A friend has a fixed handle A2 style upper with a scope installed on an SK mount that attaches to the carrying handle. I sits really high and since it sits so high above the bore, he asked me what was the best range to zero it, so that the point-blank point of impact wouldn't be too high or too low. As I am not an AR guy, probably a symptom of my years attached to an M16A1 while working for Jimmy and Ron, I have no idea.
I figured that if anyone would know, someone or multiple someones here would know.
Point blank impact will be low with a scope, there is no way around that. He just needs to shoot it and see where it hits at varying ranges. Get him to set his zero at 100 yards and shoot it to see where it hits.
The best thing for him would be to buy a A4 receiver and use a standard mount or rings. Mounting a scope on top of a carry handle is just not ideal at all.
A carry handle adapter with low ring should work. Something like this....
http://www.amazon.com/Leupold-Mark-AR15-Handle-Mount/dp/B000YA81OI
The mount that he has is physically different, but functionally about the same. Do you think that a 200 yard zero would keep the bullet between 5" low and 5" high out to ??? yards?
I think it will be difficult to get a decent cheek weld with his current set up. I'd nix it for one of these:
http://www.andersonrifles.com/product/ar15-a3-upper-with-upper-parts-kit-installed/
Cheek weld isn't doable, more like jaw weld.
The best idea would be to buy a flat top upper and screw the barrel in to it.
Barring that, one of these improves the consistency somewhat:
http://www.cheekpieces.com/delta.htmlUsing M193 out of 20" specs(.243BC/3259fps) and a SWAG'd sight height of 4.0"(irons are app. 2.6") with a 200 yard zero it looks like he would max at +0.7" high(other than the +4" at the muzzle) at 150yards and be at -5.8" at 300 yards.
http://www.hornady.com/ballistics-resource/ballistics-calculator
I tried the carry handle scope mount....for one range session. 4+ inches of distance between scope center and bore center is a clusterfugg, no matter how you slice it.
Go irons, or go flat-top.