VLD bullets - short range vs. long range accuracy

[mikecr]

I believe what orkan is talking about(shrinking MOA with distance) is a completely different subject.
I'm one of the 'special' kind of people that shoot better moa with distance. It's a bell curve for me(peaking ~500yds), and I do it with every gun and every bullet, including FB bullets just the same.

I think in my case it's combined parallax and prescription lenses.
If you wear glasses try reading through that horrible little spot in your lens, as you might while aiming a scope.
It can't be good for this endeavor..

 

[Caleb85]

I have a old 7mmRM sendero that I'm shooting 168gr Berger and at 100 yards, three shoot is about 1.7 moa but if I move back to 200 she will shoot in the .3's and at 600 it's 1/2 moa and at 850 it's a 6" group. I haven't understood all this jumping around yet either but I'm not complaining to much on how she shoots.

 

[300 RUM]

Brian Litz explains all the physics in his book. While I believe the phemonom exists, I think it is very minor. At least I haven't seen anything noteworthy in my limited personal experience. I don't believe crap at 100 yards turns to gold at 1000. My rifles that will shoot VLD's hold 1/4" 5-shot groups at 100 yards. The ones that don't shoot VLD's well don't shoot them well at any distance.

I used to believe this very same thing. The theory was sound, but I never saw anything first hand that would prove it to me.

Then I started working with 375CT, and routinely see groups of 1.5-2.5" at 100yds, that will stay 1.5-2.5" at 500yds. I have seen 2.5" groups at 100yds, produce 5" groups at 1000yds more times than I can remember.

It would appear that the longer the projectile is, the more pronounced the effect. The projo's I've worked with most are the 352 solids from cutting edge, and they are 2" long. I've not yet worked with anything else that exhibited that extreme behavior.

I have done load development for around a dozen 375CT rifles in the last year, and all performed similarly. While I do not proclaim to be able to completely explain what is going on mathematically, clearly there is something going on there.

I hate to rain on your parade but, If you shot a 5" group at 1000 you have no way of knowing what it would have did at 100 yds. It had better shoot small at 100 and repeat it to be a good 1000 yd. gun. It can not stay the same size from a 100 to 500yds. it is a cone of dispersion and it had better shoot small at 100. Conditions at 1000 dictate far more with group size than you will ever know,because you can't see it……… jim

By profession I can tell you if it don't shot good at 100 but shots at 1K or farther your twist rate is not sufficient for the bullet your using. I would also like to add that if it shots at 1K and not at 100 it is not that the load likes 1K better it is just that other influences are helping your MOA, the explanation is more then I care to wirte. Simply put if it dont shot at 100 the bullet your using is not optimal for your rifle/twist rate.

 

[orkan]

Interesting. I'm hesitant to go changing things, as one of my customers just set 3 separate ELR records at the whittington center last weekend during the Professional Marksmen ELR course. His rifle exhibits the same behavior of 1.5-2.5" groups at 100yds, which easily produce sub-moa groups at 1000yds. If it's not stable at 100yds, how can it carry on and be stable at 2800yds? (distance he made 3 consecutive hits)

The bullets aren't yawing on their way out that we can tell, as no key-holing is present. Something tells me there is a bit more to this than twist rate not being adequate for bullet, but I could be wrong.

 

[mikecr]

It's NOT twist causing it.
It's a crazy notion to think that a bullet deviating from path(for any reason, any amount), remembers the original intent, and then later in-flight calculates correction course & heads back over to the original path just in time.... :
What would steer the 'god bullet' back?

The damages done with twist issues are irreversible before the bullet even leaves your chronograph. A jump, a loss in BC, muzzle slapping disturbance, whatever. There is no in-flight counter to recover that loss, much less exactly counter.

 

[MarkS]

StemCell,
I have heard the same as you, But I have never had a long range load that would not shoot at 100 yds. My benchmark is low 0.2" at 100 and sub 0.40" at 200yds. It would be very common to see low 0.1's at 100 and 0.2" at 200yds. 200yds is the longest I would shoot groups, too many factors in longer ranges.

Mark Schronce

 

[M-61]

It's NOT twist causing it.
It's a crazy notion to think that a bullet deviating from path(for any reason, any amount), remembers the original intent, and then later in-flight calculates correction course & heads back over to the original path just in time.... :
What would steer the 'god bullet' back?

The damages done with twist issues are irreversible before the bullet even leaves your chronograph. A jump, a loss in BC, muzzle slapping disturbance, whatever. There is no in-flight counter to recover that loss, much less exactly counter.

