Could somebody give me a little info on this series of AR.
What would be a good price to pay for it?

It should have the Sporter Rollmark, and  20" Govt Profile barrel  

Used I have seen them go for as little as $800.00 and NIB around $1,200.00

Here's the rollmark for a Colt Sporter R6551.  Note serial no. starts with ST not GS.

GS pre-fix = Government Sporter

20" Chrome lined A2 barrel
1/7 twist
Forward assist
a2 upper
fixed stock
A2 Comp
A2 Handguards
NO Bayo Lug
Large pin

Brand new, around $1,000

Used , from 700-900 depending on condition

Id estimate a bit on the low side of that, based on A2 configuration, pins, and bayo lug

I'd post a copy of the roll marks if I could figure out how to do it. Grrrrrrrrrrr!!!!!!!!!!

Quoted: I'd post a copy of the roll marks if I could figure out how to do it. Grrrrrrrrrrr!!!!!!!!!!

This may help--------->Photo posting thread

or email one or 2 to me and I'll put it up 'til you figure it out

Here is my Colt. Guess I have the same thing. I paid $1000.00 for about a year ago. It's alot, but for here on guam, I felt it was a good price. I have since added a free float tube.
What is the difference between "GS" and "ST" ?  
Gene
*not the actual ser#..

If it has a GS serial number ("Government Service Model"), it is most likely a 6550, not a 6551 Sporter Target.  The Government Models are fairly rare and bring a good deal more money than the 6551's.  the 655p had a large pivot, standard FFCG pins, slab side lower and a bayo lug.   They were available in 1988 and 89, with a few more trickling out when Colt decided it was safe to sell to civilians again.  Only around 10,000 were sold, so that serial no. is near the end of the line.  Depending on the exact confguration of the rifle, it may be anything .

Here's my lower rollmark:

Good answer above, but may I add that my GS# had the bayo lug milled off, [factory] came with 3x9 armored scope, colt mount, colt cheekpiece, and a Colt .22 conversion unit in a case that was designed for an AR as the fasteners are set up for one. It also has NO block whatsoever for the FA bits. Check to see if that one has the steel insert, with the GS# It might not have it. Mine is very accurate, surprised the heck out of me when I bought it. It remains the most accurate AR I own. Very good fit and finish on it, much better then you will find nowadays on about any AR out there. Mine also has the small FC pins.  Mine is marked Sporter Target on the lower and the SN is in the GS10xxx range also. It should have the dark grey finish that Colt was using at the time. If you find one at a decent price, it is well worth it to buy it. Mine has been flawlessly running since the day I bought it.

Here are HKMP5SDS' Colt 6551 rollmarks.

(impressed with the apparent condition and of course- color. The front takedown helps date it)