Riflescope height

[airplanejoe]

Ballistic software always ask, as one of its inputs, the scope centerline height above the bore centerline. 1.5 inches seems to be the standard answer. But, what is the best way of physically measuring this dimension?
aj

[NightFisher]

I don't think the measurement need to be super accurate so I normally use a 6" steel scale and measure from the centerline of the scope to the centerline of the barrel.  I believe you can eyeball that to within 1/16".

[2 clicks low]

Depends on the rifle.

1 stick a rod down the bore and holding it centered measure to the bottom of the scope, this is A.

2 measure the diameter of the scope and divide by 2, this is B

3 take the diameter of the rod and divide by 2, this is C.

The number you are looking for is A+B+C

[airplanejoe]

Depends on the rifle.

1 stick a rod down the bore and holding it centered measure to the bottom of the scope, this is A.

2 measure the diameter of the scope and divide by 2, this is B

3 take the diameter of the rod and divide by 2, this is C.

The number you are looking for is A+B+C

Makes sense, but why would it depend on the rifle? The rifle in question is a rem 700, but I see no reason why it wouldn't work on an AR style rifle??

aj

[2 clicks low]

Depends on the rifle.

1 stick a rod down the bore and holding it centered measure to the bottom of the scope, this is A.

2 measure the diameter of the scope and divide by 2, this is B

3 take the diameter of the rod and divide by 2, this is C.

The number you are looking for is A+B+C

Makes sense, but why would it depend on the rifle? The rifle in question is a rem 700, but I see no reason why it wouldn't work on an AR style rifle??

aj

Try getting your caliper in the ejection port of a 10/22.

[airplanejoe]

Why would I want to create a computer software ballistic profile for a 10-22?
aj

[2 clicks low]

So you can adjust your sights for MOS (minute of squirrel) from 5 to 200 yds with a variety of ammo.

[jmdavis]

Go to the Hornady Ballistics Calculator and run the numbers, making changes of 1/16 or 1/8 to the scope or sight height. What do the results look like out to 600 yards?

Ballistics data is good. But the only sure way to know "your" rifle is to learn your rifle. I have run KD events where a participant shot his 10/22 out to 300 yards. At 100 and 200 his accuracy was at least as good as anyone shooting a Centerfire that day. He did this with nothing more than a 25m zero that he had obtained the day before at the RBC we were doing. At 300 yards the drop was too much (it is around 12 feet) but he wasn't the only person to fail at 300 yards that day. He was just the only one shooting a .22lr.

Learn how your rifle performs in various conditions. Do you know the effect of heat on your zero? Do you know the effect of light on your zero? Do you know the effect of a 7 o'clock wind switching to 5 o'clock on your zero? What happens in high humidity? What happens in the rain? What happens with a target 1/2 in deep shadow? You won't learn these things from a ballistics program. You learn them through practicing the act of shooting a good shot.

Ballistics data is good. But it is at best only part of the story.

[NightFisher]

I went to the ballistic calculator I use and changed the Sight Height from 1.5" to 1.6" and with the .308 load I use it changed the predicted drop at 600 yards by about a half inch.  That is less than 0.1 minute.  I am usually pleased when my actual results are within a couple of minutes of the "predicted" trajectory.

[jmdavis]

I went to the ballistic calculator I use and changed the Sight Height from 1.5" to 1.6" and with the .308 load I use it changed the predicted drop at 600 yards by about a half inch.  That is less than 0.1 minute.  I am usually pleased when my actual results are within a couple of minutes of the "predicted" trajectory.

That's the point. Minor variations in measurement aren't that important inside 600 yards.

[navybowhunter]

I've experimented with this and the "Shooter" App for my LR UKD stuff.  It does not need to be exact.

Prove it yourself, by changing height measurements, and then checking dope.  It won't make much of a difference if you are within a quarter or better yet 8th of an inch.

What is also cool, in the "Shooter App" full version (not sure if in free one or not), is once you shoot at different distances, you have the option to enter "true drop", meaning, if the software said for 800 yards come up 11 MOA (just making #'s up here), and your TRUE shot dead center was 10.25 MOA, you can adjust for the MV variation.  It is best to use all 5 (you'll see if you play with it) and give it say a 350/500/650/750/900 TRUE come up.  Then see what happens!  Way cool.

R/
Chris

R/
Chris

[crobjones2]

Chris
can you give more details on the "shooter app?" who is it by? version?
I would like to see what it gives me for .308 hpbt 168gr with a scope over bore of 4.5"
Thank you
Chris

[navybowhunter]

crobjones2:

The "app" called "Shooter" can be found in the App Store on your smart phone.

