TERRY DON'T BE MAD AT ME!!!

[rjohns94]

Ballistic charts show bullet drop estimates for an assumed 1.5 inch sight over cl of bore. What if the sight plane is higher? How is this factored into the equation? For match federal 308, a 1 inch high at 50 yds yields dead on at 200 yards. But that is sight plane at 1.5 above center line of bore. I have a 56 mm objective on my long range 308 rifle. It sits on top of a bull barrel (tapered) and I want to know the drop difference. The scope has forward parallax range compensation based on the match 308 ammo and the rifle centered to POA at 200 meters. I need to figure out how high I have to hit at 50 yards with the added height if I am to hit at 200 meters. Mixing units and deviating height above bore is confusing me at this late hour. Height of line of sight above bore is 2 inches.



Thanks

[w30wcf]

Mike,
PLug your data in here including the scope height and it will give you the information you need.
http://www.hornady.com/ballistics-resou ... calculator

w30wcf

[El Chivo]

By all means check the ballistics calculator; however, my understanding is the scope height difference only comes up once; meaning, if your new scope is 1/2" higher, then you subtract the same 1/2" regardless of the range. Meaning you adjust by 1/2" at 50 yards, 200 yds, or 1000 yards.

If you adjust a tang sight or receiver sight to be 1/2" higher, that cants the rifle, and your point of impact changes more for longer ranges and less for close ranges. But the scope is still level, it's only sitting 1/2" higher. The trajectory is exactly the same as it was, only it starts off 1/2" higher.

Here's a brain teaser for you - shoot offhand with both feet on the ground, and note your settings. Then rest your left foot on a brick, raising it 2 1/2" off the ground. What adjustment do you have to make to your point of aim at 200 yards to hit the target?

[AkRay]

Us math challenged types would probably find an answer to the question by sighting in to hit dead on at 200, then we'd tack up a target at 50 to see where it hits there, while wondering why the original question was asked in the first place.

[AJMD429]

Draw an arc representing the bullet path, and draw a straight line intersecting it in two points. That is your line of sight intersecting the bullet path, first where the bullet 'rises' to cross it close-up, then where it drops to cross it again out where you're "sighted-in".

Then draw another line starting 'higher' at the muzzle-end of your trajectory, intersect it at the same second place. That represents the change in sight height. Theoretically it would slightly increase your 'point-blank range', as it will be ever-so-slightly more 'parallel' to the downslope of the trajectory beyond that point.

...theoretically...

[rjohns94]

The question as to why ask the original question is answered here. I have a 50 yard range indoor that I belong to. The longer range is further away. It is only 100 yards Therefore, I would like to start the process of getting the scope where I want it. It requires the scope set at 200 meters. So my process would be set the scope up at the 50 yard range, setting it IAW ballistic predictions then check it at 100 yards. When I get to a longer range, I can then get it dialed in at 200 meters. So, in theory one half inch high at 50 with the higher scope would put everything right Scopes range calibrations are in meters so probably will need fine calibration but not much after that. Thanks

[El Chivo]

well, I believe here if you aimed normally with the higher scope your shots would be 1/2" lower at 50 yards, and also 1/2" lower at 200 yards.

BUT if you adjust the scope to hit as before at 50 yards, you'll be canting the rifle, and the difference will be 4x greater at 200 yards.

So if you want to move your 200 yard point of impact by 1/2", then move your 50 yard point of impact by 1/8".

AMJD I'm still trying to imagine the effect of what you pointed out. But the trajectory is independent of the line of sight, so it seems to me it wouldn't change unless you started changing the angle of the rifle to the scope.

As to why the question was asked in the first place, I think it's always good to understand the theory behind what you're doing. This is exactly the kind of question that should be asked on a shooting forum.

[Pitchy]

Where is that old geezer, i miss the ole grump.

[Nath]

So,,,,,,,, if the scope tube is pararell to the bore,,,,,,,,, how does the bullet cross the sight plane?


N.

[rjohns94]

Took the rifle to the range today. Got it shooting one hole and sighted it in at just under 1 inch high at 50 yards. The rifle is a GA Precision Crusader, built on their Templar action, Bartlein ss 5R rifling #7 GAP contour 1:11.25 twist, mcmillian A5, badger ordnance trigger guard with box magazine. Contour of the barrel is 1.200 inches @ 3 inch from action to .920 inch at muzzle. Remington x2 trigger at 2.5#. All finished in matte Cerakote OD. Springfield 4x14x56 first gen scope mounted in badger ringers on a solid 20 MOA rail. Ammo was federal match king 168gr Overall, was super pleased and impressed with he operation. I have on order a silencer with quick mount that will match the barrel contour and color, titanium of course. This is going to be a great ground hog rifle. Lol

[Griff]

Mike,
IMO, that's no half-baked question. My math skills are shy at best, and I know I couldn't explain it without the help of the specific equation, nor demonstrate it except to say that without that bit in the equation, you'd only get measurements of deviation from the bore-line, not the line-of-sight. If you'd like, I can send you a copy of a "basic" (don't remember the specific version) ballistics generator program I wrote back in the 70's. But,, you'd have to find a 'puter than can still run "Basic" programs.

[rjohns94]

Used the ballistic program w30wcf recommended. For standard measurements, it came out .6" above 0 at 50 yards for a zero at 200 yds. Since I need meters, plugged it in and it came to .708" at 50 meters. 50 yards is 45.72 meters so I zeroed the scope at .7 inches above zero. Should have it close.

