Variance in Ballistic Dope

[Awesymoto]

I have a question.
Gathered some dope on my rifle at R/O to 1000 yd.
>.308 175's smk
when i dial in the velocity to match my dope at 1000 and 900 yards almost all the other dopes are off from what i really shot.

can anybody offer an explanation?

 

[Lowlight]

Re: Variance in Ballistic Dope

Couple of questions,

1. Did you calibrate your scope,

2. What were the winds like when you shot 900, 1000,

How much is off? 1/2 MOA , 1 MOA ?

 

[OutlawTen5]

Re: Variance in Ballistic Dope

You need to determine TRUE VELOCITY and TRUE BC. I do this by shooting thru the chrono at say 12 to 15 yards and then again at distances like 250 to 300 yards. You will also find that all chrono's are not exact and do have a margin of error.

What optic/adjustments?

.308 w/ 175's should be simple to figure and you shouldn't be off too much.

 

[Awesymoto]

Re: Variance in Ballistic Dope

.1 mil is .1 adjustment. did a box test

wind was low 7-9mph at shooter from 7 o'clock and downrange had a stiffer 10-12mph 4 o'clock wind.

off as in 3 tenths of a mil at 400 & 500
6 tenths @ 800.

my real dope was always more than the projected dope. I found this odd as i had a tailwind so shouldn't it be less?

i'm using .243 G7 scale.

 

[OutlawTen5]

Re: Variance in Ballistic Dope

Outside temperature? What powder?

 

[Awesymoto]

Re: Variance in Ballistic Dope

outside was 70 ish. powder Black hills factory

 

[Lowlight]

Re: Variance in Ballistic Dope

There are a couple of things possibly at play,

One, certain lots of BHA have had some pretty noticeable variances in elevation. Two, the plates at RO are 1.5 & 2MOA in size depending what targets you shot. So not being exactly centered up can push you off the mark. Three, there is a terrain feature at 900 which with winds around 10MPH could also cause changes in elevation throwing the overall numbers off.

Without chronographing your load, you basically only have data that works there, small variations in the distances, like Foxtrot is not exactly 600 yards can create small compounding inconsistencies in gathering your data. Not to mention you didn't say how much of a box test you did, more than 20MOA I hope. Gathering dope on paper will yield better results.

I would shoot more and chronograph your rifle before coming to any conclusions.

 

[Rifles Only]

Re: Variance in Ballistic Dope

Foxtrot is 604

 

[Lowlight]

Re: Variance in Ballistic Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Rifles Only</div><div class="ubbcode-body">Foxtrot is 604</div></div>

Maybe, lol

 

[Graham]

Re: Variance in Ballistic Dope

Could there be an anomaly in your ballistics program at 800 yards?

We've been here testing DOPE for three days, and we are crunching numbers now to include computing different DKs in FFS.

Of three programs tested (Ballistic FTE and FFS and Patagonia), using the G1 and G7 BC, all were within .1 Mils to 1000 yards on all data, except at 800, when the data from all programs was .4 Mils low.

Interesting....

 

[CoryT]

Re: Variance in Ballistic Dope

One major problem can be a lack of enough GOOD data when comparing field data to the computer. For example, shooting two or three groups on steel does not tell you with a high degree of reliability the actual comeup required at a given range. It's even worse when the groups are separated by time and location.

I find it difficult to believe that three programs are all wrong by the same signifigant amount at the same range, but correct 100 yards closer or farther. Have you plotted the curve for the dope vs the curve for the computed data? I'd bet the dope curve shows a spike or dip at 800 that is not in the computed data. Odds are that the 800 yard groups used from the range session were invalid in some way, even though the group size may have looked usable. It's even possible that the scope has a 'jump' in it's travel, stranger things have happened.

When the OP collected his data, how many shots at each range comprised calling the comeup good? This is one reason I spend three days on the KD range and have a seven day class for PR, most people just don't shoot enough rounds under enough conditions to get REALLY good dope. When I see people making sight corrections after two shots on steel at 600 yards I wonder if they really understand what they are trying to do.

Making an instant correction on a field shot is totally different than getting zero data. Shooting steel is easy, but the reality is it's not generally very precise, as there is rarely a perfect aiming point and we seldom go downrange with a tape measure, but rely on a view through the scope. Shooting paper is best, and DON'T observe the strikes while shooting, the holes will be there when you go get the target. Watching seems to encourage attempts to 'place' shots. Let someone else watch and plot shot/call if you like, but keep the shooter in the dark until the group is complete.

Ideally, you'd be best off if you fired a twenty round group at a given distance, with plenty of wind flags to get a good idea of the actual conditions, with a chronograph in place and each shot plotted by an observer. How many people do that? Not many probably.

All that's need to have a problem is some groups fired at 800 on a colder/warmer day, some updraft/downdraft gone unseen, a little out of center but unnoticed, and those particular rounds just happen to be the fast/slow ones within your SD for average velocity. Frankly, I've developed a lot more faith in the computed solution, given a reasonable calibration effort. If it's dead on repeatedly at 400 and 800, it's probably closer than you can shoot out to supersonic max range. This of course presumes you actually enter the real conditions. Certainly the bullet tells the tale, but if it's not matching the the computer when the same bullet/shooter/rifle matches at three or four other widely separated ranges, guess what, it's not the program, it's the data entry. Some condition that's affecting the strike has not been entered correctly, you just don't see it.

