I have a 6520 with a fixed carry handle, and am wondering if I can install a 6721 HBAR complete upper on top, or does the sear block interfere somehow with it ?

Whether or not the lower is large hole or small hole is more important.  

But if it's small hole, then any standard upper receiver should fit just fine.

What the sear block itself will interfere with will be the use of a standard (AKA "FA") bolt carrier, and you need to use one of the modified commercial variants, either in the SP1 (AKA "SA") profile or "half moon" or "cut" carrier.  

~Augee

Thanks.

I may just leave it as is.  Funny how in just a few short years, we all think about mounting optics right away.

It is a small pivot pin/large sear + hammer pin lower. ( I measured with calipers).

The 6520 has a large hole upper and lower and the 6721 has a small hole upper. You would need an adapter bushing to mate the two. When you have a pre-ban lower with a sear block, you are limited to the type of BCG you can use. Typically, the half-circle BCG is what is to be used unless you go to the extreme and grind down the protrusion on top of the sear block which I do not recommend.

A lot of 6520s out there with small pivot pins, and lots of "pre-ban" small pivot pin lowers with sear blocks.  

The 6520 was one of the more popular models, being produced, off the top of my head, from about 1988-2013 in various forms and prefixes, with various combinations of lowers, pins, BCGs, stocks, handguards, etc.  

~Augee

The R6520 has the slickside large hole lower and the AR6520 has the A2 small hole lower. R6520s have the A2 upper. The early model AR6520s came with an A2 upper and the later ones have the A2 (flat-top) upper.

I'm with Augee, I think you are mistaken.  I believe I've seen them with normal ole A2 lowers before.  And IMHO, in order for it to be a 6520 it is an A2 upper.  That part I'm not completely sure about though.  Maybe the OP could give us a pic to help our argument.  

And I agree, I have a sear block and I've used other uppers on it before.  With the semi auto BCG it's fine.  Or the neutered one of course.  You just can't use the FA carrier.  As I understand it. I don't have an FA carrier, so can't confirm if it actually is too long and the sear block would mess with it.  But that's how I understand.

Pics from the OP:

I think that's a normal sized hole.  Dang, is it me or does it look like that barrel tapers a hair bit from the FSB to the muzzle?

Thanks for posting my poor cell phone pictures.

If I wanted another bolt carrier, can I use a non-FA carrier ?  I have seen pictures showing the differences.

That is the AR6520 which as the small hole receivers. You would be good to go. As far as another BCG is concerned, you should be able to use a semi-auto carrier if you want to. I believe that there is enough clearance behind the sear block for it to work. In my opinion, there is no real advantage running a semi-auto carrier that what you have now.

Thanks for all the help.  My only other Colt is a 6721, with no issues.  

Should I consider a heavier buffer, given the lighter bolt carrier ?  Or is it just more theory, rather than practice ?

As far as the buffer goes, you should not have to change it at all.

Yeah, I have a good amount of rounds through my 20" a2 and the neutered bolt with no problems.   I'm not sure it's that big of a deal.   But going up in buffer weights might not be the worst thing in the world.  But if it runs......  why even bother?   Although it could be a bit overgassed.  I think 16" carbines generally are.

She's purdy ... Thanks for sharing the pics with us Lawman

::shrug::

I'm not the huge collector type, and the 6520 isn't even something like the SP1 that it might be too sacrilegious to modify even a little bit... once again, the 6520 has been available in various forms for quite a while, and there are a lot of them out there.  

Based on the lack of a drain hole in the lower receiver "fence," I'm betting it's not drilled for a pivot pin detent, though it could be, if it were mine, I'd spend the extra cash and send the lower out to SAW or ADCO, have them mill the sear block flat, and if necessary, drill a pivot pin spring/detent channel in there, and use a full weight carrier and "H" buffer.  

::shrug::

It's a minor issue at best, and chances are, you'll never notice any difference whatsoever if you don't do so, but a) I just don't like the cut carriers, they chew can up the buffer face, and depending on the carrier, can hammer the firing pin retaining pin, and b) the 6520 was something of a parallel development to the RO727/M16A2 Carbine family, which, through a very circuitous means that I won't go into in depth here, eventually culminated in the M4/M4A1 Carbine as the most mature variation thereof, and while the M16A2 Carbines were very close, the M4 introduced some refinements that would later get integrated into commercial/export carbines.  

Given that it's a pre-ban, too, I'd probably swap out the extractor spring and buffer, because chances are it's the older version, before the ECP to the M4.  

Minor changes, but ones that would essentially do little more than to update the functional components to the "last gen" 6520 specs anyways.  

That being said, I probably wouldn't touch the factory staked two position receiver extension/lock ring assembly, which might negate the lower work depending on the fixtures, but that's just my own picayune tastes.  

~Augee