6.5 PRC Ballistics - drops don’t match ballistics chart

[Mram10us]

Copy. Thanks. Just wanted to verify you shot groups and not just one.

 

[WiscGunner]

One shot at each distance. Then I set the scope turret to 9.75 MOA and shot 3 rounds prone off the bipod. 3 Shot average impact was 12" high

What bullet are you using?

What BC are you using for that bullet?

 

[dfanonymous]

I still don't understand why the drops do not correlate with the ballistic trajectory. Since the scope wasn't dialed between shots, it was not part of the equation. The only thing I can guess is that my bullet was shooting flatter out to 500y and the 600y drop was an anomaly. If I had shot groups I might have gotten more confidence in the data.

There's a lot to it when you really get into.
Your charts are always a prediction (key word) to drop. Mathematics do not take into consideration: human error, and changing conditions.

Human error can be broken down to info put in into ballistic software, technique in shooting or affecting the harmonics of the gun.

Changing condition can be node changes on a cold barrel, environmental or bore conditions, ammo etc.

Next is even if your chrono is a magnetospeed or labradar and your bc is taken off the box, doesn't mean that any of it is without errors. Hence why everything needs to be verified and trued to meet real world results.

The bullet doesn't lie. It's better to true out to 700-800 yards and work your way back in. Then again we'll beyond 1000 in my opinion.
Lastly, taking accurate environmental readings is also important to assist in quantifying your charts.

 

[Canhunter35]

One thing about a thread like this, when you get an antagonist like x47guy, he really pulls out great tidbits of information from the wealth of experience on this site. Despite the ensuing argument derailing the thread, the op is still getting a debate and information being shared that otherwise would've remained silent

 

[Rflshootr]

Here is a little video that may help understand why......remember, not all rifles are the same. Small things that you wouldn't think about such as your mount system, scope height above bore, make a difference. Chronos aren't always dead on the money for velocity. Small differences change things. B.C.'s aren't exact from lot to lot, die to die, or even within the same box there are slight variations because of manufacturing tolerances and production methods. If they were, you'd never have to sort bullets. You have to shoot to get accurate dope.
http://bulletin.accurateshooter.com/2019/11/practical-d-o-p-e-video-offers-tips-on-ballistics-data/

 

[jdmecomber]

I went the 100y range today. Temp 83F. I started with a POI test with the last zero I used in the colder temp and higher elevation. The impacts were high by the same amount that I altered my zero for the elevation and temp change.

I did a tracking test on the VX - 6 scope. At 10 MOA the POI was 10.75", 20 MOA = 22', 25 MOA = 27.375". I also tracked R 5 MOA= 5.375", L 5 MOA= 5.5". Returned scope to zero and POI was good.

I also checked MV of frozen cartridges vs warm. The MV cold 3047, 83F warm 3089. The POI shift between warm and cold was 0.5 Up and 0.8 L.

My take on all this is that nothing is wrong with the scope, rifle, or mounts. The environmental changes caused the POI shift. I will need to re-zero again when I get to Montana.

I still don't understand why the drops do not correlate with the ballistic trajectory. Since the scope wasn't dialed between shots, it was not part of the equation. The only thing I can guess is that my bullet was shooting flatter out to 500y and the 600y drop was an anomaly. If I had shot groups I might have gotten more confidence in the data



It looks like your scope is over travelling. Make sure you adjust the turret values in your ballistics program. Ballistics aps are not greated equal when adjusting these values.

It looks like your scope is over travelling. Make sure you adjust the turret values in your ballistics program. Ballistics aps are not greated equal when adjusting these values.

 

[Ingwe]

There's a lot to it when you really get into.
Your charts are always a prediction (key word) to drop. Mathematics do not take into consideration: human error, and changing conditions.

Human error can be broken down to info put in into ballistic software, technique in shooting or affecting the harmonics of the gun.

Changing condition can be node changes on a cold barrel, environmental or bore conditions, ammo etc.

Next is even if your chrono is a magnetospeed or labradar and your bc is taken off the box, doesn't mean that any of it is without errors. Hence why everything needs to be verified and trued to meet real world results.

The bullet doesn't lie. It's better to true out to 700-800 yards and work your way back in. Then again we'll beyond 1000 in my opinion.
Lastly, taking accurate environmental readings is also important to assist in quantifying your charts.

I don't know if you realized it or not but you posted a great quote:

"The bullet doesn't lie."

You should trademark that one

 

[ForgivenAll]

An interesting thread, to say the least...

If my math is right, it does not look like your click values match up to what they should be. Another way to check this is to use your etched reticle shooting via hold-over and compare that with the equivalent turret adjustment.

