Wish I had a Steyr or Harris .50...
I have a USO ER25 on a DTA HTI. If that goddamn thing breaks I'll be shocked. IMO, they make awesome BMG scopes and have a lot of elevation, which is usually a big deal with BMG optics. They were $3000+ scopes new and can be found on the PX for a good deal pretty often. LR17 is another, I have that one on an SR25 and love that 3.2-17x range. Plenty of Mk4's too. Given the range of that beast I'd be all over a 5-25x. I'm not much for mag either, but in this case it kinda needed it.
MK4 is okay, when Mr. Barrett came to Ft. Lewis to talk to us about his new XM107 and give us the class, that's what they came with. The Mk4 didn't have sufficient elevation, even in a +MOA mount, in order to keep the reticle on the target at far ranges. FOV ain't exactly the best either but it ain't the worst. I like Mk4 --on my recce rifle and a 20" Grendel.
USO makes a new 2-20x --that's gotta be damn near perfect but I don't know the specifics on it. Yes, SWFA scopes are cheap and decent scopes that will take a beating. I know a guy that took a beat up 10x to Iraq to top off an M14/21 and it survived Mosul and dropped bad guys so I think it'll survive suburbia.
DTA recommends Kahles --their recommendations have been pretty much spot on IMO/E (the Elite Iron can is peerless for the BMG and the price is impossible to beat, they make great cans and this one is no exception, the 1 inch thick baffles look like art!)
I have that ACOG you mention, I use it on an M2. For FOV, it'll certainly get the job done and the reticle has a drop indicator that works out to just over a mile. I wouldn't want it on a rifle myself, but on the M2 it is the tits and there is no faster. I figure on putting an EoTech on that top rail, the one with the .50 BMG reticle. I suppose an ACOG on an M107 in a sentry role against light armored targets would be ideal but not on a "sniper" rifle.
If you DO get the ACOG, I'd wait until during/after Thanksgiving and see if Trijicon is gonna slip another one onto Amazon for half price. I no-shit scored mine for $1400 last year --asked if I wanted to "buy another" (sure, why not?) but price changed to $3k+. The green M2 reticle isn't worth the increase in cost IMO. It's by far the nicest ACOG and given it costs as much as a S&B it better be.
Finally, there's the Leupold Mk8 CQBSS (I think that's it?). It's 1-8x and they make it with a .50BMG reticle for the M2HB but you gotta ask. Note that these reticles are generally good for API and ball loaded to milspec velocity from a 45"bbl., but not so much custom loads using, say, 750gr. solids.
Non-mag but works with magnifiers:
Trijicon reflex sight (forget the name but it's the only one that's $4k+) --I really wanted this for the M2 but the cost was astronomical and besides, the ACOG plus an EoTech is probably a better option anyway.
EoTech: they make a regular reflex sight, look for the 50BMG reticle. They make or made a larger one for the M2. Technically, these will be your FOV kings. Also your cheapest option by far.
Aimpoint: they make what is basically an oversize $3k M68 red dot for one.
Dong In Optics (sp?) (the one you see military use sometimes) is a POS and overpriced IMO, I'd steer clear.
Last but not least, don't forget rings, I pretty much stick with Badger these days --solid and don't slip under recoil, don't fuck up the tubes doing it.
Hope that helps. BTDT in the BMG optics dept.
I can't help but think you'd be better served with an M2HB based on what I'm gleaning here, but I could be wrong. It's a LOT of fun though.