Angle cosine and horizontal cant indicator for 762 gas gun?

[JoshNC]

Guys, I'd appreciate recs for an angle cosine and horizontal cant indicator to go on a precision 762 gas gun. I purchased one from Horus that incorporates both features, but found it to be too bulky and I think the mounting options leave much to be desired.

This rifle is my precision 762 rig with a 4-16x S&B that takes up quite a bit of real estate. I would like options that are as low profile as possible without compromising function.

[a0cake]

Guys, I'd appreciate recs for an angle cosine and horizontal cant indicator to go on a precision 762 gas gun. I purchased one from Horus that incorporates both features, but found it to be too bulky and I think the mounting options leave much to be desired.

This rifle is my precision 762 rig with a 4-16x S&B that takes up quite a bit of real estate. I would like options that are as low profile as possible without compromising function.

I've always had these issued and they've always worked well. In addition to the ACI, they make an ADI (Angled Degree Indicator) which may be useful if your preferred PDA ballistic software only takes degrees. I prefer the ACI because it's quicker to not have to do the conversion when you're just working off a chart and don't have batteries in the PDA. If I have plenty of batteries, I'll have a LRF with built in inclination / declination correction anyway, so the ACI is really a fail-safe. Thus, it makes sense to have it in the configuration that is best suited for having no electronics, which is clearly the ACI:

http://www.snipertools.com/aci-and-mounts/angle-cosine-indicator-mil-spec-version-water-proof.html

The Badger mount for it:

http://www.snipertools.com/aci-and-mounts/badger-ordnance-gen-ll-aci-picatinny-rail-mount.html


I know you didn't ask for it, and you may be tracking fully already, but here's a write-up I did previously that I'll post here just for anyone who might want to read it.

--------------------------------------------------------------------------

In this post, I respond to the following question, originally posed on another forum: "Would you please explain incline / decline shooting in a paragraph or two?"

--------------------------------------------------------------------------

Gravity obviously has a different effect on trajectory when shooting on a flat, level plane than it does when shooting up or down. When an angle between the shooter and the target is introduced, either up OR down, gravity will have less of an effect.

There are a few ways to account for this.

The first is called "The Rifleman's Rule." It uses the shot angle multiplied by the measured range to come up with an "adjusted range."

Range X C = adjusted range

5 degrees from horizontal C = .99
10 degrees from horizontal C = .98
15 degrees from horizontal C = .96
20 degrees from horizontal C = .94
25 degrees from horizontal C = .91
30 degrees from horizontal C = .87
35 degrees from horizontal C = .82
40 degrees from horizontal C = .77
45 degrees from horizontal C = .70
50 degrees from horizontal C = .64
55 degrees from horizontal C = .57
60 degrees from horizontal C = .50
65 degrees from horizontal C = .42
70 degrees from horizontal C = .34
75 degrees from horizontal C = .26
80 degrees from horizontal C = .17
85 degrees from horizontal C = .09
90 degrees from horizontal C = 0

So, if your target is 500M away, but at a 60 degree angle (up OR down), you could using this first method, multiply 500 by .5 and come up with an adjusted range of 250M. You would then treat the target as if it was 250M away and adjust accordingly.

The next way to solve the problem is to simply apply the cosine to the adjustment instead of to the range. This is called the Improved Rifleman's Rule. So if you need 5 MOA of adjustment to make a 500M shot with no up or down angle...but then use the scenario above where you have a 500M distance with a 50 degree angle, you would simply multiply 5 MOA x .5 and use 2.5 MOA of adjustment.

The two methods will usually come close, but DO produce different results -- and the second way is preferred.

The problem is that neither of these methods are all that accurate, especially as the angles become more extreme.

Good ballistics software is able to account for an incredible amount of variables, including environmentals and the ballistic performance of individual rounds. The best solution is to plug the data into a PDA for a guaranteed first round hit.

For general purposes, just keep in mind that you will have to aim lower for a given distance if there is inclination or declination involved.

Here's a graphic representation of what's going on:

http://www.exteriorballistics.com/siteart/article_images/art1_1.gif

http://www.exteriorballistics.com/siteart/article_images/art1_2.gif

[JoshNC]

Thanks a0cake. Much appreciated info.

[TheBelly]

http://www.amazon.com/gp/aw/d/B003V04JGW/ref=mp_s_a_1?qid=1361692630&sr=8-1&pi=SL75

Two-fer. Make sure your reticle is level, load your bipod, drop the hammer.

[TiroFijo]

An good accuracy comparison of the methods commonly used for correction of vertical angle:

http://www.exteriorballistics.com/ebexplained/article1.html

[JohnnyC]

Flatline Ops (http://flatlineops.com/products/levels) has combo cant/cosign/degree indicators.

That's the levels, and you pick which degree/cosign indicator you want in the accessories page.