very confusing drop data results- help

[joshmartin8]

right guys today i thought i would do an exercise to see how far i could step out staying in a kill zone area of 3inches. will be using all my drop data and changing accordingly to each range,

 

1st shot--100-- 1/4 inch high-- 0moa

2nd shot--150-- bang on-- 1/2 moa

3rd and 4th shot--200-- 1inch high-- 1.25moa

5th and 6th shot-- 300-- 2.5inch high-- 3.75 moa

 

7, 8 , 9th shot--350--2inch high, 5 moa

 

10th and 11th shot--400-- bang on --6.5 moa,

 

155 lapua scenar @ 2820fps,

 

i just don't understand how my drop data can as close as zero as pos at 100 go up in impacts up to the distance of 350 then be back on track for 400 just don't get it, any thoughts please guys

 

[jcampbellsmith]

One of the things I have come to appreciate recently is the terrain that I shoot over. I rarely shoot on a formal level range. I now expect there to be various wind impacts on my trajectory when I'm shooting at a target in the field.

 

Some questions:

• What kind of terrain and wind conditions were you shooting in?
• Was the wind blowing right to left with respect to your line of sight?
• How many shots did you chrono to get your average velocity?
• Which ballistic calculator do you use?
• What weather data did you record?

Can I suggest you go back and try your scenario again? Two shots at each range?

 

Regards

 

JCS

[srvet]

How did you develop your drop data? Was it developed under identical environmental conditions and in the same location? Was there any land features that may cause air currents up or down that may influence the bullet?

[M99]

You need to supply more info.

 

Rifle, barrel length. Bullet, velocity. Weather details.

 

What app did you use for the data? What BC did you use.

 

Does your scope track correctly? Have you measured your clicks are actual?

[srvet]

Damn, JCS pipped me to the post!!

[brown dog]

One of the things I have come to appreciate recently is the terrain that I shoot over. I rarely shoot on a formal level range. I now expect there to be various wind impacts on my trajectory when I'm shooting at a target in the field.

 

Some questions:

• What kind of terrain and wind conditions were you shooting in?
• Was the wind blowing right to left with respect to your line of sight?
• How many shots did you chrono to get your average velocity?
• Which ballistic calculator do you use?
• What weather data did you record?

Can I suggest you go back and try your scenario again? Two shots at each range?

 

Regards

 

JCS

 

Yup, I'd also underline srvets 'same place, same time, same conditions?' question and add:

 

Difficult to judge without knowing the natural dispersion of your shots at each distance. eg was your assessment of 7,8,9 as being 2 inches high at 350 based on a 1" group or a 5" group?

If the former, your data point is probably valid, if the latter, you'll probably just be chasing your tale whilst misidentifying your MPI due to insufficient shots to iron-out the random dispersion (for example, if the group size is 'big' the first 3 shots may simply be the print of the top half of a group that -had you fired more - would have centred on the tgt).

[tisme]

I've not found any BC calculators to be spot on between 225 and 350 unless I change the BC or FPS but then they are out at longer distances. I put it down to the bullet traverling faster so the BC is out. Serria say that the BC of a 39 gr blitz king is 287 at 3600 fps and above .270 between 3600 and 3400 fps .255 between 3400 and 2800 fps .236 between 2800 and 2300 fps .210 between 2300 and 1900 fps .180 At 1900 fps and below. So the BC goes down as the speed goes down. http://www.sierrabullets.com/documents/BallisticCoefficient-rifle.pdf

 

We have to pick a BC that will work at all speeds but this is not easy as the BC doesn't always go down it can go up. Match king 69 gr is 301@2800 305@2200-2800 and 317 below 2200.

 

Best thing is find something close and then test and make your own chart .

[gbal]

Josh,this is an interesting topic-it overlaps with the current posts on "point and shoot'-ie no detailed correction for drop/drift...and the real field answers suggest more 300 than 400y-and hotting up the load makes very little effective difference,with a 4 inch criterion.

 

OK,you are asking,I think,to what range can you put all your shots in a 3 inch circle,with click adjustments as necessary.Well,all these things are somewhat variable with actual rifle/shooter/etc but we can get reasonably close.There is ultimately no alternative to field testing though.

