I am sorry to have to ask this but I have utterly failed to find this on this web site - where can I read about all the differences between the Colt AR's and everybody else?  WHich parts are proprietary, which are universal, etc.

That topic should be very easy to find information about... Click the search button at the top of this page and do the following:

"Keyword" = COLT

"Board" = AR-15

"Date Range" = last 3 months  

Click the search button and you should come up with 8 pages of COLT information...

Here's the scoop. Colt made a few changes from mil-spec in the past on some of their rifles:

1. Large pivot pins. If you have a large-pivot Colt lower, you'll need a conversion pin to use a small-pivot upper, and the opposite holds true as well.

2. Large firecontrols. Colt increased the size of their hammer and trigger pins from .154" to .170". Obviously, these pins and the firecontrols associated with them will not interchange with standard mil-spec parts.

3. Sear blocks. Colt put a block in the rear of some lower receivers years ago that sticks up out of the lower. This is so that you could not use an M16 bolt carrier in your rifle. If you have one of these sear blocks, you will not be able to use standard AR-15 bolt carriers.

Everything else interchanges just fine.

Thre is no really Colt Vs all other AR15. Some company offers the same specs as Colts, and some offers some of the specs but has other options.

Find out what you want, and get it. Dont let a brand name stop you.