievable accuracy ?

cheers

dave goodridge

australia

 

[alinwa]

THANK YOU Dave for posting these!

I've never observed what you're seeing.... and I will now be looking more closely!

I have NO/zero/NADA input on the subject, just Thank You

I'll Be Watching

Al

 

[watercam]

Did Bryan Litz not write that there is a certain amount of yaw combined with rotation in flight?

 

[dkhunt14]

Loose paper target on backer or leaning target can also cause this. Maybe on the edge of stability. Do you have enough twist for this bullet? Matt

 

[goodi]

hi matt

all targets were vertical and supported firmly on coreflute backing sheets or tensioned with gaffa tape.

pm sent

cheers
dave

 

[Immike]

Did Bryan Litz not write that there is a certain amount of yaw combined with rotation in flight?

Harrold Vaughn did write that a bullet tilted in the chamber would stay tilted throughout the bore, possibly during flight also...

 

[DaveTooley]

This goes back to my testing in the early 2000's that showed the uniformity of the meplat affected in flight yaw. I made a meplat trimmer for a number of years and now the rest is history with the trimmers and pointing dies out there. At Hawks Ridge it was confirmed over and over again that uniforming meplats decreased the yaw. Decreased yaw equates to more consistent BC's as tested and confirmed over an Oehler 43. Just part of the accuracy equation but I believe a meaningful part.

 

[Jennb]

How do you quantify thats it is not the shooter or the conditions? IMO testing done outside of a perfect environment and rifle not in a rail had too many outside factors.

 

[alinwa]

How do you quantify thats it is not the shooter or the conditions? IMO testing done outside of a perfect environment and rifle not in a rail had too many outside factors.

Huhhh???


Seriously???


The SHOOTER?


So Y'er tellin' me I got's ta' Quit Pullin' my NOSE over??


You Yankin' My Meplat?


Or is my WIND yankin' my meplat......?






off to find the "perfect environment" here....... I GOT'S the rail......










gives a whole nuther spin to "ya got's ta' hol' yer mouth right"

 

[johara1]

Dave, do you trim and point?..... Jim

 

[Laurie]

Did Bryan Litz not write that there is a certain amount of yaw combined with rotation in flight?

Yes, he covers this in detail in Volume I of 'Modern Advancements in Long Range Shooting' and photos on page 44 of four bullet marks on paper show the effect clearly exactly as per those on this topic. These are of the 0.308" 180gn Berger BT Juggernaut fired from a 308 rifle IIRC and are captioned as being at 977 yards and in transonic velocity from (IIRC) a 1:10 twist barrel. The chapter looks at factors which affect Stability and Drag, first at supersonic then at transonic speeds. The tests were repeated with super spin imparted by a 1:8 rifling pitch barrel and the yaw factor decreases not only showing a centring of the meplats in the marks, but through greater terminal velocity through drag reduction. Bryan Litz ascribes it to what is described as 'the yaw limit cycle'. It's quite a hard read for the layman like myself - the trouble is that some months on, I'd have to sit down and do it all again to really draw out the conclusions. The main one I do remember is that we may see benefits in going to much faster twists than previously seen or seen as being needed with stability factor coefficients much higher than the now recommended 1.5. (As always in such cases, I'll believe it when somebody starts winning shooting his 200.20Xs in a 1:7 twist barrel in F-TR. Until then, I'll happily let someone else risk the cost of a match barrel and possible wasted match travel, ammo and entries expenses to find out if it does work and there are no downsides that outweigh any marginal benefits.)

It shows how much attitudes to spin rates and rifling twists have changed in as few as 10 years. Not only have we ditched the old as-slow-a-twist-as-you-can-get-away-with-at-maximum-range philosophy developed to cope with poorly made and out of balance military bullets, not only have we changed the advice on optimum minimum Sg value from 1.4 to 1.5, but I remember Mr Litz himself writing of winning long-range sling matches with the self-same Juggernaut which he had designed and Berger had only just introduced in a 1:13 twist Palma rifle with the advice that 1:12 would be a better choice and saying that's what his next barrel would have. At 2,800 fps in standard conditions that 12 twist barrel (Sg = 1.43) is no longer recommended and the latest edition of Bryan's 'Ballistic Performance of Rifle Bullets' recommends 1:11.7 or faster for optimum stability. (I was shooting them in the now ubiquitous 1:10 when they first reached us in the UK and people were telling me that I'd rue this choice as it was 'too fast' a twist for best performance. )

 

[goodi]

thanks for the replies.