 

All the answers so far have raised good points-JCS and BD about terrain,srvet and MR about environment.rile l/load;tisme onBC/bullet speeds. BD s key point is critical-are your data representative-simply put,given that there has to be (substantial-see below) shot to shot dispersal,are enough shots fired at each distance to be representative-it seems not;there is no magic number,but 5 minimum is often accepted as 'group' indicating-not quite defining-10 would be better,but maybe unrealistic. This isn't always grasped in the 'rubbish in,rubbish out' generality.

 

OK,how about some data to set the scene: commercial loads for 308 with 155g drop at increasing ranges:

 

Corbon 155 scenars@2900fps muzzle -1.5 100y 2.1 200y 0.0 300y -9.2 400y -28.0 inches drop

 

Lapua155 scenar@2822 -1.5 2.1 0.0 -9.8 -29.9

 

 

BlackHills 155 Amax @2750 -1.5 1.9 0.0 8.3 -24

 

OK,all 200 zero and BCs are similar-in the mid .45 region-so we get,with small velocity and BC diffs,close agreement.

 

So thats what you should expect.....unless your load is very different,but it isn't.

 

Now,Bryan Litz has done a persuasive and well documented theoretical analysis that bears on this too:What is the maximum range at which a !/2 moa at 100y rifle with 6mm 80g @3000fps is 'guaranteed' to stay within a 6 inch circle?('guaranteed' inverted commas,since human shooting error is excluded!)Well,taking the worst possible scenario shots-when each of the dispersal variables are at their extreme,and that happens-then the answer is a perhaps surprising 248.5 yards.Consider : wind reading error of 2mph: that's elev error 1.01 and wind error 2.51 inches;Coreolis elev .23; 10fps vel changes .16;air density changes frozero .29;gyroscopic drift windage .4,coreolis windage .13......and there are other minors

That cumulatively-on some shots -means Elev is 1.68 and Windage 3.04 out.Remember too some errors will be possible in either direction (over/under errors on wind eg).

You might try to quibble about the exact detail-on some-eg wind error,but it takes a very astute shooter to guage wind vagaries to within 2mph and collate their effects over 250 yards.

 

And add in the rifles intrinsic accuracy-1/2 moa dispersion.....let's assume a perfect shooter!

 

OK,the general point is pretty clear-there is a fair bit of dispersal going on.Some ballistics programs might have the sophistication to incorporate some of he variables....and maybe not.

 

On the 'balistics' program issue-useful ball park,often BUT....we do get a lot of 'discrepancies' being reported on here.....I'd not want to use one initially,and you can't just fiddle velocity to fit.....no,no ,no-you just have to use true velocity-otherwise why measure it (accurately-another issue).Using G7 BCs will reduce,at least,the BC variation of G1 with speed. (speeds here will be around 2200 at 200y,but 1800 ish by 400yso it's an issue.)

 

Now,I have no doubt that VERY savvy and experienced shooters can do better than 6 inch hit zone at 250y-and occasionally at 1000y-but they will have access to some very good data,from their rigs and have honed wind reading etc.

 

I also note you were not considering 'hit zone' to include windage-which is major,but again the general points are clear enough.Very good and representative data-across the likely dispersal range is needed-the fancy version of BD's 'are those holes the top two,or bottom two if you'd shot more".Ditto most of the other variables.Note too,this is 'precision' ie group,not per se 'accuracy'-all hit the bull,though that's a simple click issue.

There is almost no use in collecting data unless it is at least 5 shots at each data point,and conditions are very good (no wind if possible) and a very steady shooting position. Field shooting will mean more compromises,but you need the basic data to be as accurate and precise as possible.

 

More of us should do it;some surprises can be expected (bench resters have a 'wailing wall' for disasters like .4 groups).But holes in paper are reality,and so are the raft of variables (more like an iceberg sometimes) that contrive to challenge our assumptions and hopes.Best be aware.

 

Good shooting.

 

gbal

[MJR]

Josh, your original 100yd zero is out - 1/4". You have already built in a 1/4 moa inaccuracy. Why no re zero and re calibrate your scope turret? Also as others have already said what is the inherent accuracy of your rifle at the ranges at which you've shot? If I'm understanding you correctly you fired only 2 shots at 400 YDS. I'd suggest that's not enough to establish a true mean point of impact.

