Ballistics Tables not Consistent?

[ndking1126]

Here's the situation. At my local range, I shoot at a 440 yard target and a 620 yard target (distances provided by range and verified by laser range finder).

I'm shooting with my 260 Remington, 140 grain, with known good scope. I have shot long range for a little while, but have only more recently gotten serious about it.

Using jbmballistics and strelok for finding MOA adjustments, I find I need to adjust 38 clicks for 440 yards and 67 for 620 yards (they don't spit out the exact same number, but are reasonably close).

Actually shooting, I find I need to adjust 32 for 440 and 80 for 620. I understand that tables are just a starting point and you need real world dope to get hits, but I find it strange that one is over and one is under. I would expect if the table is off, it would at least be off with itsself. Example, for every two hundred yards, you have to adjust an extra 3 clicks to get the correct elevation.

I've played with all the temp, pressure, elevation, fps etc and can't get the results to fit without making entries that are obviously not correct. And even then, only one of the two distances will be correct.

Any thoughts? Am I expecting too much from the ballistics calculators? Just shut up and shoot? Haha, thanks.

[ndking1126]

Using JBM Ballistics, and inputting everything as accurately as I know the largest unknown is velocity. I have a 26" barrel and am shooting Remington off the shelf PSP hunting ammo (can't find powder!!). If I use 2460 fps as my velocity, I get 80 clicks. They advertise 2750fps.

The problem is, now 440 yards is 46. Again, I only need 32 to be on target. It's just strange.

[taylorce1]

You've answered your own question and don't realize it. Buying a chronograph and shooting over it will answer all the questions you're asking. Without knowing the speed of your bullets it becomes more time consuming to come up with accurate data. You have to send a lot more bullets down range to get an idea of what your set up is actually doing.

Investing in a chronograph will make your life so much easier. I find it amazing how many people try to get buy without one. They aren't a terribly expensive tool for any serious shooters toolbox.

[JohnKSa]

Are you sure your scope is adjusting uniformly over its entire adjustment range?

I wonder if the clicks are moving the reticle smaller and smaller amounts as you get closer to maxing out the adjustment range.

[Snyper]

Ballistic tables are only accurate with the numbers used, and don't take into account variable BC's and velocities, nor do they make up for disrepencies in scope movements

They are just a ballpark figure in most cases.

Your results WILL vary

[Blue Grass]

To paraphrase Col. Cooper: ' Ballistics tables are entertaining to peruse during a session on the porcelin throne.'

[emcon5]

Most likely muzzle velocity, but sight height is also important.

[johnwilliamson062]

You get clean holes at 620? Shooting across a level plane?
I too find it strange the difference is not consistent. Is the scope extremely old? I have seen some well used mics that will vary slightly over areas of their adjustment where they have been used the most.

[Bart B.]

Consider the following variables one contends with when comparing published data to what they get with their stuff assembled to what's listed.

Muzzle velocity:

* will be a few dozen fps faster for a given load and barrel when the barrel's clamped in a fixed standard receiver hard mounted in an unmovable stand compard to when shoulder fired with the same barrel in a rifle.

* average with the same rifle and ammo can vary 100 fps between two people.

* with a given load can vary 100 fps across several barrels of the same length; they all have different bore and groove diameters.

* varies with different amounts of neck grip on the seated bullet; more grip = more velocity.

* can be 50 fps or more between two lots of the same powder used in identically loaded ammo in the same rifle shot by the same person.

All software does not give the same ballistic numbers for a given bullet's muzzle velocity and environmental conditions.

All internally adjusted scopes, even the same make and model, do not have exact measurements of some angle per click. Their optics' lenses are not all the same exact focal lengths and their adjustments are not exact for click units' mechanical change of the erector tube's angle inside the scope. There's easily as much as a 5% or more error that is seldom noticed when making small changes. Measure you scopes' adjustment changes per click by clamping the scope solidly with it focused on a yardstick or ruler some distance away, then moving and adjustment 50 clicks then see if they reticule moved what's needed to mathematically be that amount on the measuring stick. The 12.5 MOA you thought fifty 1/4 MOA clicks would move it may well be 10.84 MOA. Or 13.02 MOA.

[ndking1126]

Lots of great feedback, I appreciate it.

Taylorce1, I should buy a chrono I agree. I just purchased a new scope for the rifle, so I'll put that next on my list. The scope was the last piece of the puzzle, so I thought... haha!

I hadn't considered the possibility of the adjustments getting smaller and smaller as I got closer to the scope's limit. This is entirely possible. It's an old Bushnell from I think the late 70s, so it's no where near cutting edge. It spent its life on a 30-06. I know its consistent as I went to the same range twice shot at the same targets and had to have the same number of adjustments even after returning to my zero at the end of the first trip. Since I am getting a different scope, I probably won't measure out this one Bart, haha, but that is a great idea. I can use it if I am still getting crazy numbers with my next one.

Johnwilliamson062, I was shooting at scuba tanks (10"x37") so i can't speak to how clean the holes were. I had a great rest, but wasn't hitting every time. I attribute this to inconsistencies of the ammo. Randomly I'd have a couple hit a foot low or so. I had a few horizontal misses that could have been wind, me, or just the fact that the factory ammo isn't extremely accurate.

Either way, I appreciate the feedback. Since I'm getting a new scope and moving (and won't have access to long range anywhere within a reasonable distance) I'll just have to guess that the changing adjustments are to blame. Thanks everyone.