It is developed by "Kennedy Development Group" and I believe they have a free version.

You can also "google" JB Ballistics, that is a free online ballistic app, that has a TON of features as well.  You could check your 4.5" bore height there as well.

R/
Chris

[VAshooter]

I don't know much about Computer "apps" but I have shot a rifle.

How does the "app" handle a variety of projectiles? In my experience the projectile has a large effect on what happens down range.

[jmdavis]

I think that one uses weight, velocity and BC to make the calculation (modified by sight height). But, only Sierra provides BC for different velocity ranges (which can vary significantly). Most other bullet manufacturers only provide one average BC across the range of velocity.

For example here is the data for the Sierra 168gr Match King.
size            wt     SD                     Ballistic Coefficient
0.308   168   .253                    .462 @ 2600 fps and above
                                                 .447 between 2600 and 2100 fps
                                                 .424 between 2100 and 1600 fps
                                                 .405 @ 1600 fps and below


Compare that to the Hornady 169gr AMax

Ballistic Coefficient (G1)   .475
Sectional Density   .253

Or another Hornady bullet the 168 BTHP

Ballistic Coefficient (G1)   0.450
Sectional Density   0.253


One Might assume better performance with the AMAX, and one might even get better performance with the AMAX, but we don't know what velocity that BC is based on.

The point is that your solutions are only as good at the data that you input. The more accurate the input data the better your solution. But all of that being said, if you are limiting yourself to 400 to 600 yards the difference between G1 and G7 will be less than the difference between 2100 fps and 2650 fps (as an example).

There is one sure way to know how YOUR bullet and load will perform and that is to shoot it and test it.

You guys should get VAShooter to tell you about the difference in group at 100 yards between his .308 handloads and Federal Gold Medal Match out of the same gun and with the same conditions.

[drifter52]

jmdavis-

That velocity range of BC's that Sierra publishes for its bullets is something I've been wondering about.

For example, for their 69 gr SMK Sierra shows data of:
Size: 0..224
Weight: 69
SD: .196
BC: .301 @ 2800 fps and above
      .305 between 2800 and 2200 fps
      .317 @ 2200 fps and below

For IMI's 69 gr 5.56 Razor Core round which uses this bullet I've measured average muzzle velocity from my 16.25" AR barrel at 2828 fps.  Hornaday's on-line ballistics calculator indicates that the bullet's velocity drops to 2200 fps at 225 yards, and drops to 1558 fps at 500 yards.  So for a 400 or 500 yard shot the bullet passes through all of those BC ranges.  Most ballistics calculators only provide for entering one ballistic coefficient, so what's your take on which BC to enter into the calculator - the BC at the muzzle velocity, the BC for the velocity range that the bullet will be traveling the greatest distance at, or some type of average?  Or doesn't it matter all that much? 

This may be more of an issue for, say, the .308 175 gr SMK shooting out to 1,000 yards where the BC changes from .496 to .485 at about 525 yards where the velocity drops from  MV of 2621 to below 1800 fps.  Or, again, doesn't it matter all that much?

I'm sure its obvious I don't handload, but I'm trying to get the get the best calculated ballistics charts to take to the range for the factory loads I'm using.  I'd be interested in hearing your thoughts on this.  Thanks.

[jmdavis]

Good question, the BC will change in flight, but it is the initial velocity that has the most effect on the path, I think. By the time that you get to 500 yards you are already over .5 seconds into flight.

The point the 69 data brings up is that sometimes BC increases as velocity decreases. This data was proven well over a century ago by Black Powder Cartridge shooters. They learned that a heavier bullet moving slower was more accurate than a lighter bullet moving faster at long range. Part of the secret to Black Powder Cartridge shooting is avoiding the trans-sonic issues of the bullet slowing below the speed of sound. They avoid that with lower velocities. That is one of the differences between BPCR and Modern cartridge ballistics and theory.

[JoeZ]

Sight height over bore can also help you calculate how much your group center should be beneath your aim point to get a 200 yd zero on any rifle only shooting at 25m.

Formula: sight height over bore( in inches) minus 1.25 inches equals distance impact group center is below point of aim.

We used this formula very successfully at a Racine WI kd shoot. All 18 shooters "zeroed" at 25m and all group ceters were within 1 moa at 200 yards.

Examples;
AR 15 with Armalite scope mount; 2.5" - 1.25" = 1.25" (Impact is 1.25" below point of aim.)
AR 10 with Burris PERP mount; 2.75" - 1.25" = 1.5"
M1A iron sight; 1.25" - 1.25" = 0" (this has 2 caveats, actually measures a bit less and if you use a 6 o'clock hold, impacts must be 2moa higher than actual point of aim.)