Have a preban Colt Sporter Target that I bought slightly used---probably had a couple thousand rounds through it when I bought it.  It was my first AR.  Has a 20", chrome-lined bore and chamber with government profile barrel.

When I first purchased it, I installed a 3.5 Compact ACOG on it---mounted on carry handle. When I originally sighted it in (at 100 yards) it was pretty accurate just off the sand bag (no sling or anything special)----was able to consistently get 2-3" groups at 100 yards----whether using the scope or just the "issue" metal sights.

Since original purchase, I have probably put 4 5 or 6K rounds through it----mostly plinking----some 'blasting' where the barrel got pretty hot----using everything from lots of Wolf to SA Battlepacks to some Q3131A and XM193.  

A couple of weeks ago I started to notice that the accuracy was way off---when shooting out at 200+ yards----I was a good foot off of target.  Checking things out, I found the scope was loose----I tightened it up and then resighted it in yesterday.

Resighting in I found I had to significantly change the adjustments on the scope. Plus, using the metal sights, I had to crank the rear sight all the way to the left to get the bullets to "kind of" center at 100 yards---it used to be accurate as can be when the rear peep was just about centered between the rear sight "guards".  Even with the adjustments, shooting SA, I was getting a dispersion around the bull of not 2-3" off of a sand bag, but more like a foot.

Not being an AR expert, is this just a case of barrel wear----or did the barrel somehow get bent---or are both those situations occurring?
I recall that there was one time that the rifle got knocked-over and fell a couple of feet-----when the barrel was damn hot----could it have gotten bent? I know that a SS brush got used on the bore once---but my initial thought is that that shouldn't "kill" a chrome lined bore, etc.

Any insights on this?  I am looking at putting a Jewell trigger into it (I'd go with the RRA, but have the sear block to contend with)---I assume I should start looking at a barrel replacement, right?

Use a good solvent inside the barrel. One that is effective against copper, in addition to normal fouling. I use shooters choice and a good stiff bronze brush on an Otis kit. But careful use of a cleaning rod works as well. Just make sure you wipe any small spills or streaks of solvent off the receiver if they occur. As they will attack you nickel accetate sealant coat applied over the anodizing.

You didn't mention what you were using, but chances are you have a slight buildup of copper residue that's effecting accuracy. This is normal after so many rounds. What's important is to not be too aggressive. You want to get most of the copper out, but you will never get it all out unless you harm the barrel itself.

If you do this, I think you will like the results next time you're at the range. Absolutely do nothing radical unless you try the small steps first, like cleaning your barrel with a decent copper remover. Or you certainly could make a small problem much worse and irreversable.

I don't think there's any chance you will bend the barrel unless you us it in high intensity bayonette drills, as a crowbar or run over it with your car, etc....... Nor will a stainless brush permanently harm it unless it is used for quite sometime.

Run some copper remover through the barrel & see if accuracy improves.

It's a long shot (pardon the pun) but make sure the barrel nut is still tight.  I have seen about one message a year where it comes loose severly affecting accuracy.

Grab the front side and see if you can twist it or the barrel in the receiver.  Remove the handguards and see if you can move the barrel nut (it has teeth similar to a gear).

Thanks for the input and ideas!  An update:  Took off the handguards and checked the barrel nut, but, it appears to be tight----the gas tube goes right through the barrel nut, ergo, I couldn't get it to turn----but then again, it wouldn't wobble and didn't otherwise appear to be loose.

Started on the bore with one pass of Sweet's 7.62 and followed-up with Hoppe's Bench Rest---I am suprized at how much blue and black 'crud' is coming out.  Letting it 'soak' a bit right now w/HBR in the bore----what is interesting----the rifling is "coming back"----I thought the fact that it didn't look sharp and crisp was an indication of barrel wear.  

It will be a couple of weeks before I can get out to the range again----but will let you know how it turns out. Thanks for the help one and all!!

I may be off on my words to describe this, but a benchrest AR guy told me not long ago to pull down your bolt and clean the passage that the gas rings run in and also lube the gas rings lightly. I believe for this same passage a couple of companies also make a carbon scraper to scrape carbon out of the BOTTOM of the recess. He told me this has a bigger affect on accuracy than most people think. Again, my description on locations may be off. My AR isn't near me and I'm going off of a good but short memory! Hope this helps.

Don't forget to grab the sight tower and give it a good pull to see if it's tight.  Once in a blue moon, one of the taper pins that hold it on COULD back out, but usually they go in tight enough that you're  lucky if you can remove one without drilling it out.   Can't hurt to check.  If one is loose, drive it back in with a brass punch and a 16 ounce hammer.

CJ

Either the copper fouling (Bad if you mistook it for wear!) or better yet, damage to the barrel crown.

I bet when you droped it you marred the barrel crown; it will cause just the sort of dispersion youmentioned.