Ballistic Calc vs. Actual, question in here somewhere...

[AccelR8]

A little background info first:

The rifle is a 6.5 Creedmoor. I have previously only shot it as far as 500 yards. At that time it had a target scope on it and I was using a ballistic calculator app on my phone. The drops at various yardages were pretty near spot on.

I began having trouble with that calculator so I switched to another calculator. I also switched to a different scope because I wanted to get away from the 1/8 moa dot and fine cross hair of the target scope. I wanted a reticle with MOA sub-tensions.

Fast forward to now. I took the rifle out to 1,000 yards. In fact this is the first time I've ever shot anything beyond 600. First I did a tall-target test. The scope tracked vertically just fine and the math said the clicks were almost exactly ¼ MOA, with a .2% error (two-tenths of one percent). I zeroed the rifle at 100 yards.

Rather than try to talk MOA or number of clicks I guess the most concise way is to talk inches. I will describe my actual point of impact relative to the point of impact predicted by the calculator. So, at 200 yards my impact was 1 inch high. At 300 it was 3” high, at 400 5” high, 500 8” high and at 600, 9” high.

These results don't worry me much as in my opinion the differences are not that great. I can say it's due to a bunch of little errors stacking up, errors in actual velocity, actual distance, actual BC, weather and so on. No big deal.

But what has me scratching my head is this; when I moved to 1,000 yards my actual impact was 114 inches LOWER than predicted. At all other distances I was high and then going from 600 to 1,000 it flip-flopped and now I'm low, and by a substantial amount.

I've been racking my brain over this. I have checked and double checked my notes, and thinking did I make an error in counting my scope clicks? I've pretty much eliminated that as a possible explanation, and also my first shots at 1,000 after I dialed up to the predicted MOA and number of clicks, my first shots hit the ground well on front of the post holding the steel target. Easily 8 or 9 feet low.

In case you're thinking I was at the top of my scope travel and was topped out, no that wasn't it. I still had lots of “up” remaining and had no trouble dialing up more to get my hits centered on the target with “up” to spare.

So I guess my question is, um, WTF? Is this normal?

 

[Lone Hunter]

You don’t have something inputted correctly on your ballistics calculator. You need to check your muzzle velocity scope sight height BC. Basically if you put in junk you get out junk. Not trying to be a smart azz but it’s generally your inputs are not correct. Try counting in MOA instead of clicks. Too much counting at distance. Go back and double check your inputs

 

[AccelR8]

You don’t have something inputted correctly on your ballistics calculator. You need to check your muzzle velocity scope sight height BC. Basically if you put in junk you get out junk. Not trying to be a smart azz but it’s generally your inputs are not correct. Try counting in MOA instead of clicks. Too much counting at distance. Go back and double check your inputs

Thanks, I've done all that. Scope height is correct. MV is correct or at least what the chronograph says it is. BC is what Sierra publishes. Every piece of info I have to input I've triple checked and if there's a mistake I'm not finding it.

One of the problems I had with the old calculator was every time I would use it I'd find my scope height had changed to 4 inches! I'd reset it to 1.75 and the next time I'd open it, yup 4 inches again. That's not happening with this calculator.

 

[AccelR8]

Chronograph data was obtained with a Magnetospeed. I am going to shoot with a friend who uses a Labradar and get data to compare to the Magnetospeed data. Will do that in a week or two. But for now, it seems to me I'd have to have a pretty big error in MV to cause this and the Magnetospeed data appears to be valid, meaning it is about what I'd expect to see given the load data.

 

[Lone Hunter]

What type of ballistic calculator are you using? I run Strelok Pro and Shooter. If I don’t put the right inputs in then the data will be wrong. Sounds like something is not set properly on your calculator. Try setting up your inputs on JBM. It’s free and pretty reliable.
http://www.jbmballistics.com/ballistics/calculators/calculators.shtml

 

[AccelR8]

Well, it's Strelok Pro but I don't want to bad-mouth the calculator. It has a very good reputation and I'm sure the trouble has something to do with me, my use of it and not the app itself. I guess.

 

[rebeltex]

Was it a for sure known 1000 yd range?

 

[cheek9075]

Are you setting the temp, humidity, altitude? Have you use the the trajectory validation feature?

Also I believe the BC will change as speed changes. My rifles usually are pretty close up until 600 yards or so, then I use the trajectory validation, it get really close after that.

 

[AccelR8]

I could not get a reading with my rangefinder, I did get good readings on the other ranges (200-600). As the day got brighter, I could no longer get a reading on 600 either. My rangefinder is a Nikon which I've had for like 25 years now. But, I've read it's 1,000 yards (Thunder Valley), I've been told it's 1,000 yards by other shooters, and Google Earth shows it as approximately 1,000.

 

[AccelR8]

I am setting Temp, Humidity, and altitude. I have not done trajectory validation. Would it make that much of a difference?

 

[Lone Hunter]

You get your true muzzle velocity by knowing how many MOA you had to dial to hit at 1000 yds. There is a chart on Strelok Pro that gives that feature. You put in what you dialed to get to 1000 and it will give your muzzle velocity. Look in bottom right of main screen and you see a target with an arrow up and down. Click on it and you will see this. It is called Trajectory Validation.

