Ballistic head scratcher?

[Grouse]

Loaded up some 80gr Berger for my 6mmBR, they are running at 3195fps.

 

Went out to verify my ballistic charts and annoyingly found out that at 275yds I need 2moa (not the 2.6moa that most ballistic apps quote)

 

then went out to 300yds - 3.25moa

353yds - 4.50moa

 

Now using a BC of 0.301 G1

 

Annoyingly cannot bodge any ballistic app to coincide with the actual data for 275yds (had a few misses over the top at rabbits and crows which is now evident as to the reason why!

 

Checked 100yd zero and did a "click" test on NXS scope, all was fine?

 

Anyone offer a reason why?

[MJR]

Are you sure of the velocity?

[DaveT]

Have you got correct sight height?

 

Use G7 bc.

[jcampbellsmith]

Is the 80 gr Berger, a flat base or boat tail bullet? What's its part number? Regards JCS

[Grouse]

Chronoed using a friends magneto, but can check on my CED.

 

Sight height checked at 1.8"

 

Flat base 80gr Berger Match Grade PN:24321 (quoted BC is 306)

 

Groups yesterday were shot in still conditions?

 

The POI corrections for the longer ranges are more or less correct as per Horus on Kestrel?

[jcampbellsmith]

One question. Are you shooting from one firing point or are you moving firing points and have a fixed back stop? Are you shooting over a flat field or is there some varied terrain between you and the target? I've certainly found some of my 'field' locations used for practising have hard to see wind effects especially when I'm shooting across a valley into a far bank. I've only got two places where I can go and shoot over 'flat' ground out to about 400 yds.

 

Regards

 

JCS

[LukeSoutherner]

Have you got correct sight height?

 

Use G7 bc.

 

 

+1 to using a G7 BC

[jcampbellsmith]

 

 

+1 to using a G7 BC

I don't agree. I think G1 BC is correct for a flat base bullet. I suspect the issue is not the calculation, but the field conditions - primarily wind and secondly terrain. I quickly ran the data supplied with

 

http://appliedballisticsllc.com/ballistics/

 

and it all stacks up.

 

Regards JCS

[Grouse]

I don't agree. I think G1 BC is correct for a flat base bullet. I suspect the issue is not the calculation, but the field conditions - primarily wind and secondly terrain. I quickly ran the data supplied with

 

http://appliedballisticsllc.com/ballistics/

 

and it all stacks up.

 

Regards JCS

Thanks for your input, does the 275 yard correction stack up too?

 

All shot from the same position, did think originaly down draft, but yesterday evening was windless?

[jcampbellsmith]

I got a 2.7 MOA correction at 275 yds when I ran your data quickly with the link above. I guessed around 5 degrees C as temp. Best regards JCS

[247sniper]

_{I had a similar if not the same problem with my .204. I corrected the problem by zeroing at 200 yards instead of 100 yards.}

 

_{Then I also used velocity banded BC figures for the G1 standard for that bullet. You bullet G1 BC according to Berger is .300, that will be for a specific velocity say tested at 3000ftps. As your range increases you bullets velocity decreases and there for as G1 is measured in "velocity boundary's" the original figure you quote of 0.300 wil drop as you range increases and velocity decrease's.}

 

_{Basically at 350 yards that bullets will not have a BC of 0.300, the velocity will be less at that range and so the G1 BC falls and decays with it, a problem when using the G1 standard method.}

 

 

_{Hope this helps you out some mate, any question just ask.}

_Steve.

[The_Dogge]

When I ran it through applied ballistics with the figures above I got the 300 and 350yd drops spot on, but the 275 is indeed showing 2.75MoA.

 

As most of the measurements are perfectly matched by the calculated trajectory I would suspect that the 275yd drop is a measurement error of some kind, possibly vertical wind as people mentioned, and/or an error in range/target measurement, or in shooting that particular group. it certainly happens to me, and when all the other measurements align with predictions I would just shoot that range again, usually the unexplained result is not repeatable.

[BlueBoy69]

As 247sniper stated, the ballistic coefficient (BC) given is for a set velocity range and local met (temperature, density, water vapour content and pressure) using a specific drag law model. At best it's a rough fit and, with only one BC given by Berger, it's never going to give you the correct figures, just an approximation, as the BC changes over the whole velocity range and hence trajectory.

 

Each drag law model covers a different standard 'bullet' (originally an artillery projectile), with a specific shape (form). The most common model system used is that developed by the Aberdeen Proving Grounds in the early 20th century (with earlier development in Europe), this covers the G1, G2, G3, etc. to G7 model laws. In my previous MOD job we had a different model drag law systems, but I can't go into that.

 

Anyway, the bullet in question is then matched to the drag curve of the model using a form (fudge as we used to call it) factor. In the case of the commercial shooting industry, this is calculated as follows BC = sectional density (SD) divided by i, where i is the form factor of the bullet. The value for i derived from firings.