[Ben_Rumson]

Just for grins plug the numbers in that program for 1.5" scope height & see if the difference is worth worrying about... Or put another way how much you would have to compensate for the half inch difference.. & let us know...

[El Chivo]

Ben, I'll try it with Remington "Shoot".

30-30 Remington with 150's, and a 1.5" scope height. Zeroed at 200 and 1.98" high at 50.

Second time same but with a 2" scope height. Zeroed at 200 and 1.61" high at 50.

Difference of .37" or about 3/8". Nearly the full half inch. Go figure.


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If I run it again staying zeroed at 50, then seeing where the bullet falls at 200 with two scope heights:

with the 1.5" scope, bullet is 7.92" low

with the 2.0" scope, bullet is 6.42" low,

which is a difference of 1.5".

Funny, I thought it would only be 1/2" different. Could it be that, with the end of the trajectory being steeper, taking it 1/2" lower allows the bullet to drop proportionally more?


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If I try it with .243, which is more of a laser-beam trajectory:

1.5" scope, zeroed at 200, and .40" high at 50

2" scope, zeroed at 200, .03" high at 50

Still a difference of .37"

Now, staying zeroed at 50, seeing where the bullet falls at 200:

with 1.5" scope - 1.61" low at 200

with 2" scope - .11" low at 200

A difference of 1.5", same as with 30-30. So it doesn't have to do with steepness of bullet drop.

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I thought that changing it 1/2" at the muzzle (and zeroing at 50) would result in a 2" change at 200. Yet it's only 1.5". Could it be that it's because the bullet is only traveling an EXTRA 150 yards, after passing 50? That zeroing at 50 negates some of the 1/2" change at the muzzle? If changing at the muzzle results in a 4x change at 200, that's a difference of .75 when you make it 50 yards (instead of the muzzle).

And when you reverse the two, the difference at 50 when you want to keep the same hit at 200, I'm still surprised it's that much. But note it's .75 of the half-inch difference at the muzzle.

COINCIDENCE? I DON'T THINK SO

I'm going to try this using 100 yards and 200 yards instead of 50, and see what the proportions are.

[El Chivo]

OK, zeroed at 200, with 1.5" scope hits 3.66" high at 100 yards


Zeroed at 200, with 2" scope hits 3.41" high at 100 yards

difference of .25" - half the difference at the muzzle


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Now, with the 1.5" scope, zeroed at 100, bullet hits 7.32" low at 200

With the 2" scope, zeroed at 100, bullet hits 6.82" low at 200

difference of .5 inch, same as the difference at the muzzle.

Well I can't figure this one out, except that it seems to have to do with range only, not velocity or bullet weight or altitude or rotation of the earth. And it's inversely proportional, note that if zero range and target range are both 200, all values are zero, no matter what the scope height.

[Nath]

So,,,,,,just out of interest, if the scope is pararell to the bore just how does the bullet cross the sight line?

Is there an inclination on the rifles action?

Is there an inclination in the scope?

Open sights induce an inclination of the sight line to the bore, no?

N.

[Ben_Rumson]

Nath...On modern scopes the cross hair adjustment causes the shooter to incline the firearm because the cross hairs are shifted downward when adjusted to raise bullet impact... You incline the rifle automatically as a result to put the cross hairs on POA...When you run out of elevation adjustment you shim up the rear of the scope...

[Nath]

Nath...On modern scopes the cross hair adjustment causes the shooter to incline the firearm because the cross hairs are shifted downward when adjusted to raise bullet impact... You incline the rifle automatically as a result to put the cross hairs on POA...When you run out of elevation adjustment you shim up the rear of the scope...

Sorry Ben, no disrespect friend but thats to simple an explanation for me!

If the scopes tube is pararel to the bore how can any cross over be achieved?

The bullet will start below the sight line and only ever fall away, no?

Is it not the case that on open sighted barrels the rear sight is higher than the front thus giving some elevation, no?.

The very notion a ring is shimed when more adjustment is needed suggests some inclination away from pararel is needed, no?

The point I wish to make is, if a scopes sightline is as open sights as in there is inclination to cause a crossover point any subtle difference betweem makes etc will cause these pooter programs to be out, no?

N.

[Ben_Rumson]

“The bullet will start below the sight line and only ever fall away, no?” Correct

”Is it not the case that on open sighted barrels the rear sight is higher than the front thus giving some elevation, no?.” Correct

”The very notion a ring is shimed when more adjustment is needed suggests some inclination away from pararel is needed, no?” Correct

“If the scopes tube is pararel to the bore how can any cross over be achieved?”
Short answer : There wont be any cross over as long as it remains parallel to the bore and there’s nothing wrong with the scope ... The scope tube must be adjusted off parallel as you say to achieve the wanted POI and then there will be a cross over.. and that’s why adjustable external mount were made...Modern scopes move the target image within the scope to adjust the sight line for drop and windage... and as I said before the tube can externally remain parallel to the bore..

"The point I wish to make is, if a scopes sightline is as open sights as in there is inclination to cause a crossover point any subtle difference betweem makes etc will cause these pooter programs to be out, no?"
Yes.. Computer programs wont help If you don’t aim true .....

[Nath]

So to sum up then, an image moving scope must all ready have the image mis-aligned with that of the scope tube in order to achieve a convergance with the bores line, no?

If so, any variance from manufacturer to manufacturer will throw a pooter program out!

N