 

[mac86951]

BC is an average for a velocity range. It's a unitless coefficient; Sierra is usually good about publishing BC for velocity ranges; but atmospheric conditions can change it.

Check all your inputs and use published bc at 100 based on muzzle velocity; then tweak BC at 1000 to match known measured MOA scope adjustment. This adjusted value is your average BC over velocity from muzzle to 1000y. Check your calculator at 400. Should be a little high since velocity average for the distance is higher.

It's Ballistics not rocket science. Accurate data makes accurate calculators. Expect all your "numbers" to change with temperature, barometric pressure, altitude, and wind, oh and the compass direction you're shooting, and spin drift. The rest of us work to get our loads and data accurate enough for the rest of this stuff to be a bigger factor than the loose but behind the stock.

-Mac

 

[AccelR8]

You get your true muzzle velocity by knowing how many MOA you had to dial to hit at 1000 yds. There is a chart on Strelok Pro that gives that feature. You put in what you dialed to get to 1000 and it will give your muzzle velocity. Look in bottom right of main screen and you see a target with an arrow up and down. Click on it and you will see this. It is called Trajectory Validation.

Thanks for that. I just did the trajectory validation. Before TV, the app had my temperature corrected MV at 2,765 fps. After TV the MV says 2,404 fps.

This is 42 grains of RL16 and a 142 SMK. My barrel is 24 inches. At 50 degrees F the 2,765 fps seems believable (I had 2,784 at 85 degrees). The TV corrected mv at 50 degrees of 2,404 seems low.

If I use the 2,404 and look at the table of elevation MOA it's now way off. For example I actually dialed up 11 MOA at 600 yards and the table now says 17.2. At 300, my actual dialed was 2.5 MOA and the table says 5.1 MOA.

I don't think the 2,404 can be correct. I think trajectory validation is not the whole answer, I've got a problem somewhere else.

 

[CharlieNC]

First of all for ballistics no one talks or thinks in inches, you need to think in either moa or mils which are angular measures. If shots are all high out to 600yd then it is most likely the 1000yd is actually further. As mentioned you can use the good known dope out to 600yd to true the velocity using Strelok, then make a range chart to compare your "1000" results to find out the actual distance. But how did you determine 114" low, that's an extremely large target.

 

[AccelR8]

Interesting, if I do my TV at 600 with my 11 MOA actual, I get Strelok gives me a MV of 2,767.

 

[gman47564]

i would verify the distances to begin with... 200 might be 175. 300 might be 250 .. 600 might be 500... exact distances is important...

 

[338 dude]

Thanks for that. I just did the trajectory validation. Before TV, the app had my temperature corrected MV at 2,765 fps. After TV the MV says 2,404 fps.

This is 42 grains of RL16 and a 142 SMK. My barrel is 24 inches. At 50 degrees F the 2,765 fps seems believable (I had 2,784 at 85 degrees). The TV corrected mv at 50 degrees of 2,404 seems low.

If I use the 2,404 and look at the table of elevation MOA it's now way off. For example I actually dialed up 11 MOA at 600 yards and the table now says 17.2. At 300, my actual dialed was 2.5 MOA and the table says 5.1 MOA.

I don't think the 2,404 can be correct. I think trajectory validation is not the whole answer, I've got a problem somewhere else.

 

[AccelR8]

I'm just gonna start all over again. Hopefully this week I can get the Magnetospeed and the Labradar together and get a consensus on MV and go from there. Check zero and shoot it again at the various distances, log the data and see where I wind up.

I tried the BC validation and it returned .866 which is obviously not right. I've got a fly in my ointment somewhere.

 

[rammac]

Actually rocket science IS ballistics.

Past 600 yards BC values will have a big effect on the accuracy of your ballistics calculator and most manufacturers don't provide very accurate BC values, but Sierra at least breaks their BC values down in to velocity ranges (0 to 2000 FPS, 2000 to 2800 FPS, a 2800 FPS and above). At a minimum you need to use a ballistics calculator that accepts multiple BC values and use them. Bryan Litz's book Ballistic Performance of Rifle bullets 3rd edition, lists quite a number of bullets and their BC values that he developed using radar. He breaks the BC values down in to different ranges than Sierra (0 to 1500 FPS, 1500 to 2000 FPS, 2000 to 2500 FPS, 2500 to 3000 FPS, and above 3000 FPS) and he lists both the G1 and G7 BC values.

I don't know which bullet you're using but if you take a 140gr Sierra Matchking as an example and compare the 1000 yard drop using Sierra's BC values and Bryan Litz's G7 BC values you see a difference of 4.5 inches of drop. That's assuming that you used all the BC values and not just an average. If you used the average G1 value vs. the average G7 value there is a 5 inch difference at 1000 yards. With your exact bullet there may be more or less of a difference but there will be a difference.

 

[AccelR8]

View attachment 1205012

Hmm, I'm sure I got .611 BC for my velocity range off the Sierra website. So there's one thing I can correct.