Computed vs Actual Ballistic Data?

[wbyfan1]

Gents,

I went to the range yesterday to verify drops/trajectory out to 500 yds utilizing a load for my new 240 Weatherby. I'm planning to order a custom dial for my Leupold scope and simply wanted to verify computed vs actual drops. I anticipated a small delta between the two, but was surprised to find they were substantially different. The actual's were much flatter than what JBM's on-line calculator came up with. My load utilizes a 95gr Nosler Ballistic Tip at 3450fps. The velocity was verified with my chronograph.

Below is what I came up with utilizing a 100 yd zero and the average drop of 4 shots at each distance from 100-500 yds. I've seen small discrepancies before, but never any this large. It was a little windy during the range session, but it was primarily a quartering head-wind of about 10-12mph, which I figured would increase the drop just a skosh.



JBM/ Actual

100 0/ 0
200 -1.9/ -1.0
300 -7.9/ -3.0
400 -18.6/ -13.5
500 -35.0/ -25.5

Have any of you folks seen a difference this large between computed and actual drops? After seeing this, I'm just a little concerned the dial Leupold sends won't be very accurate. It's my understanding they take your actual muzzle velocity(if you know it), bullet BC, avg temp, elevation
and create a dial. If this is the case, mine will be wayyy off. What do guy's think?

 

[c. schutte]

How far is your scope over the center line of the bore? I've never used the custom dials from Leupold so keep that in mind, what I say could be totally useless. It could be that your trajectory is a little high further out than what you computed data is expecting or better said you are a little higher further out. Try entering your 200 or 300 yard drop value so that both the computer and your actual drops equal at that range and see if that solves it? If you could get honest drops past that then you could work backwards coming closer.

While I do not use those programs I do go out and verify actual drops, I trust that data. And I would also do that during the anticipated weather conditions of when you expect to hunt.

 

[OU812]

I only use computed drop values as a starting point to verify actual drops. Real actual drop values trump computed. Shoot the actual drop values a few times before getting your turret cap made. I don't use the CDS dials as they are only good for 1 bullet, 1 speed, 1 BC, 1 altitude. MOA is what I use and it's good for every speed, bullet, bc, altitude ect...

 

[wbyfan1]

I only use computed drop values as a starting point to verify actual drops. Real actual drop values trump computed. Shoot the actual drop values a few times before getting your turret cap made. I don't use the CDS dials as they are only good for 1 bullet, 1 speed, 1 BC, 1 altitude. MOA is what I use and it's good for every speed, bullet, bc, altitude ect...

Agree on the CDS comment, but I picked up this scope at a price I couldn't pass on, besides, once I work up a load, I rarely change to something else.

 

[Rocketvapor]

Your drop @ 200 and 300 is pretty remarkable.
Just think if you zeroed at 250 to 300yd.

 

[MAinAR]

Have seen this before with 100 yard zero with ultra fast loads. Very slight difference in what you measure or call zero can have great effect at longer ranges. Zero at least out at 200-300 and you may see this go away. Dial down to 100 then check.......

 

[Elkman]

I buy the standard 1/4 per click dial, then build a range card to my rifles spec's. I have my data memorized out to 400 yards, and if I shoot farther than than that, the animal is usually standing, which gives me time to look at the card.

 

[TackDriver284]

How far is your scope over the center line of the bore?

That would be the first thing I would check.

 

[Polaris]

I think your 300 is an outlier. Doesn't follow the calculus (magnitude of change in both predicted and actual). Something else happened there. Maybe a deflection updraft. What was your mirage doing?

And counterintuitively, a headwind will actual raise your POI. Wind crossing into your rifling twist will also raise POI up/right (assuming RH twist), tailwind will also raise POI. Crossing LXR will change POI down/left.

 

[wbyfan1]

How far is your scope over the center line of the bore?

That would be the first thing I would check.

1.5 inches

 

[Rocketvapor]

With 4 shots at each range averages to give a drop, what was the grouping like?
Would more shots give a better center?

 

[.300winmag]

What BC are you using? Nosler'a or someone else's? What chronograph? Did you use elevation or air pressure or both? Also do your best to measure your actual scope height, if you have telescoping gauges, it's quick and very accurate.

Finally, set a target at 100 yards, shoot a group and then dial up 10MOA and shoot another group using the same aim point, then dial another 10 MOA up and shoot another group. Measure the distance between groups, many scopes don't move what they're supposed to.

Ballistic programs are very accurate tools, but they're only as accurate as the data you put into them. Measure everything and don't assume or take anything for granted.

 

[Rocketvapor]

.300winmag

Take your favorite ballistic program and try and find a trajectory that comes close to the Actual (not the JBM values) of bullet drop in the first post.
I mean like make up some values and get those drops.
I don't know the environmental conditions but use some thin air like 85F, @ 3500ft.
Use the B.C. for the 105gr Berger instead of the 95gr Nosler (0.545)
Use a higher velocity like 3650.
With a hundred yard zero the calculated 500 yd drop is close to actual drop at 500 yds.
Better bullet, thinner air, higher muzzle velocity, is needed to get the same 500yd drop.

What's the program drop at 300 yds? More than 1MOA?

Try and find a program fit for 100=0, 300= -1MOA, 500= -5MOA.

I think something other than program values is going on.
Bullet lift, POI determination from 4 shot groups, ranging?

 

[.300winmag]

.300winmag

Take your favorite ballistic program and try and find a trajectory that comes close to the Actual (not the JBM values) of bullet drop in the first post.
I mean like make up some values and get those drops.
I don't know the environmental conditions but use some thin air like 85F, @ 3500ft.
Use the B.C. for the 105gr Berger instead of the 95gr Nosler (0.545)
Use a higher velocity like 3650.
With a hundred yard zero the calculated 500 yd drop is close to actual drop at 500 yds.
Better bullet, thinner air, higher muzzle velocity, is needed to get the same 500yd drop.

What's the program drop at 300 yds? More than 1MOA?

Try and find a program fit for 100=0, 300= -1MOA, 500= -5MOA.

I think something other than program values is going on.
Bullet lift, POI determination from 4 shot groups, ranging?

I see what you're saying, and I don't disagree with you. What I was trying to convey, is the ballistic programs are dang good, and to be off by such a huge margin, something has to be off. What, I don't know, just suggesting starting with start with the program and the inputs you put into them make sure a simple typo isn't the culprit.

 

[wbyfan1]

What BC are you using? Nosler'a or someone else's? What chronograph? Did you use elevation or air pressure or both? Also do your best to measure your actual scope height, if you have telescoping gauges, it's quick and very accurate.

Finally, set a target at 100 yards, shoot a group and then dial up 10MOA and shoot another group using the same aim point, then dial another 10 MOA up and shoot another group. Measure the distance between groups, many scopes don't move what they're supposed to.

Ballistic programs are very accurate tools, but they're only as accurate as the data you put into them. Measure everything and don't assume or take anything for granted.

I'm using Nosler's G1 data of .379 and a CED Pro Chrono. Yes, I used my local elevation and also input the baro pressure and temp from an airport weather service nearby. As for measuring scope height, I don't have a telescoping gauge to measure it, but feel the method I'm using is somewhat close.