that appears to be a late-80s Colt AR-15
if you were going to buy one new, I would go with a Colt LE6920 or a Colt LE6940. the difference being the proprietary monolithic rail system on the upper with flip-up irons. you'll likely see SP6920s and SP6940s around and they're the same thing as their respective LE equivalents save for the markings -- Colt Defense and Colt MFG (or something), respectively. they'll run you something like $1000 (SP6920), $1200 (SP6940), $1200 (LE6920) and $1400 (SP6920). you have to decide if the extra $200 is worth the rollmark or not.
this is a LE6940 that I picked up when they first came out (2009, I believe):
^that's a LE6920 sitting on the box in the lower left
I haven't shot it yet and I likely never will as my shitmachine is still going strong but I'm a huge Colt ***** so I just had to have it.
the LE6920/40 is a great rifle and I don't think you can go wrong with either
THAT BEING SAID:
is it worth saving $250 or so by buying a non-Colt AR?
there will be a few people who will jump in this thread and talk about different brands but let's kick the ballistics. I've fired exclusively Colt for nearly 20 years and I've only ever experienced a half dozen issues, all of which could easily be attributed to shitty ammunition with the most recent being a fte (of TULA, OMGZ) in my DRY and DIRTY Lightweight Sporter.
some people like the midlength gas system because the extra 2" is supposed to keep your rifle alive longer or somesuch **** (LOL), but I highly doubt anyone can scientifically prove a quantifiable advantage of a midlength over carbine setup; if you're really worried, run a heavier buffer
whatever you do, there's a few things you're going to want...
be sure whatever you buy has a 1/7 twist barrel (6920s and 6940s do as do all other colts, I believe), I cannot stress the importance of this enough. anything else and you won't be able to stroke your **** with excitement as you accurately shoot 70+ grain ammunition
do not worry about m4 feed ramps or any of that ****, I haven't had a problem with any of my older Colts feeding ball, jhp, etc. they're nice and most rifles nowadays have them so...
do not be a retard and buy a bunch of MOE **** before you even shoot your rifle and don't worry about "mil-spec" stuff because it's usually not
DO NOT BUY SOME BULLSHIT LIKE THIS:
I've been meaning to get rid of that trash since I bought it, I wouldn't use it for anything more than fixing a wobbly table
I'd pay the extra $200 for the Colt and call it a day.