Hi I have a 20" preban Colt AR with a 1:7 twist barrel. I puchased the gun a few years ago and recently began shooting it alot. I've been tring to site the gun in at 100 yards and I can't get a good grouping. I shoot 6 controled shots from a bench on a sandbag. I might get 1 bullseye but the rest go all over within a 8 inch circumferance. There is no pattern nomatter what I do. 1 could be 4 inches low and left the next 2 inches high and right ect. I've been shooting for many years and can get excellent grouping with all of my other guns but not with my AR. I called Colt and they suggested using a 62 grain bullet but that was their only suggestion besides sending it in for them to look at it. I bought the gun used from a dealer and it appered to be well taken care of.
Does anyone have any suggestions or ideas on what could be wrong.

Thanks in advance for your help.

Try shooting at 50 yards. I llike to use paper plates as targets. Very easy to see the black sight on the white background

Used guns always have a potential for problems, but usually the gun is OK, or something minor needs attention.  Good information is always helpful, get, and read "The Competitive AR-15" by Dezeeker Press (sp) and you will learn all you can absorb about AR accuracy and how to get it.

The first thing I would do is a detail inspection, starting with the muzzle crown.  If someone has been cleaning the barrel from the wrong end, it is possible to nick the muzzle crown, which has a very bad effect on accuracy.  Remove the bolt assembly and look at the muzzle crown inside the flash hider.   Any nicks or signs of uneven wear anyplace on the crown?  If so it may need to be recrowned to restore it's accuracy.

If this seems OK, then a really good barrel cleaning is in order.  Make sure to use a Dewey or othe high quality coated rod with correct, high quality brushes and jags and an AR15 Bore Guide in the reciever.  Clean first with a regular bore solvent to remove carbon.  Clean the chamber throughly with a GI M16 Chamber Cleaning Brush.  In the barrel, once you get clean patches with ordinary solvent, switch to the new Remington Barrel Cleaner/Rem Oil combination or use Brownell's JB Bore Polish with Kroil Oil.  Both use a very mild, non-embedding abrasive to remove ALL contamination from the lands and grooves.  They are what the military Service Rifle Match Teams use and will not harm your bore.

Now buy a selection of some really good ammo in various bullet weights - say, Federal 69 grain match, some Black Hills match in various weights, etc.  AVOID Wolf Russian and other cheap junk.  US made "White Box" Winchester or Federal is a much better alternative.  Colt actually recommends against Wolf in their Armorer schools due to many dismal reports of gas system and chamber fouling from Police Armorers.

Use a careful bench technique if you are skilled.  Keep in mind that the weapon does not touch the sandbags - the hands that hold the weapon do, but not the gun itself.  If in doubt, find one of your local Service Rifle high level shooters and let them target it for you.  This will confirm or deny your own results.

Again, get the book, clean the rifle and use good ammo.  I suspect you will get better results this way.

Warmly, Col. Colt

Certified Law Enforcement Trainer
Certified Glock, Colt and Remington LE Armorer

Thanks Col Colt for the info. I'll be going out on Saturday to shoot again. I for got to ask , What size grouping should be expected at 100 yards from that gun. Maybe I'm expecting to much from it.

A well maintained AR should shoot 2" groups at 100 yards with quality FMJ ammunition.  

Ok , Last night I completely stripped down the gun and cleaned everything. One thing that I also noticed is that there is some play between the upper and lower. How much is acceptable?

A little is OK.
It should not effect your aim as both the front and rear sight is on the upper.
If it bothers you look for the buffer that goes between the lower reciever and the back pin tang on the upper.

Play between the upper and lower receiver affecting accuracy is over rated.  I have a couple Colt 614s where the upper and lower will seperate enought to push playing cards through.  Both with standard chrome lined uppers are capable of 2 MOA at 100 yards.