Gunsmithing AR accuracy issue and question

[ekaphoto]

I have a Colt 6920. I like to keep it simple no rails accessories etc. Its accuracy is crap. I have tried all types of ammo (including match 77 gr) etc no good. Accuracy is 1.5 to 2 inches at best at 50 yards ie 3-4 moa. That is with a scope BTW. if that at times. only mods i have done:

replaced grip with aftermarket one to fit my hand better
CMC 2.5-3 lb trigger

I dont want to dump a lot of money into my AR nor do I expect to get sub MOA with it. granted it is combat accurate enough but that kind of accuracy drives me nuts. I would like to get consistant 1.5 to 2moa area with match ammo.

I am thinking about having the upper lapped to square it. I know with bolt actions that helps but with the AR am I pissing in the wind or would my money be better spent some place else.

Any ideas?

 

[KittieKatt]

Could be a few things:

Barrel. I would contact colt and ask them the expected MOA out of their barrel for that model. I am sure if you give them the serial number they could tell you exactly who manufactured it and all the specs for it. If it is a barrel there are many companies that make wonderful barrels that are very accurate.

Optic. You could have issues with the mounting being off or your reticle drifting. If this is the case either work on your mount or replace/repair the optic. one way to test this would be try shooting it with iron sights and see if you can get more accuracy out of it..

Hand Guard. I am guessing this is running on a captured hand guard? if so possibly try putting the old one back on see if it helps. in a captured system the hand guard can throw things off a bit as it changes the harmonics of the barrel.

Barrel Nut. Ensure your barrel nut is torqued to proper spec and is not loose. If you do anything with your barrel nut be sure to use an action rod not vice blocks so you don't stress fracture your upper.

Ammo. Ensure you are matching the ammo to the twist ratio of your barrel. 1:9 use lighter bullets mainly 55gr.. 1:8 mid weight.. 60-70gr ish.. 1:7 heavier rounds 68+ gr bullets.. You can shoot bullets not matched to your twist ratio, but a lot of the time the accuracy will suffer a little as the twist ratio helps stabilize the bullet's flight.

Those are the ideas that come to mind to look at first.. Hard to say without being able to see it in person... Possibly see if you have a shop in your area that specializes in custom AR's and is honest.. They may be able to see something right away.. Always much easier to diagnose in person..

 

[Blutroop]

Could also be your barrel twist rate.. I recently had a barrel with a 1-12 twist that only liked 45 go and under bullets. I decided if I’m gonna have to buy expensive ammo I might as well get a 224 Valkyrie barrel for it.

 

[ekaphoto]

Thanks for the quick replies.

1. It is 1:7 twist. I have tried several varieties of ammo from 55 to 77 and in between. This includes match ammo. Basically it is the same as Mil Spec M4 from colt as you can get according to Colt, except for no select fire and 16" barrel vs the shorter M4 barrel.
2. Hand guards are stock I just changed out pistol grip and trigger but good things to check along with barrel nut.
3x, I have tried different optics including iron sights. With my old eyes optics are a better choice. I currently have an eotech but used primary arms 3x prison and Leopold VXII 3-9x40 still a no go. I even have an issue M4 carry handle floating around here I tried.

Has anyone here tried lapping their upper? If so did it make a difference and if so how much? I don't want to dump a bunch of money into this. I have a Tac Ops for precision shooting and an M1a loaded for a DMR type.

Thanks

 

[GH41]

"I am thinking about having the upper lapped to square it. I know with bolt actions that helps but with the AR am I pissing in the wind or would my money be better spent some place else"

The first mistake you made was buying a 6920 and expecting it to shoot better than 2-4 MOA. You are pissing up a rope if you expect lapping to make it measurably better. You don't mention what scope and mount you have but it really doesn't matter. A $3,000 scope won't make it a target rifle either. Another thing... Shooting an AR, even a target grade one is not something most people get good at without a lot of practice. Some people never get good at it. I am one of them. I'll take 3 MOA all day long shooting of of an improvised rest with a duty grade AR.

 

[hino895]

What GH41 said 6920’s are junk.

 

[OLD308]

I have a 6920, Colt haters aside, its a good AR. Lapping the receiver barrel interface will do little to nothing, mostly nothing. I have the tool I’ll quickly sell you. Or you can pay a machinist to square it up. Either way it’s not the issue. So I would not bother. With factory ARs it’s most often the 4-8 lb trigger throwing you off. Especially if your used to bolt gun tuned triggers.

Check the muzzle devise for signs of impact. Again highly unlikely.

Measure run out on a factory round. Then chamber the round and extract. Measure run out again. Unlikely this is your issue but it was on on MK12 I did years ago. Bad chamber.

It’s a Crome lined barrel, some shot decent, most don’t. Your is awful. Sounds more like an AK.

If all else fails, WOA and Lothar Walter sell a very accurate replacement barrel for cheap. Or you could put a Red Dot on it and call it good.

Mine shoots 1.5 at 100 with a red dot and Larue trigger/hammer with CS springs. That’s slow fire in perfect conditions with a bipod and rear bag. I’m not a believer of match triggers in AR rifles. Hope this helps, good luck.

 

[moosemeat]

My 6920 groups about the same. I even had a service rifle shooter check it out. I'm going to replace the barrel. Mine is already freefloated and the barrel nut is torqued.

 

[KittieKatt]

If you wan to fix that one up I would say new barrel, something like a Ballistic Advantage or another company known for accuracy. Personally I love Ballistic Advantage. I worked for a manufacturer for a while, during that time we made custom AR's for many SWAT teams around the country. We used Green Mountain, Noveske, and a few other companies in these builds, but the Ballistic Advantage seemed to always be the most accurate of them. Considering the price point was cheaper than some of the other brands claiming to be the same they became even more of a favorite. I currently have an 8.3" BA barrel on my .300 black build and can get groups I can cover with a quarter with it..

The thing I would do after that is free float it. Put on a decent hand guard. All in all for about $300 ish could have a gun that is reliable and shooting about MOA more than likely. if you upgraded your upper and BCG you may get a bit more accuracy..

Personally I will never buy and off the rack AR again.. You can build one for just a little more that is far superior and has everything you want from the start.. You just have to do some research and make sure you have the right tools for the job when it comes time to assemble it.. lol