[James K]

Shhh! Be quiet and I will tell you a little secret. Most data in those ballistics tables, especially at long range, is generated by computer, not by actually firing. Further, the firing that is done may be in an underground or indoor range, with no wind, cold, heat, or whatever else might affect a bullet. Then every rifle is different, and every lot of powder is different. Powder companies don't just whip up a new batch of powder using the same recipe; they actually make a general type of powder, then test each batch. The one that matches the closest to the standard for canister grade (the kind we buy) in that type is packaged up for sale. The rest goes to the ammo companies which have their own labs to test for the type and charge to be used to meet their published ballistics figures.

Jim

[Bart B.]

Jim, I've used Sierra Bullets software for several of their 30 caliber match bullets shot at different muzzle velocities (2400 to 3100 fps) to calculate down range zeros past the 100 yard ones. Calculated sight settings for 600, 800 and 1000 yards were no more than 1/2 MOA off reality. With both scope and aperture sights.

I have no idea how good other ballistic software is.

[Jim Watson]

I think the advantage of Sierra software for Sierra bullets is that it uses their sliding G1 BC vs velocity scale. Nobody else does because they want to advertise that big high velocity number. Same reason they don't use the boattail based G7, it is a smaller number.

Sierra toughs it out because they deal with a lot of target shooters who want good dope.

[kraigwy]

Ballistic tables are fun to play with and the MIGHT get you close, but nothing beats shooting different ranges and getting zeros for those ranges, writing the data down.

Hard to be a data book where EVERYTHING is written down.

[AllenJ]

I'll just have to guess that the changing adjustments are to blame.

No reason to guess at that as you can test that pretty easily. Sight your rifle in to be zeroed at 100 yards. Adjust the scope 8 clicks right and shoot a 5 shot group. Adjust your scope 12 clicks down and fire another 5 shot group. Adjust 8 clicks left and fire another group, then finally adjust 12 clicks up and shoot your final group. You should be back to being zeroed at 100 yards and by measuring the distance each shot in each group is away from where the rifle was originally zeroed, you can get a statical average of how much each click represents with that scope.

I would guess that your problem is not a single issue but rather a few. I ran into the same problem when I first started using ballistic calculators and have come to this conclusion: Instead of trusting what bullet company's, chronographs used in uncontrolled conditions, and computers are telling me, I work up my own dope for each rifle and load. It is more time consuming but in the end I know it is right.

[johnwilliamson062]

I asked if the holes were clean because if the bullet is destabilized for some reason the ballistic coefficient will change significantly. If you have a pretty stable bllet at 440 and a destabilized bullet by 640 the FPS might be faster than the charts at 440, but much slower at 40.

Just an idea. The odd misses wouldn't support the idea, but they also wouldn't disprove it.

[hartcreek]

Maybe you should get a different scope...one with mildots or multiple cross hairs. I went with a Gameseeker 3 so I just use the different crosshair instead of wearing out the turrets.

[Bart B.]

Watched a test some years ago comparing Berger's 30 caliber 185 VLD against Sierra's 190 HPMK. 20 or more rounds were fired alternately by the same shooter so the barrel's bore fouling and temperature were virtually identical for each pair of shots. Both left the .308 Winchester 26 inch test barrel at about 2560 fps. Two chronographs were used; one about 15 feet in front of the muzzle and one about 15 feet in front of the 1000 yard target; about 995 yards down range. The bullets went through the two 1/4" thick, 3-foot square foil-covered foam boards (start & stop trigger connections for the down range chronograph). Both chronographs tested less than 5 fps different in a velocity comparison for a given shot.

Subtracting down range velocity from that at 15 feet gave the velocity loss for the bullet at 995 yards; the important number. I don't remember what their down range velocities were, but somewhere in the upper 1100 fps range as I remember. Average velocity loss difference down range between these bullets was 50 to 60 fps.

Berger's 185 VLD G7BC is .284 and its G1BC is .549 as calculated by Berger.

Sierra's 190 MK G1BC's are .515 to .533 according to their calculations.

Why did the Sierra 190 have a 50 to 60 fps less velocity loss than the Berger 185 with a lower G1BC that Berger's?

Each company's software set for equal environments with bullets leaving at 2560 fps, their bullet going through the 995 yard screens are:
* 1082 fps for Berger's 185 VLD; 1478 fps loss.
* 1245 fps for Sierra's 190 HPMK; 1315 fps loss.

[30Cal]

Use the table to get on paper. Hang on to the wind drift numbers, but may as well throw the elevation tables away at that point.

Real data is real. Ballistic models are not. It's comical that they publish BC's to three significant digits.

[Bart B.]

30cal,

Calculated data from Sierra's software with their 30 caliber, 168 through 200 grain HPMK's, was within 1/2 MOA of my own actual shooting zeros at ranges from 100 through 1000 yards with both a .308 Win. and .30-.338 Mag.

The reason BC's are calculated to 3 places is because it lets people see the long range drop differences between perfectly balanced bullets and those very slightly unbalanced ones with less than 1% more drag. That causes elevation shot stringing.

Wind drift numbers with 4 digits (12.34", for example for a given bullet drift at 1000 yards in a 1 MPH crosswind) are ten times more significant than 3 digit BC numbers.

[44 AMP]

You should be back to being zeroed at 100 yards and by measuring the distance each shot in each group is away from where the rifle was originally zeroed, you can get a statical average of how much each click represents with that scope.

Yes, but that number will only be valid in the range of clicks that you used. It should be valid for all the clicks in the scope, but you can't be certain it is, unless you check them all.

The point about the scope possibly not being precisely uniform throughout its entire range of adjustment is a valid one. Particularly when you look at the extreme ends of the range and older scopes that weren't made to be target scopes, so it was rare for people to use that part of the adjustments.