Home Forums Hunting & Shooting Ask The Gunwriters How "accurate" are online ballistics calculators?

Checking my Jarrett rifle today and saw some indications (at least for me) that my calculated ballistics did not match my real world ballistics by more than a little bit. I understand that they are only a guide and you should check the data against downrange results, which is what I did today. My rifle shoots a 280AI, 150 grain BT at an average of 3075 fps at the muzzle. I went back and re-checked all inputs to ensure correct figures and something is just not adding up.

I was 2" high at 100, and according to several calculators, I should be in the neighborhood of 13"-13.5" low, or thereabouts, at 400 yards. My 400 yard target today was sub MOA and exactly centered at 9" low. The difference between 9" and 13" at that range would be a clean miss if holding on the line of the back of a deer and dropping the shot into the vitals. Just curious if your verifications of ballistic data showed up with that much real world difference.

Couple of ideas....

Sometimes the published BCs are not as accurate as they might be.

There is a big difference between the G1 and G7 drag models.

I'm sure you already checked altitude, temperature, and humidity.

Are you sure that you were exactly 2" high at 100? Being 2 1/4" high at 100 will make a big difference at 400.

If all that is in order, then a 4" discrepancy is probably about as good as the math model will do.

An incorrect velocity when chronographed can do that.

Godogs, to my way of thinking a hold on the back w/9" drop would result in a high shoulder shot killing the animal. With the 13" drop it would result in a low shoulder shot which would also anchor the animal. Lots of variables to consider and they must be as exact as possible. powdr

Altitude has a surprisingly large impact on trajectory.

Some online resources are very detailed, and allow you many inputs. I have found them to be very close to actual trajectories.





P

Relative humidity and temperature influence how dense the air mass you are shooting through is and can be a source of error if either or both are a default setting on your ballistic calculator (I mean you can't enter them because the program doesn't ask for a value).

You're off by more than the all the normal variables would indicate. You certain the target is 400, and not 375ish? Beyond that, I have no idea.

Godogs,

As Denton mentioned, published ballistic coefficients aren't always accurate. For one thing, a single BC really doesn't work well for most rifle shooting beyond about 200-300 yards, because velocity changes BC.

Many listed BC's from bullet companies are derived from chronographing at the muzzle and at 100 yards. The difference in velocity is used to calculate the BC, but since muzzle-to-100 yard velocity is the highest for the bullet, the listed BC becomes less accurate at longer ranges.

This may be more than you want to do, but for my longer-range shooting I've found two books by Bryan Litz to be invaluable. First is APPLIED BALLISTICS FOR LONG-RANGE SHOOTING, which explains many things, including how complex BC really is, and why. The second is BALLISTIC PERFORMANCE OF RIFLE BULLETS, which lists the range-tested BC's for hundreds of bullets--but it isn't as useful without reading the info on BC's in the first book.

Neither is an inexpensive book, but together they've saved me LOTS of ammunition and time when testing loads beyond 300 yards. As an example, a couple of weeks ago I was finishing up some tests on a modest-magnification "dialing" scope. First I tested the scope at short range to see how accurate and repeatable they clicks were, then I tested it at the local 1000-yard range--but because of the magnification limit I only tested to 600.

I used the Litz info on that particular bullet and its chronographed velocity, instead of the manufacturer's published BC, because I'm pretty sure it's from 100-yard testing. The scope and load ended up being dead-nuts out to 600, hitting exactly where the ballistic program predicted, thanks to using very specific BC info.

You may or may not want to invest in the information, but it saves an awful lot of time and ammo in my work. And I would bet the Litz numbers would have resulted in far more accurate program results for your load.

Not telling, but rather asking, would a difference in scope height make a significant difference?

Depends on the difference, but here's an example, using a 165 Sierra GameKing from a .308 at 2700 fps in typical fall hunting conditions in my part of Montana, with the rifle sighted-in 2" high at 100:

1.5" scope height: -24.9" @ 400 yards
1.8" scope height: -24.0" @ 400
2.0" scope height: -23.4" @ 400

The range I shoot at has laser confirmed distances to the benches. The range is inside the rows of a pecan orchard which makes for great shooting conditions...no wind, consistent light, etc. My Talley bases and rings bring the scope up to exactly 2" over the centerline of the bore. Elevation is 250 above sea level. I'll do more range testing, but it sure seems like the load is outperforming the expected results since it is dropping much less at specific ranges. I didn't expect that since various bullet makers have been called out in the past for being overly generous with their published BC's.

Something is screwy, but it's hard to tell what.

A difference in 100-yard zero of 1" (+3") would make that much difference at 400, as would the 400-yard target actually being around 365. But apparently that's not what's happening, and your chronograph data would have to be WAY off to make a 4" difference.

It's not at all clear, given the variables of published ballistics, just how accurate these calculators are but this much I do believe.....it's a darn good starting point. One might have imperfect data to submit but it's still better than nothing at all. No matter what the calculators say, it's simply no substitute for actual shooting at distances

Years ago I simply sighted my rifles in at 2" high at 100 yards.....these days I sight them in to be 8" low at 300 yards.....

It's not at all important how you sight in your rifle.....only important that you know where it prints at the distances you are shooting......and if the range of the target is beyond your shooting data....just get closer or don't shoot at all......as the old axiom....trust but verify.

vapodog,

The three ballistics programs I use are all very accurate, and given the same inputs all produce the same results. That's because they're all based on the same ballistic information, which is pretty sophisticated these days, in fact far more than needed for 400-yard predictions.

Godogs,

I am assuming the CENTERLINE of your scope is 2" over the centerline of the bore. Is that correct?

In theory, both temperature and humidity affect air density but at 400 yards, the humidity will have no appreciable effect at all. Temperature will have some effect. For example, dense air at 10* F compared to thin air at 90* F would result in a 1" difference at 400 yards with that bullet. Those are extreme shifts too. I cannot think of anything that would make the difference he is having. My software shows a 15" drop using his parameters. To get to 9" I would have to have a parallax shift in my scope. You cannot correct the BC enough to make that up ( I used Litz G7 BC for my calculations). Velocity would have to be raised to 3450 fps or so. I don't think you can get their without a 7RUM.

I'd reverse engineer it by setting the "zero" to 400yds and -9". Then shoot the intermediate ranges and see how close you are.

So in essence, your off by 1moa, 13" - 9" = 4". 4" at 400 yrds is 1moa on the glass or in come-ups. IME, within the margin of error for most ballistic apps found on the computer...I use Hornadys. And that`s why I always shoot to comferm, sometimes it`s exactly on. most times I need to add or subtract elevation.
But it gets you on paper IME.

Hmm. I mostly use Berger's or Sierra's programs, and have never encountered a 4" difference in elevation at 400 yards--as long as I used good BC info. Before Bryan's book appeared, good BC info wasn't nearly as available.

I agree 4” at only 400 yards is a lot, which leads me to believe the input was incorrect.