[joshmartin8]

thank you so much guys and thanks global for the time to reply with such a detailed response, all of you have raised good points, i am going to try to answer all of your questions if poss and will bullet point them now,

 

- was shooting from a barn off some very solid bails,

- wind was full value right to left at 10mph ish

- i have over many trips out chrono my rounds and they are always around the same speed 2810-2825 ish

- I use istrelok on my iPhone

- weather data was 5degrees, 9995hpa 335ftsea leval, it updated from my phone,

 

- ACCURACY INTERNATIONAL 24'' with lapua scenar @2820 @ 0.460bc

- IOR 6-24-50 always been perfect

 

- i have not found if i can adjust the bc on istrelok to take a different bc rating so stuck with the .460 input,

 

- you guys are correct i should be doing more shots to form an accurate group which is what i am going to do next time i go out, min of 4,

- the rifle is and has always performed outstanding normally achieving 1/4-1/2 moa groups , couple days before it achieved at 2.5 inch 4shot group on a bottle at 560 so thats why i went with one or two shots as i thought the accuracy of it and me were pretty predictable and reliable,

 

hope i have answered all question that i read,

 

next course of action will be to test velocity once more, double check 100yard zero, have new targets for each distance and fire 5 shots at each of them letting barrel cool between each distance increase, document all data, i.e.clicks exactly, wind speed, temp,altitude, pressure etc,

 

maybe consider changing to a different phone app that will allow me a different bc input rather than the g1,

 

thanks again guys and i look forward to your responses they really have helped,

 

JOSH

 

 

 

 

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[gbal]

Josh,thank you for the data for your rifle.

 

Have you not a reasonable answer to your question (How far can you shoot with a 3 inch hit zone?)

 

Your 4 shots in 2.5 inches at 560 yards seems to be more relevant than anything alse.

The question is not a 'ballistic program' question-there may be other questions that are--it's a"Maximum effective range ' question-ie abour precision/accuracy,not ballistic per se.(Of course for deer you would need 'terminal performance' -energy/penetration etc-above some limit-your load will be about 850 ft lb at 400y and 500 ft lb at 500y-not great for large deer.Bullet construction is another consideration-though with scenars,you are not likely to be stalking!

 

I'd load some more,and fire at least three 5 shot groups-as you say- at 560 yard,as you have that doped..(or 500 and 600 )If you get close to your previous group size,centered on your target,is this not your answer?

You'd need a whole lot more data to do a balistic program MER (max eff range) -and it simply is not so good as field firing (those are the actual variables that matter,and include shooter ability!)Most programs have to "approximate' actual rifles etc,and have no input for the nut behind the butt.

I keep repeating the point,but it needs repeating-G1 BCs are NOT constant.(G7s are a bit better)-here are Sierra's honest figures for 155 Palma bullet in 30 cal (scenar will be a similar pattern):velocities and BCs:

 

2700+ BC is .504

18-2700 .470

15-1800 .43

under 1500 .38

 

Most makers give only the best (MV) BC.Set aside any detail about accuracy etc of the BCs-the point is that BC reduces with velocity,and we have Sierra data for the same bullet.

 

Your rifle is shooting 155 scenars-BC similar to Palmas- and velocities similar to below(Lapua data):

 

MV 2820

100y 2468

200 2141

300 1841

400 1571

500 1340

600 1161

 

It's very clear,as with ALL bullets,that the BC over flight distance varies quite a bit-from .504 at the muzzle to quite a bit less than .38 before 500y. Sophisticated programs might 'know this'-or not. G7 is rather more stable-hence preferable.

But as above,you don't need this to answer your : what is my "Maximimum (three) inch accuracy hit distance ' question. You do need to confirm it's around 575y...should be a pleasure to do so!

 

Why not ten shot groups!!

 

(note,when this is done seriously-eg by Bryan Litz-you do need more shots-because the 'worst possible scenario' case shots don't happen every time (obviously) -and can be off in several directions(L/R;up/down) so a decent sample is needed. Matters little for tin cans etc,but it may do for live targets at distance-and remember,the theory is actually ballistics only -it cannot factor in shooting errors-though of course field shooting can (euphemistically refered to as 'fliers' often-well,some are ballistic,some are shooter-the more 'flier' the more likely the latter.Ballistics can be 'bendy',but not too stretchy-though the ultimate cause is typically something physics like a 10 fps velocity variation-pretty good,but it all adds up...or down... or right ..or left;and every so often,cumulatively-bigger group...or more simply: a 'miss" ! :-)

 

gbal