I need to get this straight. An upper, and a lower + a BCG = a rifle?
Also, as far as compatability goes, if I have a colt lower, can I fit like a LMT or BCM upper?
Also, how hard are handguards to replace?
LMT and Colt make great bed partners. Replacing handguards is cake…after you get the hang of it that is
I’m not exactly sure which ones, but some (maybe all?) Colt lowers have larger pins than normal. As far as I know all other brands use the same size. Since you have a Colt lower, you should check the pin size. I don’t really know much about your options to make a large-pin lower work with a smaller-pin upper, but I’m sure somebody around here does, I’m definitely not a Colt expert.
Some upper and lower combinations might be tighter than others, but they should all fit together fine, apart from the pin issue.
As for replacing the handguards, it depends on what you want to replace the regular handguards with. 2 piece rail systems are generally pretty easy to install, but most one piece ones require removing the flash suppressor, front sight base and barrel nut.
I thought the pins that are different are for the fire control group? 
Yep, FCG pins, not take down pins
Well actually… both.
Older Colt AR-15’s had large pivot pins that required you to remove a screw before you could separate the upper from the lower.
This was done to prevent lowly civilians from using a military, full auto upper on a lowly commercial model.
For the life of me, I simply cannot understand why folks even buy from Colt. :rolleyes:
You want quality? Plenty of good research has been done by folks ON THIS BOARD. Decide what you want exactly, and go get it. But geez, Colt’s are made from alumininum just like all the rest…
Glad they got rid of that B.S. Picked up a pony rifle yesterday(6920). 
Well, this will be my first AR and I have done quite a bit of studying.
After much consideration the Colt 6520 fits my purpose to the tee except for the lack of the tactical handguard, which I find is not that hard to replace.
As far as value, I cannot find a better value than this model of Colt for $1050. I have no problem at all with the non-m4 barrel, and the fixed A2 handle. Everything else about the gun keeps it on the left side of the chart stock from factory. Everything right on the chart would have to be upgraded making the cost about the same except for DPMS and Olympic, which I do not see many good comments about.
The reason for the question, just in case I might want to upgrade later to a flatop upper and/or m4 barrel, would it be compatable?
I also asked the upper+lower+BCG question because I was thinking about going with LMT also, just not sure how to put the thing together or if it requires tools, but either way, it is still the same price if not a little more after I buy a carry handle, handguard, H-buffer.