The above makes sense to me. The bullet is not a guided missile and I could never understand what force would make it correct and return to the intended path. What would make it "KNOW" what the intended path even was? The whole theory is lost on me. And the 'going to sleep' thing sounds like backwoods gypsy mysticism. It is, I guess, beyond me.

 

[orkan]

The above makes sense to me. The bullet is not a guided missile and I could never understand what force would make it correct and return to the intended path. What would make it "KNOW" what the intended path even was? The whole theory is lost on me. And the 'going to sleep' thing sounds like backwoods gypsy mysticism. It is, I guess, beyond me.

It's actually a fairly well documented aspect of bullet flight.

[youtube]http://youtu.be/4pF8W5liSRc[/youtube]

Gyroscopic stability will cause lateral movement in opposition to the initial direction of the objects path. Think a bowling ball as its thrown with some spin... or a motorcycle without its rider. Bullets have tremendous gyroscopic force.

 

[Erik Cortina]

But how can 2+ unstable bullets all become stable at the exact some point and become accurate at longer ranges?

 

[yukonal]

the benchrest shooters (some) say bullets "go to sleep" after an initial wobbling much like a child's spinning top...they wobble, then straighten up and spin stable, then wobble again and fall. i believe this especially applies to boattail bullets. there are some suggestions that a 200/300 yd group can have a smaller MOA than at 100 yds.

This.


I have 2 .284 Tactial rifles that shoot smaller moa at 400 than at 100. It's not a myth...

 

[orkan]

But how can 2+ unstable bullets all become stable at the exact some point and become accurate at longer ranges?

I must be misunderstanding what you're saying, as I didn't say they become stable at the exact same point. More importantly, I didn't say it was a "stability" issue at all. They are perfectly stable, yet exhibit epicyclic motion due to their length in proportion to their width and RPM. Basically it is a characteristic of all spin-stabilized projectiles. I would guess that the shorter the projectile in relation to its width, the less the effect presents itself if all other things are equal.

Trust me Erik, when I noticed this happening with 375CT, I spent a great deal of time trying to identify the cause. It didn't make sense to me at first either. It happens every single time, and I can replicate it on command, anytime I shoot my 375CT. Bryan Litz maintains that it happens with all bullets to some degree, and I agree.

Also, just to make sure you completely understand what I'm describing: I'm not saying that linear group size gets smaller. I'm saying that angular group size gets smaller. So a 2.5" group at 100yds can easily be a 3.5" group at 600yds, thus the size in MOA at 600 is considerably smaller.

 

[Erik Cortina]

I get what you are saying, but my question is this: Bullets leave from the same point (muzzle) and 100 yards away they have deviated enough to print a large group, what makes them at that point turn back and print a small group at longer distance?

 

[orkan]

I get what you are saying, but my question is this: Bullets leave from the same point (muzzle) and 100 yards away they have deviated enough to print a large group, what makes them at that point turn back and print a small group at longer distance?

The epicyclic motion dissipates faster than the gyroscopic force, would be my guess. Though as I lake the equipment necessary to find out for sure... it's just a guess.

 

[mikecr]

The epicyclic video doesn't demonstrate spiraling bullet path. It's just the tip settling out a few degrees to align with established bullet motion.
This hurting accuracy and BC, and not just up close.
Any damage done isn't going to be undone.

Keep in mind that NOBODY has ever proven decreasing moa of accuracy with distance.
And even though I myself shoot better with distance, I know it has nothing to do with bullets, and it is not decreasing moa of accuracy at distance. It's just me shooting better there.

 

[amlevin]

And even though I myself shoot better with distance, I know it has nothing to do with bullets, and it is not decreasing moa of accuracy at distance. It's just me shooting better there.

Does the larger target have anything to do with it 8)

 

[Turbulent Turtle]

The reason we spin the bullets in the first place is to use the gyroscopic force to keep the bullet from giving in to the overturning force exerted on it by the resistance of the air. The higher the velocity, the greater the overturning force so that you must have a fast enough twist to make sure the bullet doesn't yaw or worse out of the muzzle.

As the bullet travels, it sheds forward velocity but keeps its spin rate as there is nothing to slow it down; so as the resistance of the air diminishes the spin rate remains constant and the bullet gets more and more stable.

It has been postulated that these long-for-caliber bullets are actually close to being unstable at the muzzle and may have the nose of the bullet spin a little off center but that irons out as distance grows and the bullet becomes very stable and remains precisely on track.

If this were the case, of course all the bullets from the same load and barrel would become completely stable at the same distance as they follow the same trajectory. They would not get any closer as a group, but they would no longer disperse at quickly as they had done until they became fully stable. So one could conceivably see a somewhat larger group than expected at close range, but the group would not be as big as you would expect it to be at longer ranges.

