Formula for adjusting aim? Specrte DR scope

[Jarnhamar]

I have an Elcan Spectre DR.  The range adjustments are built into the scope and the scope is designed to be used with an M4.

Is tere some kind of formula I can use to adjust for the difference when using the scope with a C7?

Like, if I'm using a c7 and my target is 300 meters away should I use the 400 meter increment int he scope since it's designed for carbines?

At ranges of 0 - 300 meters will it make much of a difference anyways?

 

[Fusaki]

A related question:

Given that a sight with a built in BDC or range dial is designed around a specific weapon with a specific barrel length and a specific ammunition, how far off is the issued C79 (presumably designed for M855 out of a 20" C7 barrel) when used with a 14.5" bbl C8 or a 16" C8A2?

 

[Fishbone Jones]

I think that this could be better and more definitively answered if one of you guys contacted Elcan sales or tech services and asked them.

However, some others of us with these scopes, would like to know also. So please post anything you find out.

Cheers.

 

[Fusaki]

I didn't call Elcan, but I did poke around a bit and came up with this:

http://www.usaac.army.mil/amu/ServiceRifle/M16-M4%20data.pdf

According to the above, with M855 and a 200m zero, the bullet will strike 7.83" below point of aim at 300m with an M16, while it will strike 10.5" below point of aim at 300m with an M4. 

The difference, therefore, is negligible.  The range dial on your SpectreDR should be good on both the C7, C8, and SFW weapons systems.

Theoretically, you should aim 2.67" high with the range dial on your Spectre set at 300m, but can you even shoot that well?  I know I can't.  I'm pretty sure with a C7 that kind of accuracy isn't even possible.

Which makes me wonder why Trijicon bothers to market the ACOG TA31 in both M16 and M4 models...

http://www.trijicon.com/user/parts/products1.cfm?PartID=476&back_row=4&categoryID=3
http://www.trijicon.com/user/parts/products1.cfm?PartID=481&back_row=4&categoryID=3

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A side note:

Given this information, I can't see myself even bothering with the range dial on the C79 while shooting the PWT3.  At the 300m mark, all I have to do is hold 8" over centre of mass and I should be just fine.  I think whether the range dial is practical for combat or not is also up for debate.