It's an AR15. This model will have exceptional accuracy. It could be a 1:9 barrel which was the standard for a long time. 55 gr or 62 gr is best for that. Because of the A2 configuration (fixed carry handle) putting an optic on it is not really feasible. It could be worth enough to sell and get a more up-to-date one.Not really an AR15, it's a semi auto M16 being that it has the full rifle length barrel, carry handle and I'm assuming the one piece non-collapsing stock. It should be a great shooter and do just about anything you need it to do.
Can you look over the barrel and find the twist rate of the rifling? Should be either 1:7 or 1:12. If it's the slower rate it'll only work well with lighter 55gr or 50gr ammunition, won't stabilize the 62gr or heavier properly. The 1:7 will shoot just about anything well.
That's the gun I have; fixed handle, sear block, 1:7 twist, flash-hider, but no bayonet lug. Essentially a civilian M16A2.A R6601 Sporter Match HBAR should have a MH serial number prefix.. Not MN it was made around 1990-1991 Would have a large front take down pin and a sear block MOST do anyway, No bayonet lug but should have a flash hider. And be a 1-7 twist chrome line heavy barrel, With A2 upper and rear adjustable sights.. And the receivers are usually a greyish color as it is a pre ban Colt and that is the color they were then..