Author Topic: scope height and trajectory  (Read 2940 times)

Offline wolffire99

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scope height and trajectory
« on: May 17, 2010, 11:07:53 AM »
Correct me if I'm wrong, but the only way you can really calculate your trajectory is if you take your scope height into consideration.  The higher the scope, the higher angled up the bore will be, thus creating more arch than a lower height scope.  Should we be doing everything we can do get every last mm out of our scope rings to get as low as possible?

Offline Orionblade

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Re: scope height and trajectory
« Reply #1 on: May 17, 2010, 12:26:31 PM »
Assuming your mount is at a slight angle to the axis of the bore, you have a given angular elevation between the bore axis and the scope axis. In order to increase elevation, you need a higher front ring, while the rear ring stays lower. If you increase the height of both rings, then you're moving the scope further from the centerline of the gun, but it remains parallel (to within whatever tolerances you can maintain) to the same axis it had a few mm lower on shorter rings. The increase of, say, a half inch between low and medium or high rings on your average weaver rail means your scope line of sight intersects the path of the bullet a few mm higher, and thus a bit further out for a given powder charge and bullet weight.

While the bullet is always falling downward as it leaves the bore, but your bore is typically inclined relative to the scope and mount, it appears to come up, cross your line of sight, and then fall back down again. IIRC, most rifles of 30-ish caliber are set up so this happens at something like 50 and 100 yards. So, with a higher scope mount you're talking about adding perhaps an inch to your first crossing point and subtracting that same inch from your second crossing point, so when the bullet passes up through your line of sight, it's actually at 49.95 yards, and when it comes back down it's at 100.05 yards. You can change that crossing point even more significantly by switching to heavier or lighter ammunition, or a bullet with a higher or lower ballistic coefficient, so I wouldn't worry about ring height eating up elevation.

Look at an AR-15 in an A1 or A2 configuration - that carry handle is monstrously high off the bore line... AFAIK the only benefit of an M4 or A3 upper (other than the M4 feed ramps) is that the flat top reciever can extend a rail from your eye all the way out along the free float tube to the gas block, so you can put a 2 foot long train of optics out there - scope, IR, thermal, or a red dot with a magnifier on the back end and infrared on the front, etc. etc. etc.

For height, though, you can improve cheek weld and make the gun more comfortable to shoot. The only time a height issue should be considered for accuracy is when you're talking hitting a target over 800 yards away and you've run out of elevation adjustment with your cheapo wal-mart scope. At that point, an elevated rail (again, at an angle) would help quite a bit.

That's just what I've learned from researching my mosin nagant scope mount project, so someone else chime in if I missed the mark.
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Offline Muddyboots

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Re: scope height and trajectory
« Reply #2 on: May 17, 2010, 12:44:29 PM »
Not bad, Orionblade, not bad at all!


The short answer Wolffire is no. We should be mounting the optic in a place where it will allow the following:
1) unobstructed line of sight (LOS)
2) reasonably comfortable and repeatable body mechanics
3) Unobstructed operation of the scope and rifle mechanics

We don't need to keep the scope low, we need to know what the scopes center hight is to help figure out WHERE the scopes LOS crosses the trajectory.

Since the internal adjustments of a scope (expressed in Minutes Of Angle - MOA) are limited, you want the zero point of the scope to begin as mechanically close to the desired rifle zero. This should also allow the reticle to be as close  to the optic center of the lens train as possible. This is why real long range scopes have things like extended adjustment range in 30mm tubes and even require mounting on sloped bases (20 moa or even more) Wow it gets complicated fast!

The reason we attach optics the way we do is operator convenience. The scope could just as easily be attached to the other end of a frame and viewed through a video setup. like, for example, an Apache chin gun. As long as everything is consistent in it's relation, it will work provided the math is correctly understood and executed.

I'm not sure that helped.

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Offline Orionblade

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Re: scope height and trajectory
« Reply #3 on: May 17, 2010, 12:50:12 PM »
Believe it or not, according to a recent interview I read with Ronnie Barrett, the guys over in iraquistan are hanging aimpoints/red dots off the side rails of their 50 cals so it's easier to engage close in targets while moving than with the monster huge scope hanging off the top. Hell, out to 100 yards it'd be hard to aim with one of those even on a bench rest, but I'm thinking the elevation would more than max out to bring it in from nice and sweet at 1500 yards to nice and sweet at 150.
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Based on thorough experiments involving kissing in the rain, exposing shoulders to direct sunlight, and dancing by the light of a silvery moon,  I have found that, within the bounds of frostbite and decency, hapiness is inversely proportional to the amount of clothing worn.

