I’ve always wanted a 6520 as a quick lightweight easy to use carbine. One downside is I’d prefer it with a flattop as opposed to the carry handle.
How does the BCM upper compare to the 6520 in weight?
It would give me a flattop and midlength which I prefer.
Have you seen the Colt 6720?
If you would prefer the midlength, wait until Paul switches over to .625" FSBs sometime this summer. 
Rob_S has a parts weight chart you could get that kind of weight info from. Looks like a 4 oz difference between an A2 and a flattop, but then you have to account for whatever BUIS you throw on.
https://www.m4carbine.net/showthread.php?t=52088&highlight=weight+chart
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I had an older 6520, back when Colt used big pins and a big front take down pin on their lowers, and other than that it was a GREAT rifle. My only complaint was slight verticle stringing after lots of rounds down the barrel. But I guess thats the trade off for a pencil barrel.
I would buy a light weight barrel again in a second.
What is wrong with the Daniel Defense 16" light middy barrel?
FN makes both it and the BCM, you know.
Really? How sure are you on FN making barrels for DD?
If you at thw quality of the parts go Colt and BCM are by far the best of the best . As I have owned both brands and the first Colt was in the middle 70s. So it is what ever turns you on.
I’m sorry, this whole “waiting for the 625 FSB” thing is lame. I can think of no functional benefit to the 625 FSB.
This is what Paul said to me in an email a few weeks ago.
My guess is it actually will not even be 1 oz lighter. The narrower barrel journal (.625 vs .750) is replaced by additional material in the FSB or generally the gas block. The FSB forgings are the same, but the one with the .625 bore will be a bit heavier, and probably offset any savings.
Combine that with the difficulty of finding .625 GBs, the ability to use an easy to find side sling swivel that actually fits properly with no modification, etc and I think that the lightweight midlength with the 750 FSB was a great idea from a functional standpoint.
.625 LW Middys in production now.
Should be out soon.
Complete rifles when the Middy MOE comes out. Grant has some set up for custom builds on the way.
BFH .625 Middys coming this fall.
I like the .625 on a carbine - only becuase it looks correct.
On middys I like the 750 (even though we are switching over), because it is easier to set up with a variety of gas blocks, FSB, etc.
11.5" LW (.625) coming this fall 
This is what I have heard, in a thread on mp.net.
Plus, the only other barrel hammer forging machine in North America is owned by Colt Canada
Ruger
You know DD has their own hammer forging machine, right?
My thoughts exactly!
Exactly! I think there’s also a video of their facility floating around.
Evil, evil man. 
Yeah, he is a real dick.
This is Ruger’s with the four hammers and chuck head pictured:
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And since nobody looked…
http://www.danieldefense.com/?page=shop/detail&product_id=250
All Daniel Defense barrels are made from start to finish in our state-of-the-art facility located in Black Creek, GA. In keeping with our heritage of not cutting corners, all barrels feature a forged chamber, chrome lining, and heavy phosphate exterior coating. They are all MP tested on-site and are available in multiple configurations which includes barrel only; barrel with Front Sight Base; and barrel with FSB, gas tube, and delta Ring assembly. These barrels will prove themselves to be more accurate and last longer than a barrel that has cut or button rifling.
remember you can get a BCM in lw middy vs lw carbine with colt.