AR15 diminished accuracy

[HRider]

My 20" AR's accuracy has gone away. It is shooting 4.5" groups at 100. I have checked the sights to insure that they are not loose, cleaned the bore and the comp, looked at the barrel nut (it didn't appear to have moved, the gas tube is still centered in the notch). This rifle only has about 2k rounds through it. Does anyone have any ideas about what may be causing this problem?

Hurley

[GentlemanJim]

Crap on the crown behind the comp can do it

Jim

[Merlin Orr]

Copper fouling, Powder fouling. loose barrel, damaged crown (unlikely with a comp.)

Guess 1 - bullet hitting comp

Guess 1 1/2 - fouled barrel - using Sweets?

Guess 2 - loose barrel

Guess 3 - damaged throat from improper cleaning

All guesses you know...

[Dan Sierpina]

Changes in ammo. Going from decent bullets to lousy bullets will do it too. You didn't say what kind of accuracy you had originally.

[Run n Gun]

Changes in ammo. Going from decent bullets to lousy bullets will do it too. You didn't say what kind of accuracy you had originally.

+1 I've got one AR that will shoot ~1/2" with certain ammo and 4"-5" with others.

Ed

[Bill H]

I recently had a new Krieger barrel that the accuracy went away fairly quickly. The shop I got it from recommended scrubbing with JB paste. It took a while to get it really clean but it shot good again when I was done. Copper wasn't the problem, carbon was.

[M.E.Anglin]

is it ss or chromed lined or???

[warpspeed]

Fouling or something has come loose.

Check the bolt & carrier too. Look at the comp, any bullet strikes ?

Have you checked the headspace ?

[RiggerJJ]

The first question I would ask is (after making sure the scope/sights were still zeroed) "What did you change just before the problem arose?"

The second would be, "did you go back to the old _____ and see if the problem went away?"

Then I would look elsewhere...

JMHO...

jj

Edited December 28, 2008 by RiggerJJ

[warpspeed]

Something else to check ; Try a different scope. Perhaps the reticles have become detached or floating and the result is not good.

unless it has iron sights in which case have someone else try to make sure it is not your eyes.

Edited December 28, 2008 by warpspeed

[Sudden Death]

+1 on the JB cleaner and polish, you would be surprised how dirty your barrel still is after you think you cleaned it good.

[HRider]

I am using the same ammunition (75gr BTHP). The rifle has a Colt MT6700C (20"compensated HBar 1/9 flattop, not chrome lined) upper on a Rock River lower. It is iron sighted. I pulled one of my old R6600 Colts out , ran some of the same ammunition through it (it shot an 1.25" group) and took it to Ft. Benning. I looked at the comp, I can't see evidence of bullets striking it. It must be carbon or copper fouling judging by the feedback on this thread. I will try scrubbing the bore with JB.

Thanks, Hurley

[Bill H]

Let us know how you make out.

[cking]

Get the bore out in the sunlight, and see if you have a carbon ring just in front the chamber. JB and tight fitting JAB.

[HRider]

I cleaned the bore with JB. I took it to the range this morning. It is now stringing shots in a horizontal line from center to six inches right. I took the comp off and the group (er, pattern) moved to the left, but are still spread horizontal. At least cleaning the bore changed it. It was throwing a large rectangle before. I will clean some more and try again.

I tried some LC M193 ball through it (just a few rounds) and it was doing the same thing with it (the horizontal line was a little more wavy though).

Hurley