Dope/real world discrepancies, high DAs

[Galloglass]

I'm noticing a fairly major discrepancy between predicted values and real world values shooting at moderate ranges over different days but similar environmentals. Interestingly, these differences hold true over different calibers, rifles, optics, and even shooters, and furthermore show up not only in my older AB mobile but also in two other programs. The average discrepancy is about 2 moa or .5 mil at 500, increasing to 2.5 moa and .6-.7 mil at 600. This correction holds to 700, have not tested beyond that to see if the correction remains constant.
I might add that these are all reliable rifles and scopes, the chronograph values are solid though I have not chronographed at this shooting position. To make the values even close to matching I have to input velocities which I know would be extremely over pressure. Calibers are .308 and .260, and I've shot many thousands of rounds of both, so quite familiar. It is true that these observations are based on shooting steel rather than paper, hence my hedging on the differences, but my conclusion is either I am missing something or the predictive algorithms of multiple programs loose their accuracy in conditions as described above. I'm always flatter than predicted.
I would appreciate any comments and/or similar experiences. am I missing some critical input here or is this an issue with the model? For older shooters like me, it just reinforces the old adage that you have to shoot your dope to be sure, good as ballistics programs are.

 

[Galloglass]

Sorry my environmentals went missing in the above post...temps from high 60s to low 80s, station pressure 22.6, elevation 7630, DAs from 10300 to 10800.

 

[FeMan]

Making sure you know your true velocity is important and often overlooked, but you have that taken care of. One thing I have found is quite often the published BCs are too high. In my testing, I have not found a bullet yet that shoots to POI as the factory BC predicts. I find I have to reduce them by 7% or more to get things lined up between reality and my ballistics. I believe marginal stability contributes to eroded BCs (like Miller numbers under 1.5) but I cannot prove that. 2 MOA disparity at 500 is a lot though. I really don't see major differences in my rifles until I get to 800 and beyond.

 

[CharlieNC]

Have you accounted for the temperature effect on velocity? Once I have taken anal care to assure ALL the other parameters are accurate, I have ended up tweaking this to fine tune the calculations. I did not measure velocity at temps, but adjust it based on dope at various conditions to handle the lack of fit.

 

[Galloglass]

Yes. And as noted in my post above, this is a consistent error over several different rifles, scopes, etc. powders involved are H4350, 8208xbr, and varget...none known for much temp sensitivity. It begs the question of WHICH temperature we should use, ambient air, kestrel in the sun, etc but what causes me to suspect the model is the consistency of the difference between the predicted and the real over the variables involved, caliber, scope, gas gun/bolt gun, different days with different winds, even different shooters...they all exhibited shooting flatter by @2.5moa at 600. I use the approximation because this in on steel, not paper, but the trend is clear. Any others please chime in, especially if you shoot at altitude or 10k+ DAs. I should add that the rifles in question all track very closely to AB mobiles predicted values at "normal" DAs, which are 4000-8000. That's why I'm baffled.

 

[XBBR Shooter]

Sorry my environmentals went missing in the above post...temps from high 60s to low 80s, station pressure 22.6, elevation 7630, DAs from 10300 to 10800.

If you enter station pressure, then you should set elevation to "0" - if you enter both station pressure and elevation (7630) you will get bullets that run higher than predicted, down range.

 

[Galloglass]

Yes, I mentioned both DA and station pressure for reference. I do set altitude to zero and use station pressure for input when using the program.

 

[fredo]

Is this phenomenon localized to one particular shooting location?
Or, are you experiencing this 'shift' at other spots, despite similar conditions?

You mentioned shooting in 10K+ DAs, does that mean mountain country? If so, thermal updrafts in mountain country can & will make a bullet shoot 'flatter' as the day warms & thermals begin moving up as warm air rises...

I've measured a DA shift of over 3,000ft on the same day, @ only 1,100ft ASL. But that's more rolling country, than it is mountain terrain...

Also, I've taken well proven dope to a new location and have experienced exactly what you describe...shooting HIGH. When that does happen, it usually happens in the heat of the day. And I've experienced it at more than one spot. Once the day warms up, all of a sudden you're shooting HIGH. But come toward evening, and the rifle starts back to 'normal'. So, best I can surmise is that there's some 'intangible' amount of lift being added to the bullet as heat radiates up from the hot ground? Dunno for sure, but I know its real!

Don't look past the use of a temp sensitive powder, as well. MV could be speeding up up as ambient temp increases, and/or ammo 'warms up' due to solar influence. That's something could be solved via chrono testing...

Nothing really scientific to quantify & correct for, but that's where 'local knowledge' helps at certain shooting spots! And I know it can drive an analytical mind crazy! But if ya shoot a location enough times, you'll begin to realize those little atmospheric nuances that a weather meter/ballisitc app can't quantify & correct for...

 

[Galloglass]

Fredo, thanks for your reply. It's gratifying to know I'm not the only one to experience this. Yes, as you would imagine from the elevation this is big mountain country, shooting area is about a mile and a half from the continental divide (@10500') and the topography is a broad (@4mi) valley and shooting position is shooting across valley facing east at 2 degree up slope into a large butte (9000'+) which stands more or less alone. As you might guess this makes for some interesting winds later in the day, but that's independent from the flatter dope. The effect I describe is generalized to the area, and it does occur as the day begins to heat up. Not a lot of shade in this country so you can imagine the solar radiation being pretty intense. I've done a great deal of shooting in mountains and am familiar with erratic thermals, this seems to be different, especially as bullet path is not crossing any canyons, arroyos, etc. worst thermal lifting I've experienced is shooting more or less at right angles across natural features like these, thermals can and do cause some very erratic bullet behavior, but this seems to be different. Whether it is due to some sort of radiated heat back from the ground I do not know, but there is no question that the effect is real.
I get the temp sensitivity for powder, I will assiduously measure vels next time at that shooting position, but as mentioned above the powders I'm using in .260 (H4350) and .308 (8208xbr and varget) are all on the low end of the temp sensitivity scale and I have shot lots of rounds in all sorts of conditions and not seen this at more "conventional" mountain summer DAs, say about 4000-8000' in my AO. I will also try keeping one lot of ammo in shade on ice and shoot that vs. ammo box covered but in sun to see if ammo temp is the issue.
I discount this however because in my opinion 2-2.5 moa at midrange distances is a great deal, and because I know I would be blowing primers and locking up the bolt in some of the rifles involved if my actual vels were that much faster. Perhaps Bryan or some of the AB wizards will add their thoughts to this...

 

[dcali]

You're not measuring something right. 2 MOA of error at 500 yards is too much. It should be much closer than that. Range, velocity, angle of fire, pressure, temperature, BC. If you get all those right, you should be pretty darn close with your run of the mill long range bullets.

 

[Keith Glasscock]

Do you have your scope height above bore line correct? It can cause an error like you describe.

 

[DocUSMCRetired]

I would take a look at a couple of articles here:

1. http://www.appliedballisticsllc.com/Articles/ChronographChapter.pdf

2. http://www.abmediaresources.com/BallisticCalibration.pdf

3. http://www.abmediaresources.com/BulletPointing.pdf - Not sure if you point or not.

4. http://www.bergerbullets.com/twist-rate-calculator/

I would first double check the inputs I am using in the calculator. For that you can find your individual user guide here www.abmediaresources.com then make sure that the BC I am using is one of the tested numbers. Then I would make sure that my SG is above 1.5. 2 MOA off tells me something else is going on.