Offline wolffire99

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Re: scope height and trajectory
« Reply #4 on: May 17, 2010, 02:05:51 PM »
maybe this illustrates it better than I can describe. 


Offline joeinwv

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Re: scope height and trajectory
« Reply #5 on: May 17, 2010, 04:02:07 PM »
Scope height has no relation to trajectory. The trajectory stays the same if the scope is mounted 1" off the barrel, 6" off the barrel or no scope at all.

From your picture, your high scope rings would have a rear ring mounted much taller than the front. A high rear and a low front would give you a strange zero.

I think what you are looking for is information on maximum point blank range - or point blank zero.

In most cases, mount the rings that give you the most comfortable and repeatable position. Set your zero 2" high at 100 yards. With most centerfire rifles, this will allow you to take a lethal shot at a coyote or larger animal from 0-300 yards with no holdover.

Offline Orionblade

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Re: scope height and trajectory
« Reply #6 on: May 17, 2010, 04:10:24 PM »
Also, from that sort of geometry, your scope rings would have to be about ten feet tall.



This is more realistic.

The front "ring" is typically "lower" relative to the bore line than the rear, but that's the incline of the rail relative to the barrel. You're talking a degree or two, not the 20 degrees or more seen on long range guns like mountain howitzers and antitank rifles that fling projectiles in large integer fractions of a mile.

If you move the scope axis up six inches off the barrel in real life, that would be represented by slightly less than one pixel in that above image. That would move the line of sight crossing points by about two pixels, if that answers your question.

The trajectory doesn't change at all, only the points at which the LINE OF SIGHT crosses the trajectory line. move that line of sight all over the place and realize that out of the 200 yards represented by the whole length of the image, only the first yard is occupied by barrel, and the scope is only a pixel or two long.
You can't run away on a world that's round.
You're only comin' back to where you'll be found.

Based on thorough experiments involving kissing in the rain, exposing shoulders to direct sunlight, and dancing by the light of a silvery moon,  I have found that, within the bounds of frostbite and decency, hapiness is inversely proportional to the amount of clothing worn.

Offline ZenGunFighter

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Re: scope height and trajectory
« Reply #7 on: May 17, 2010, 05:50:34 PM »
The higher the scope is above the bore, the flatter the trajectory appears.
When the Chevy Truck Sportsman's Challenge was popular, tricked out 10/22 with scope risers of 6-8" were common.

I'm not a rifle guy, so I haven't studied the reason it is so. There is an explanation on page 23 here; http://viriato.net/airgunning/bfta_setup_manual.pdf
 
If anyone wants to check it, use a ballistic computer that allows input for sight height above the bore. put in two differnt heights, say 3/4" and 2" and look at what it does to the flight of the bullet.

I'll dig out my Pact computer later and run some numbers for you.
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Offline Orionblade

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Re: scope height and trajectory
« Reply #8 on: May 17, 2010, 10:25:00 PM »
It shouldn't do anything to the actual flight of the bullet, though I know what you're talking about and I think that's the point of confusion with the OP.

The apparent flattening is the closing up of the distance between the two crossing points, or "zeros" - if you zero a rifle for a certain distance, there's a second, longer range, where the rifle is also zeroed. If you raise the scope up six or so inches, then those two points might only be tens of feet apart, and very far down range - that might be really kick ass for when you're shooting targets that are all a fairly set distance away, especially if they're steel knock-down targets that you don't need a bullseye on, and whose angular radius is less than the MOA difference encountered in the range between the two zeros.

Shit.

I might have to try this out, but that sounds like it could be REALLY effective.

SHIT!

OK so like +1 and stuff. I have math to do....

 ;D


But in terms of making your scope hug the bore vs. stand an inch above, you're not going to notice any ballistic or range performance difference except that which comes from getting a more comfortable cheek weld. A zero is a zero until you start stacking monster spacers in there like he said.

DAMN maybe this would be a good scope setup for olympic shooters? or do they just use iron sights... *ponders*
You can't run away on a world that's round.
You're only comin' back to where you'll be found.

Based on thorough experiments involving kissing in the rain, exposing shoulders to direct sunlight, and dancing by the light of a silvery moon,  I have found that, within the bounds of frostbite and decency, hapiness is inversely proportional to the amount of clothing worn.