JBM has a nice calculator for bullet stability. The goal is to have the stability factor at 1.5 or above, but bullets will be stable under that value; they won't keyhole or yaw, but they are not completely stable.

I like to have a little faster twist than the minimum recommended by some bullet manufacturers for the same length bullets so that where they recommend a minimum of 1:12 twist, I elected to go with a 1:11 twist. This puts my bullets at a stability factor of 1.54 according to JBM.

You are of course, free to disagree and present other hypotheses but this one makes sense to me and the results have not infirmed the theory.

I also do not want to overspin the bullets lest a sub-standard one escape my inspection and fall prey to dynamic instability downrange and produce one of those WFT results on paper. I just want enough spin to have a stability factor right at 1.5 or so.

 

[Catskinner]

Litz had a visual of this. I've seen it posted here before. It shows the spiral motion of the bullet on a time elapse of the bullet flight.

Basically, the bullet is gyrating on it's flight axis and the gyration diameter decreases over the flight time.

(forgive me if my scientistical vocabulary chopped this explanation to bits)

This is what make's a rebated boattail VLD interesting ( .15 thou step ) in the shank like the Lapua D46 goes to sleep as it leaves the barrel due to the lack of laminar gas flow over the top of the bullet. It is said to have to have 10 to 15 % flatter trajectory with this type of design.

 

[johara1]

Erik, I see no way they can validate the statement, because they have no idea what it fired at a 100 yds when it printed smaller at longer range…….. it is a guess. I have fired too many small groups at 100 and they open up to 2" at 1000 yds. When your bullets are stable the hole look like a smaller caliber shot them,you can't shoot sub .1 groups @ 100 or 2" groups @ 1000 with unstable bullets. I would like to see this say 5 - 10 shot groups in a row for me to believe it is not a once in awhile fluke. You back it up and i'll believe it, and i would like to see the bullets spun on a Juenke machine and see if they were any good to start with. To me it seems they need tuned. I've shot too long and to and seen wind at long range blow a bullet into a group and shooter error all come together…..I jerked it, bucked it, sun was up and sights down and call it in the middle…. and it was. C. Hathcock ………. jimO'Hara

 

[orkan]

Erik, I see no way they can validate the statement, because they have no idea what it fired at a 100 yds when it printed smaller at longer range……

I think there is a misunderstanding as to what is being said here. I'm not saying it will print smaller groups at longer range, when measured in linear inches. I'm saying it will print smaller groups at longer range when measured in MOA or Mils. Meaning a 1.5" group at 100yds does not become a 1" group at 500yds. It means a 1.5" group measuring 1.5 MOA at 100yds will become a 3" group at 600yds measuring 0.5 MOA.

I had a long conversation with Russ at DTA in regard to this. He has vast amounts of experience with 375CT, and he confirmed that it will happen with solids in 375CT almost without exception. They did a PR piece on shooting 375CT at 3080yds, and were able to produce some extremely impressive sub-moa groups. Russ told me that ammo from the same lot used during that test was fired the day before and was shooting over 3" at 100yds. Yet the next day it was sub-MOA at 3000+.

As for replication, yes, it will do it every single time, without exception... so I could replicate it with ease. Every single 375CT that I've worked with, using solids, exhibited this behavior. Oddly enough, when using jacketed bullets like the 350SMK, the effect is far less visible, once again going down to the point where it would be tough to determine what the cause of the larger groups is. I've not experienced it on smaller cartridges, as even my 338LM shooting 285 HPBT's doesn't seem to do it to a degree where I could be certain the root cause was not me. However, most of my rifles will shoot .25 MOA or so and I have no issue getting them to do it. So when a 1.5-2.5" group presents itself at 100yds... you can bet I'm going to take it seriously. When I started with 375CT, this vexed me for a long time. It wasn't until I got more experience with them that it became apparent what was happening. I can virtually guarantee you, that if you are shooting a 352CE from a 375CT, or a similarly constructed solid bullet, that you will see this exact same behavior to some degree, and it will be obviously apparent. Once upon a time, I believed just as some of you gentlemen believe, but experience has shown me otherwise.

I'm not nearly intelligent enough to completely competently explain the "why" of it, but the effect is an absolute fact. No doubt it is a fact that is not very visible in most rifle/cartridge/bullet combinations, but it is a fact none the less. I don't expect you to accept it without seeing it for yourselves, but if you run into the same situation in the future, you'll at least have this thread to fall back on and someone to call and talk about it with. I'm available anytime.

 

[Erik Cortina]

If this were the case, of course all the bullets from the same load and barrel would become completely stable at the same distance as they follow the same trajectory.

Bayou shooter, my point is that they don't follow the same trajectory, if they did they would print small at 100 yards.