 

For the bullet in question, the 80 gr Berger FB Varmint, its SD is 0.194, its G1 BC is 0.300 and its G1 form factor is 0.647 (the form factor calculated by dividing the SD by the BC, the actual figure not given by Berger). As mentioned, this is for a specific velocity and met conditions, again that is not given Berger. A form fact below 1 means that the bullet is far better than the reference G1 bullet. Not really surprising when the reference bullet is flat-based 3.28 calibres long with a 2 calibre radius head (CRH) nose, the Berger is 3.76 calibres long and has a 7.5 CRH (presumed tangent) nose.

As your trajectory doesn't match up, it suggests their figures need updating, or they should give a better range of BCs to cover multiple velocity regimes (boxes).

[jcampbellsmith]

Grouse This forum would also be a place to pose your question - http://forum.accurateshooter.com/forums/ballistics-bullets-board.21/Regards JCS

[The_Dogge]

This is <300yd, the fine accuracy of the BC makes no practical difference, seriously.

Try it out, I just put in the figures above, then increased the BC by 10%, the difference at 275m was 0.08MoA, 0.22", less than the width of a bullet hole. The bullet just isn't in the air long enough for the difference in drag force to translate to a difference in time of flight and therefore drop, at these sort of ranges. This observation is not due to an inaccurate BC, I can be 100% sure of that.

Lets go and bark up a different tree. :-)

[LukeSoutherner]

So, I'm wondering if it's what both Dogge and Jcampbellsmith have both suggested around wind, and the terrain itself? Although the OP did say it was windless.

 

Worth another go to see if the same results occur but in a different location to shift the variables of wind and terrain maybe?

[Grouse]

From experience in the field I am aware that at below 350yard it's not a BC issue, even by fudging the BC in any app it still doesn't translate to my field results.

 

The range in question is right outside my house, so it's an easy one to replicate, I have shot rabbit and fox in the same situation over the last 8 years there, but never experienced the issue before. Admittedly it's only after missing several rabbits and crows over the last week that I did a test to see why I was missing over the top?

 

To my way of thinking a miss by 0.75 moa at 275yd is frustrating on pests, but it might have just been how the animal was positioned that gave the result,

 

Could be a ranging error, I will range back to house when I'm next at target, but could also be a bad hold by me? Ruled out atmospherics as it was a rare still day for Derbyshire last night!

 

Appreciate your thoughts and inputs...

[Swarovski1]

There is so many variables when u miss.atmospherics is enough to miss without human error in your shooting nevermind your ammo making.

[247sniper]

Like I said earlier in this post, ive seen and experienced it several times and you have a false zero!

 

Try zeroing at 200 yards ans the get back to us.

 

Steve

[gbal]

The posts have raised some likely sources of disparity.

A very common one on here often turns on the input data to the balistic program not matching the programs default setting-scope height is a common one,and some programs don't allow it to be changed.

The rest are equally familiar-velocity is not always accurate enough,and the now well known BC issues (BC is a continuous variable correlated with velocityand which BC to use.)

 

The second and perhaps less accepted issue is the firing data input may lack precision. Typically three shots are fired at some given range(s) and then the drop measured (as the average,or estimated 'group' centre). As we all know,shots disperse,and unless your rifle is a proven 1/4 moa-even if it is- then three shots is just not enough-5 minimum is better,near essential beyond 100y ( a 1/2 inch error may skew your results-suppose that three shots are fired at 200y (Steve is correct in prefering a longer distance) and they show just over an inch of vertical between them-just where would the fourth shot strike-we tend to assume (hope really) within the outer two,but that is wishful and really should be checked with more shots,because

if it's another 1/2 inch out,then our assumed drop is compromised....(an inch at 200 is very good for most rifles under field cconditons,perfectly possible for a semi custom rebarrel of course).

But you only do this once,best shoot a few more rounds and get more robust data to input,along with very careful measures of all the variables. There is really no other reliable way,and no short cuts.

(sometimes-as here perhaps-it's worth checking the 'non fitting' data (275y) which may be unrepresentative for all sorts of reasons,as replies have suggested.

Shooting is probabilistic-but each shot is determined by the physical conditions of/at the shot,some of which vary-as we all know but seem to begrudge reducing the error, with better samples;even two more shots (5) is much better,and it's false economy to stop at 3,at least for the initial critical settings.Best do it right (if you have to go back,conditions will have changed.......doh!)

 

gbal

[jcampbellsmith]

Like I said earlier in this post, ive seen and experienced it several times and you have a false zero!

 

Try zeroing at 200 yards ans the get back to us.

 

Steve

Agree. I've also found my 100 zero wasn't spot on sometimes. Sometimes I will check my zero at 100 yds before trying the gongs. Other times I like to try to hit the gong with my first cold bore shot.

 

Regards

 

JCS

[The_Dogge]

Like I said earlier in this post, ive seen and experienced it several times and you have a false zero!

 

Try zeroing at 200 yards ans the get back to us.

 

Steve

It's possible Steve, but the OP says that he has shot 4 groups at different ranges and 3 of them, including the 100 yd, are spot on the calculated trajectory. That isn't evidence that the zero is off to me.

 

I am a fan of zeroing at longer ranges, since errors in the short range zero are multiplied down range. Flip side is that unnoticed wind while zeroing has more effect.