 

[Graham]

Re: Variance in Ballistic Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: CoryT</div><div class="ubbcode-body">Have you plotted the curve for the dope vs the curve for the computed data? I'd bet the dope curve shows a spike or dip at 800 that is not in the computed data.</div></div>We did. The lines track straight (except, of course, for the anomaly itself).

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: CoryT</div><div class="ubbcode-body">It's even possible that the scope has a 'jump' in it's travel, stranger things have happened.</div></div>Agreed, so that's the first thing we ruled out. We tested dialing vs. holding using a Horus reticle and the scope tracks.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: CoryT</div><div class="ubbcode-body">When the OP collected his data, how many shots at each range comprised calling the comeup good?</div></div>Our sample was 74 rounds.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: CoryT</div><div class="ubbcode-body">All that's need to have a problem is some groups fired at 800 on a colder/warmer day, some updraft/downdraft gone unseen, a little out of center but unnoticed, and those particular rounds just happen to be the fast/slow ones within your SD for average velocity.</div></div>One set of data was from a 75 degree day, 72 rounds; the other data was from a 45 degree day, 74 rounds.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: CoryT</div><div class="ubbcode-body">Frankly, I've developed a lot more faith in the computed solution, given a reasonable calibration effort. If it's dead on repeatedly at 400 and 800, it's probably closer than you can shoot out to supersonic max range. This of course presumes you actually enter the real conditions. Certainly the bullet tells the tale, but if it's not matching the the computer when the same bullet/shooter/rifle matches at three or four other widely separated ranges, guess what, it's not the program, it's the data entry. Some condition that's affecting the strike has not been entered correctly, you just don't see it. </div></div>Perhaps, meaning that I believe you. But I also believe the bullet.

The problem is that the bullet is talking out of both sides of its jacket.

Yes, there could be an unseen wind issue. I'm not knocking the programs I mentioned, as they work well, I like them and I use them.

 

[Lindy]

Re: Variance in Ballistic Dope

The wind was blowing pretty good out of the north, i.e., from behind the berm. It's possible that a downward deflection was caused by that, but, IMO, unlikely, as a .3 mil deflection at 800 yards is 8.6 inches. That's a lot of deflection for what would have been a short exposure.

And it's not a range error, as I have checked that range more than once with a PLRF 10.

I don't have an explanation for that anomaly which occurred only at that distance. I'd be inclined to hold a little high when shooting that target.

 

[Awesymoto]

Re: Variance in Ballistic Dope

my 800 projected is 8.9 my real dope was 9.2 on the dot. so yes the 800 has a 0.3 mil deviation. however re-inspecting my dope, it is possible that due to the large vertical targets we fired on, that the ballistic dope was correct at 600 and less. 800 still doesn't match though.

 

[CoryT]

Re: Variance in Ballistic Dope

Graham and Lindy, certainly there is enough data to be called good. I'd like to see chrono numbers for each 70+ round group for each of the days. It would also be interesting to shoot at a different 800 yard range, just to see if the collected data holds up. One of the nice things about living here at Gunsite is I can, on any given day, shoot several different locations/directions at a target so things like wind, light, angle, position, etc. can get evened out to a good average.

Awesymoto, it would seem you don't have enough GOOD data. It's certainly possible that the program is good to 600, then deviates a little up for 700-1000. That's usually a minor tuning issue of bc/velocity. When 700-1000 are within .1 mil, the rest should be as well.

 

[Lindy]

Re: Variance in Ballistic Dope

>I'd like to see chrono numbers for each 70+ round group for each of the days.

So would I - but I have lived far too long to be that anal retentive. One of the things which made my life far easier a couple of decades ago was making a decision that I didn't have to know everything.

So, I'm perfectly happy to not know what caused that deviation. Now I'm late for a nap. Ciao!

 

[Lowlight]

Re: Variance in Ballistic Dope

The original Werewolf deviated from the ballistic solution only at 500 yards... I had serious hard data for every yard line and matched it up to several ballistic calculators and everything match BUT the 500 yard berm which was at least .3 mils off the track.

Personally looking at my data from RO and comparing to other locations I think the wind had a much bigger effect than we gave it credit for in the elevation department, at least it appears to be flushing out that way. Could be a mind thing, or could be real... I don't doubt some of it was ammo related either, I think their variations moving from lot to lot might have played a measurable roll.

 

[Awesymoto]

Re: Variance in Ballistic Dope

I'm unsure of how accurate ballistic calculators really are but I figure under 500 shooting IPSC plates .2 mil error doesn't matter much. The DA changed from -2000 below to roughly 1200 above through the week but I have my finger on I need to tune my bc on FTE

 

[Lowlight]

Re: Variance in Ballistic Dope

3000ft is a big swing and will certainly matter.

 

[Awesymoto]

Re: Variance in Ballistic Dope

Data book is at home but from what i can remember:

dope for 400, 500, 545, 600, 800, 900, 1000

Real dope: 2.8, 4.0, 4.8, 5.4, 9.2, 11, and 13.5 respectively

Ballistic FTE dope (dialed so 1000 yd matches real dope)

2.5, 3.8, 4.5, 5.2, 8.8 (largest deviation at 800), 11, and 13.5
this is hard to read.... sorry

edit using some of this the chart i got is this... looks odd.

i think my data i shot is wrong or there is some thing going on.. the bullet reduces its drop rate... seems impossible.