Trajectory differences between 5.56 ammo

[MTMilitiaman]

Something I've always wondered was how much the trajectory differences between different types of ammo mattered with some of the BDC sighting devices on the market. Specifically, say you're running an 18 inch barreled AR or other 5.56mm carbine with a 4x RCO calibrated for the standard 62 gr M855 ball round with a 300 yard BZO. You want to run 77 gr OTM rounds at about 2600 to 2700 fps, and you want to be able to use the optic's BDC to place rounds on steel plates and torso targets out to 500 yards or so. Are the trajectories going to be different enough to make this impractical or is it simply a matter of maybe adjusting zero range a little on the 77 gr load?

I've ran some numbers through some exterior ballistics software I have, but getting numbers I can compare to what I am finding for the M855 is difficult. Things like barrel lengths, sight heights (I remember the RCO on an M16 being pretty tall), and different atmospheric conditions make finding apple to apple comparisons a little difficult.

 

[Zak Smith]

It takes a lot to be off an E silhouette at 500 yards.

If you want to hit 12" plates out to 500 yards, you need to figure out the hold points for your ammo from your rifle at your locale/temperature.