Once turret tracking is weeded out, since you have measured your muzzle velocity accurately and have entered all of the other pertinent info in your ballistic solver, then, have you ever considered that the published ballistic coefficient is slightly different coming out of your barrel compared to the factory test barrel? Each bore is different.

Since the bullet doesn't lie, once you have established your proper elevation hold (whether by reticle or turret adjustment) then tweak your ballistic coefficient to match your point of impact.

 

[John Spadaro]

I did notice that the POI for each shot when testing the scope matched the subtention on the reticle. Meaning if I had dialed in 10MOA, the measured distance between the POA and POI was 10 MOA as measured by the reticle.

I am not sure how to make use of this information.

 

[ForgivenAll]

Hey, that's good news! That potentially weeds out your turret tracking question.

Now, on to getting your ballistics solver to match what your bullets are actually doing... what I have had to do on my rifles, and I think consistent with others, is use my measured muzzle velocity in the solver (since that value is a known quantity and is highly accurate with that Magnetospeed), then just modify my ballistic coefficient until my calculated drop matches my actual drop. At least then you are covered out to the farthest distance tested. Extrapolating beyond there is closer than before but ultimately requires field verification.

 

[WiscGunner]

I am not sure it has been discussed yet but if the OP is using a SFP BDC reticle and comparing this type of Guesstimate Reticle to fixed value (even with error) turret clicks this is just another added issue to the compounding issues.

 

[WiscGunner]

The TMOA reticle is only intended to be "accurate" at max magnification. At ANY other magnification will be off and math will be required to compensate.

It is also important to note Leupold
has had issues in the past figuring out which MOA they use. In my opinion, to be confident in a Leupold, a person needs to measure the reticle click values AND measure the reticle values to see what is actually installed in the scope and IF they match. Most people know Leupolds have a poor track record of accurate clicks but the math gets thrown right in the trash when your have an MOA reticle and Shooter MOA turrets (with error). When the OP knows these things for certain from testing himself then he can decide what to do (ignore the error, record the offset in the APP or get a new scope).

All too often assumptions are made that longer ranges are just a click away, and they can be but only if the correct data is used and not assumed.


Hopefully the OP can get it figured out.

As ForgivenALL, BC needs to be trued more than velocity. The Magneto Speed has an error of up to .5% or 15fps in this case. Start with this as a given and adjust the BC (correctly entering G1 or G7 value) to more closely match your trajectory. Beyond 1000yrds the muzzle velocity will also need to be slightly tweaked.

The OP has not said which bullet/BC combo he is using but I am sure if he does plenty will chime in with data that worked for them. Some manufactures like Berger, Hornady and Sierra provide fairly accurate BC's that only need a little tweaking to compensate for muzzle velocity (BC changes in flight), muzzle break, barrel (bore size, rifling, chamber). Other brands need major adjustments to compensate for the optimistic marketing of BC's


As the OP is now aware, the powder will produce different velocities at different temps. This temp data should be used to build separate "rifles" in his APP to be used in corresponding weather.

If all other data such as scope height over bore, actual twist rate (can vary) and bullets dimensions are input in the APP correctly then the last assumption to remove from the data is weather. I highly recommend the WeatherFlow wind meter for under $100. It works with several apps such as Hornady 4dof but I prefer the user interface of BallisticsARC (free).

 

[Chase723]

I am sure Aaron does not know how to shoot...

Seems like there is plenty of good info in this thread, within it I'm sure you'll find your answer. This post is in response to what has become a subtopic of it. POI change with change in position. It does happen but it's not because of magic, and it can be mitigated. More than anything it's a perceived change in POI from the POA. It's not actually changing because it's migrating or the barrel whip or harmonics are changing.

Experience has taught me that it's dictated by a couple things. 1) Cheek weld/sight picture 2) paralax 3) Shoulder pressure/bipod loading pressure and subsequent resultant MV variation/recoil management. 4) Cant can also effect it but the horizontal impact difference is much more pronounced.

The larger the cartridge/more recoil energy the more #3 matters. I've found that once I'm conscientious of all those things the POI shift between positions is pretty negligible. Personally, I do load development and zero in the prone and never look back.

In the Gunwerks video posted, that difference at 1000 yards can be due to a 15-20fps MV variation... which can happen when you go from prone to the bench or vice versa.

 

[John Spadaro]

John

Here is a Vx6HD out of the box with a solid zero....

It was dialed 32 moa and travelled 35.38 inches. 32 moa is 33.504 inches.

My scope dialed to 25 MOA shot 27.375" 25 MOA = 26.175", so my scope is dialing 4.6% high. How do apply this knowledge in a field situation? I haven't used a ballistics calculator that has this correction. I have the Sig BDX rangefinder, the Hornady 4DOF, and now the Strelok Pro.

It looks like the Strelok allows for an adjustment of the MOA click value. So in this case a 1 click = .27375" is that correct?