How Much Shim Under The Scope Mounts?

[D.B. Cooper]

If your zero'd at 2 that wont change, since thats your known input.
Truing the data will change ALL the drop #s the calculator spits out before and afterwards, but they should corelate more precisely than previously.

I don't understand how that is possible, unless I'm (obviously) not understanding the "trueing" process.

For example: If I increase the MV so that my 300 yard hits are spot on using the 1.5 MOA reticle marker, then all of my downrange velocities will be incorrect. The app will apply the new MV across the board. Same is true if I change BC. It will apply the new (false) BC across the board, and that will change velocities and points of impact throughout my dope card.

I do get that 200 yards will still say "zero" and I will still spot on at 200 yards. but everything else will change.

I guess I'm most concerned about down rang velocity because, at least for hunting, I'm using that to determine my outer limit to take game. For example, if my new "true" dope card shows 2000 fps at 450 yards, (currently that's at 400), and I shoot a caribou at 450, the projectile won't expand because it's not really going 2000 fps.

 

[Varminterror]

It seems to me that, if I change either of those data points, it will upset the dope card up and down the scale.

That’s the point. If the estimated trajectory is not perfectly aligning to your actual trajectory, when you KNOW your environmental inputs are correct, then it points to an error in your ballistic inputs. Either your MV is wrong, or your BC, or both. The math really isn’t so complicated, and MOST of the variables we input into our ballistic engines are only used to look up or calculate (depending upon app structure) the air density applicable to the input conditions… very little math is actually being done to calculate trajectory, and nothing terribly complex.

So if that math is missing the mark, we tell it information - empirical inputs - to let it goal seek the proper ballistic inputs for us to align the calculated trajectory to our actual trajectory. That’s why we call it “truing,” and not something like “fudging.”


In other words, there is SOMETHING about what you are putting into the calculator which is wrong. If you narrow that search down to the ballistic inputs, then it’s really only MV or BC. The calculator can help you true your BC to align with your trajectory.

Adding: if you have short range disconnects, VERY commonly it is because you have an incorrect scope height in the calculator, OR because you really aren’t zeroed at your zero.

 

[Walkalong]

There are some good videos out there that explain how to do that. Since I use a Kestrel, I watch the videos that deal with it using the Kestrel.

Garbage in garbage out, but good data in gets excellent results.

 

[D.B. Cooper]

...VERY commonly it is because you have an incorrect scope height in the calculator...

I'll start there. I think I used the default height of 1.5", when, according to the mount manufacturer, it's 0.92"

 

[LoonWulf]

.92 for low reapers is top of reciever to center of scope. You need to add 1/2 your receiver dia to get your height above bore.....
So bout 1.6" unless i screwed something up......

 

[D.B. Cooper]

.92 for low reapers is top of reciever to center of scope. You need to add 1/2 your receiver dia to get your height above bore.....
So bout 1.6" unless i screwed something up......

I'll take your word for it. Those kinds of measurements are hard to make, which is why I went with the default.

 

[D.B. Cooper]

I might be chasing my tail on this. That first subtension is 1.5 MOA, not a generic 300 yards. 1.5 MOA for my rifle and bullet is, according to my ballistics app, 275 yards. The difference between 275 and 300 yards, again, according to the app, is 2.16" and I'm about 2" low, so...0.16" is not much of a concern.

Guess I could try verifying at the next two subtensions, 4.5 and 7.5 MOA which works out to be within .07 or .08 MOA of 400 and 500 yrds., respectively.

 

[LoonWulf]

Thats what id try, if its within acceptable spec for you then your good to go. I realize impact velocity is a concern, and if your drop figures are close then you can be fairly confident your impact velocities will be close as well.

Oh do you know your actual MV?

 

[Varminterror]

I'll start there. I think I used the default height of 1.5", when, according to the mount manufacturer, it's 0.92"

That .92” is the height from the top of the rail to the center of the scope.

The height you need is the distance between the center of the bore to the center of the scope. With the .92” ring height, your input dimension is quite likely right around 2”.

 

[Varminterror]

Oh do you know your actual MV?

