Ballistic apps - something missing

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Scotchmo

Scotchmo

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Jan 9, 2016
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Most ballistic apps have scope location settings for horizontal (X) offset and vertical (Y) offset. They can be set to actual inches of offset if needed. Though I don’t use those, what I would use, but what’s missing is a similar setting for (Z) offset. That would be how far away the scope is from the muzzle. Drop is measured from muzzle, while angular adjustment of the turrets is measured from the scope. When that distance is large, it makes a difference. I have a couple of long range airguns where that scope to muzzle distance exceeds a yard. There is no way for the ballistic app to correspond at all distances without taking (Z) into account, yet they don’t do it.
 
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Gerry52 said:
Do you mean distance from bell end of scope to end of barrel? At first I thought you meant height of scope above barrel. Just wanted to clarify.
Gerry
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The “bell” is kind of arbitrary, so (Z) offset is measured from the erector tube gimbal to the end of barrel. The gimbal is where the focal plane and reticle image is located. It’s also the only location along the LOS that doesn’t change height when you adjust the elevation turret.
 
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Hmm , in all reality, yardage given to the app is typically given by way of range finder from the shooter. Which in most cases I would say is behind both, the muzzle and focal plane. I know I don’t add for the length of the rifle when taking a range for a shot. Maybe I’m weird, but that has never been something I’ve thought about.

In another note, ballistic curve is based on angle of barrel intersecting sight line. That angle is a constant in the equation based on whatever zero the shooter has chosen.

Just thinking out loud here and trying to conjure a reason that offset Z would be useful.
 
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Exodus said:
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Just thinking out loud here and trying to conjure a reason that offset Z would be useful.
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I measure my distance to target from the scope. Here is a hypothetical to explain the affect of a change in (Z) offset. If you add a yard to the barrel length but keep everything else the same (including velocity), the starting point of the muzzle will be higher, and the amount of drop will be less (shorter flight time). That throws off the calculation for clicks/holdover.

In an actual situation, the zero would be reset and that would bring the barrel angle down in relation to the shorter barrel, but it still leaves the one yard shorter distance and less drop.

It’s a small amount, but measurable.
 
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Correct me if I am wrong and not on the same subject, you think that if some type of measured relationship from the scope lense to the muzzle end is given it would make the ballistics apps more precise in relation to scope height? You have brought up something that I've been thinking about for a while now... Scope height matters and is hard to pin down with a precise mechanical measurement when angled rails, moa mounts, erector tubes, and lense location and dialing are involved, but it may be figured out backwards...

Albeit maybe in a bit different context because I have not been thinking about the relationship to the end of the bore for this, but instead somehow using the actual starting point of the arc to then get a true scope height measurement to plug into the app. This "beginning" point of the arc, somewhere just behind the shooter, seems like it is the basis for the app's ability to figure out the math and the scope height somehow gives it a reference to this point in time. A complete arc along a line. Line of sight being a constant, the arc starts somewhere by or behind the shooter and ends at the given target yardage at point of impact, the scope height sets up this arc's relationship to the trigonometry math in the app so a precise dope solution can be given to adjust the angle of attack of the arc to meet POI.

So what I was thinking, but haven't done the math on because that's not my suit, is true scope height in relation to the bore can likely be found by doing "backwards math" from the point of impact back to the scope using other known yardages, and thus finding the start of the arc. And so somewhere just forward of the start of the arc is the true scope height.

Backwards math from point of impact, and impacts at other closer known distances, coupled with projectile speed and bc, should give the location and height of the scope to bore centerline at the lense, and could also give you the location of the bore end and it's relationship to the scope line of sight. If I am thinking about this correctly, it's going to be in the math.
 
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Probably why I always have to slightly change inputs in the app to get actual drops to correlate as close as possible with the app. Though it seems drops are slightly off somewhere in the trajectory anyway as distance increases to ELR.

Airgun velocities and BC's are so low that a distance of a few yards can mean a miss due to the LRF being off or picking up something more reflective.

Yeah way above my wheelhouse in thought too.
 
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18.13 said:
I don't remember which version but I had a version of Chairgun that had what you are looking for
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I’ve got V2.2.2 but couldn’t find anything.

If you’re ranging from the scope turret and the end of the muzzle is one yard past that, isn’t the issue just the distance you’re using? IE rangefinder distance is 100 yards but trajectory distance is 99 yards.
 
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lbc_PSI said:
I’ve got V2.2.2 but couldn’t find anything.

