Re: This Acog on an M4 - Any downside?
IMO, the BDC on the ACOG is convenient when firing M855 out of an M4, but is otherwise unnecessary. If you zero at 300m, then your bullet trajectory will only be a couple inches above your POA within that range. In practical terms, just aim for the belly button and you'll at least hit the target every time even under stressful conditions.
When considering an ACOG, I look hard at its optical dimensions. The part I like least about the ACOG is that the eye relief is about 1.5". But that's not the whole story, so look at the size of the objective lens, the width of the eye box, etc. Compared to Leupold's HAMR, the ACOG has what I would describe as shorter and wider optical dimensions. This can be good for an imperfect cheek weld.
When engaging targets within 50m, getting a sight picture on an ACOG may only slow you down. A reflex sight would be beneficial here. I've also been able to confidently engage silhouettes out to 300m using an Army issue Close Combat Optic. If you are shooting from a bench or from the prone, you have all the time in the world and don't need a reflex sight.
So in my opinion, when you are shooting silhouettes, an ACOG is best suited to a range of 150-400m and a reflex sight is best suited to a range of 25-300m depending on the size of the dot. If you are shooting paper plates, I would go with the ACOG for improved precision.
I say all of this as a deployed Cavalry Scout in the U.S. Army. My issued weapon has an ACOG and given a steady position I can hit a head sized target at 300m.