^this

 

[Walkalong]

Oh do you know your actual MV?

That is huge if that is off, and published numbers aren't close enough.

 

[JFrank]

Just to stir the pot a bit more, the last time I checked the value of sub tensions, the scope magnification had to be set at a particular power.
Not all scopes are the same in this regard.

 

[D.B. Cooper]

...do you know your actual MV?

That is huge if that is off, and published numbers aren't close enough.

Yes. I have a chronograph, and I used a 10 shot average to arrive at the MV I put in to the ballistics calculator app.

 

[D.B. Cooper]

Just to stir the pot a bit more, the last time I checked the value of sub tensions, the scope magnification had to be set at a particular power.
Not all scopes are the same in this regard.

I'm aware of that, and such is this case with this scope. However, the way I use a scope is to set it to maximum zoom (in this case 12x) and leave it. I use it, essentially, as a fixed power scope. (I actually would have preferred a fixed 10x or 12x scope but the big name companies no longer make them.) At any rate, this shouldn't be a problem in this case.

 

[D.B. Cooper]

That .92” is the height from the top of the rail to the center of the scope.

The height you need is the distance between the center of the bore to the center of the scope. With the .92” ring height, your input dimension is quite likely right around 2”.

Changing that input to 2" changed the difference between the ballistic app's prediction and the actual amount of drop between 275 and 300 from 0.16 to 0.19, so it went in the wrong direction.

OS: It looks like we've this discussion going across two different threads.

 

[Varminterror]

Changing that input to 2" changed the difference between the ballistic app's prediction and the actual amount of drop between 275 and 300 from 0.16 to 0.19, so it went in the wrong direction.

OS: It looks like we've this discussion going across two different threads.

It sounds like we have a lot of “garbage in, garbage out” problems going on. Lay a ruler on the side of the rifle, measure between the center of the bolt to the center of the scope. Putting the wrong number into the app for scope height will never be fruitful, and is not an apt means of attempting to true your trajectory - because it NEVER will true up.

As I mentioned in the other thread, I think something is wonky here in how you’re trying to use the app. The conversions you mentioned for linear displacements are not aligning with the corresponding angular displacements, so none of it is actually making sense.

 

[D.B. Cooper]

It sounds like we have a lot of “garbage in, garbage out” problems going on. Lay a ruler on the side of the rifle, measure between the center of the bolt to the center of the scope. Putting the wrong number into the app for scope height will never be fruitful, and is not an apt means of attempting to true your trajectory - because it NEVER will true up.

As I mentioned in the other thread, I think something is wonky here in how you’re trying to use the app. The conversions you mentioned for linear displacements are not aligning with the corresponding angular displacements, so none of it is actually making sense.

I don't thin I have a reliable, accurate way to measure sight height. I can put a ruler on the rifle, but I won't be able to definitively say I measured from the center of the scope to the center of the bolt face. I just don't have a tool that will determine the center of those two objects.

 

[Varminterror]

I can put a ruler on the rifle

That’ll get you within ~1/16”, and that’s all I typically do for my rifles.

Opening the bolt and measuring from the center of the bolt body to the center of the scope can give you a better view, if you can’t quite tell where is center on the action/bore.

Or…

For round/tube type actions, pull the action from the stock, measure from top of the scope to bottom of the action (A), then measure diameter of scope at the same position (B), and measure diameter/depth of action at same position (C), then your scope height is H = A - B/2 - C/2

Or…

Measure from the center of the cocking piece at the back end of the bolt to the bottom of the scope objective bell above it (A), then measure the diameter of the objective bell at the same position (B), then the scope height over bore is H = A + B/2

But laying a ruler on the side of your action and scope will work just fine. Being off by a tenth of an inch isn’t terribly influential, being off by a half inch can be so - you’re effectively adding that height to an offset correction at your zero distance, so having the wrong scope height by 1/2” is the same influence to your trajectory as zeroing your rifle 1/2” high instead of actually zeroed… but your calculator wouldn’t KNOW it was zeroed at an offset. 1/2” error at 200 isn’t much, but it IS a click in your scope, and equivalent to a 2.5” error at 1,000.