If you’re ranging from the scope turret and the end of the muzzle is one yard past that, isn’t the issue just the distance you’re using? IE rangefinder distance is 100 yards but trajectory distance is 99 yards.
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If you always use the same equipment together, (the rifle, range finder, and app profile) and they were calibrated together to set up a profile in the app, then it would not matter what the actual distance is because the app program figures out the solution based on your prior inputs from the rifle setup and the ranging. However, if you mix up the data being inputted by using a physically measured 100 yards for setting up the app and then happen to use range finder that may call the measured distance 95.7 yards, then the data inputted won't give the correct answer for that particular shot. A range finder that is not precisely repeatable can the biggest effect on long range accuracy with air rifles; each yard matters at long range.
 
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steve123 said:
Probably why I always have to slightly change inputs in the app to get actual drops to correlate as close as possible with the app. Though it seems drops are slightly off somewhere in the trajectory anyway as distance increases to ELR.

Airgun velocities and BC's are so low that a distance of a few yards can mean a miss due to the LRF being off or picking up something more reflective.

Yeah way above my wheelhouse in thought too.
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I've noticed this as well. Particularly when setting up a "close" 40 yard zero and then getting the turret dialing range out to say 200 yards. I figure out the bc to there and have a fairly repeatable input at long ranges. However coming back into say 100 yards the point of impact may be 1 moa high. I have not dug too deep into that, like setting up multiple bc's yet, but I think that may be the correct way to go.
 
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Airgun-hobbyist said:
Correct me if I am wrong and not on the same subject, you think that if some type of measured relationship from the scope lense to the muzzle end is given it would make the ballistics apps more precise in relation to scope height? ….
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I have found the ballistic apps to be precise in relation to scope height. Though not so in differentiating target distance vs muzzle and target distance vs scope.

If you have MOA compensation built into the scope mounts, scope height should be measured at the scope gimbal. If you have straight mounts, then measuring height anywhere along the scope tube is fine. That assumes that the barrel is straight.

I have switched to better matching drag models in some instances and that eliminated the need for multi BC. Multi BC can be used to help true up a trajectory but it won’t solve the scope to muzzle (Z) problem.

I can subtract out the drop difference manually from information given by the ballistic app. Reducing my holdover by ((drop_at_target_distance) - (drop_at_((target_distance) - (Z)))). That puts my POI on the bullseye at all distances.

But that would be better handled by the app. I do have an older version of ChairGun that has a scope base entry, but not sure if it’s implemented correctly and it does not exist in other versions. It was limited to 36” max and I have two airguns that are greater than that. I’ve never seen it implemented in any other ballistic app. I still prefer Strelok for other reasons.

I can get closer results (but not exact) by taking the target distance measurement from the muzzle for far shots (past my zero=trajectory apex), and taking the target distance measurement from the scope erector tube gimbal for close shots (closer than my zero=trajectory apex).

I’m just talking about squeezing the last click (or two or three clicks) of accuracy out of the application. It’s not a deal breaker, but I don’t understand why it’s not included.
 
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Speaking to the current guardian of the MERO ballistic app (the original developer had to go into hospital) there is a barrel length input on that which he thinks is used for trajectory as well as the internal ballistic part of the app.

It may be worth trying to see if it makes a difference. When you download MERO some virus software objects to it, but thousands have been using it without problems.
 
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Any time I've had a problem with close shots being what the app says but not the far shots it was scope height was wrong. The best way I have found is not to measure from scope to barrel but say you are sighted in at 40yrds then measure exactly 10yrds from the end of the barrel see where you are hitting at 10 and ajust your scope height in your app to match
 
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Airgun-hobbyist said:
If you always use the same equipment together, (the rifle, range finder, and app profile) and they were calibrated together to set up a profile in the app, then it would not matter what the actual distance is because the app program figures out the solution based on your prior inputs from the rifle setup and the ranging. However, if you mix up the data being inputted by using a physically measured 100 yards for setting up the app and then happen to use range finder that may call the measured distance 95.7 yards, then the data inputted won't give the correct answer for that particular shot. A range finder that is not precisely repeatable can the biggest effect on long range accuracy with air rifles; each yard matters at long range.
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I would think the easiest solution would be to find the trajectory from two or three sight in points. After entering basic data like scope height pellet weight and drag model, the app should be able to calculate a velocity to match your sight in points.
 
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18.13 said:
Any time I've had a problem with close shots being what the app says but not the far shots it was scope height was wrong. The best way I have found is not to measure from scope to barrel but say you are sighted in at 40yrds then measure exactly 10yrds from the end of the barrel see where you are hitting at 10 and ajust your scope height in your app to match
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For field target purposes, I would suggest measuring to the scope bell. In FT, the barrel has to extend over the firing line and the trigger should be behind the firing line. If you calculate scope height based on measuring to the muzzle but then you extend the muzzle a foot or two towards the target, your dope will be off on the close shots. When I set up dope and yardage marks on a scope, I use the scope bell as my reference because it's easily repeated and it won't change the yardage marks if I move the scope to a different gun.
 
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