I appreciate your comments and thoughts.

jim

pm sent - it does not seem to matter whether the projectiles are standard or trimmed and pointed etc

since 2009, Australian IBS LRBR competitors have been extremely fortunate to establish a unique and wonderful relationship with the IBS and the Virginia 1000yd BR club.

http://www.virginia1000.com/aussie.page.htm

as can be seen on the photo of the Australian Light Gun Group record, pin hole offsets are apparent.

although not noticeably obvious in the original posted photos, the holes are not round but slightly oval, with the 6mm bullet holes having a slight elongation of approximately .005-.007 " or more, directly in line with the axis of ovality.

it should be also noted that when using vld projectiles (of various calibres) that, other Australian LRBR competitors have reported similar slight instability and pin hole offsets in their targets.

if possible, is anyone able to provide a photo of a 600yd or 1000yd BR target shot with VLD projectiles, in which all the pin holes are central ?

 

[Alex Wheeler]

I looked through a lot of target pictures I have. Its rare for them to be centered. Here are 2 small ones shot at 1k. Same bullet. Different barrels and shooters. The second one does appear to have centered holes or at least more centered than usual, while the top one obviously doesnt. Both of these groups measured in the 1.0xx" range. With the size of these groups I just dont know what to make of off set holes.

 

[alinwa]

OK, this is fascinating..... I'm a complete newbie at all this but I did go down and paw through a stack of 600yd targets to find some with clear bullet holes "in the white" (I hate bullets in the white.... probably never looked at them closely! LOL I certainly never ironed them closed and looked...) Bullets in the white at 600 are embarrassing...

Sure enough... a couple are crooked!

 

[alinwa]

Dave Goodridge (goodi, above) postulates that maybe windy conditions have something to do with it???

I'm kind of agreeing..... my good group targets, typically shot in these calm West Coast conditions, seem perfectly centered up the meplats...

The wind is howling here tonight, I think in the morning I'ma go down and bang some windy groups at 600 and see if they're flying wabbly!

 

[Keith Glasscock]

If it was gyroscopic precession, I'd assume that they would all point the same direction. Looks like something is oscillating to me.

 

[dcali]

It could be what’s known as yaw limit cycle behavior. Some bullets have a relatively pronounced yaw at range that never damps out. I don’t know if this effect is big enough to create holes as far off center as people observe, but I would look there first. It can be a few degrees.

 

[Laurie]

It could be what’s known as yaw limit cycle behavior. Some bullets have a relatively pronounced yaw at range that never damps out. I don’t know if this effect is big enough to create holes as far off center as people observe, but I would look there first. It can be a few degrees.

That's what Bryan Litz postulates in Modern Advancements in Long Range Shooting Vol I. At the risk of sounding like a lobbyist for Applied Ballistics llc (which I an certainly not, just a satisfied customer), I'll say - as I have said before - that it and its Vol II successor (Volume, not edition and the two books deal with different topics) are very valuable collections of hypotheses and subsequent range experiments that should be of interest and value to any L-R precision shooter or handloader and contain the sort of gems you could only find in the old PS magazine, but with a scientist and well resourced team doing the work.

 

[Immike]

I guess we’re still throwing rocks...I’d like to hear from Bryan on this.
Just sent a pm to applied ballistic board... @DocUSMCRetired

I got a feeling that the tail is wobbling not the tip.

 

[dcali]

That's what Bryan Litz postulates in Modern Advancements in Long Range Shooting Vol I. At the risk of sounding like a lobbyist for Applied Ballistics llc (which I an certainly not, just a satisfied customer), I'll say - as I have said before - that it and its Vol II successor (Volume, not edition and the two books deal with different topics) are very valuable collections of hypotheses and subsequent range experiments that should be of interest and value to any L-R precision shooter or handloader and contain the sort of gems you could only find in the old PS magazine, but with a scientist and well resourced team doing the work.

I think it's very likely. I don't shoot long range benchrest so I haven't seen enough of these holes to really say, but something like a 3 degree average yaw would be typical for the average yaw in a bullet that exhibits this behavior. The direction would be random, and the offset of the point would vary slightly, as you still see the epicyclic motion. If that's what folks are seeing, then I would assume that's whats going on.

The interesting part here is that not all bullets do it, and I would imagine that bullets that don't would have a ballistic advantage at long range because of the dependence of drag on yaw. Depending on how BC is measured/calculated, there could be a small but significant difference between two bullets with the "same" BC at long range.

Unfortunately, I'm not aware of any easy way to determine what, exactly determines when a bullet will do this. I have seen literature that suggests boat tail angle plays a role, as does the sharpness of the heel. (Unfortunately a sharp angled heel is better than the radius we must put up with when using jacketed bullets.) And I would assume boattail length and other factors contribute as well - anything that impacts the magnus moment. It's sort of like dynamic instability - easy to define, but hard to pin down. In general, I'm told by people far more knowledgeable than me that this is in general a bigger "problem" for long bullets. I put problem in quotation marks because there's no reason that I can think of that this behavior would be detrimental to the precision potential of a bullet.

As always, I reserve the right to get things wrong.