Gunsmithing 6.5 CM barrel fitment dissapointment.

[Halfnutz]

After putting this together last year and taking it out a few times, I was let down by the accuracy. It is well over 1 moa with both 140gr Hornady match as well as 130gr. Federal GM Berger.

After doing some browsing I saw some threads about leaping the receiver face and "bedding" the barrel extension.

So tonight I broke it down to give it a look. As you can see in the attached photos there is maybe 30% contact. There is a significant amount of play.
My old bench calipers are suggesting the upper ID is 1.184 and the barrel extension OD 1.180.

I guess I'm going to take a stab at the "thermoset" technique with some shim.

Any other suggestions?

 

[supercorndogs]

I wouldn't see it making much difference on a 2 moa gun. Or at least I wouldn't think its the main reason its a 2 moa gun.

 

[padom]

A listing of the parts we are discussing would allow us to help tell you what direction to go. I agree though, 2moa gun isnt going to get fixed by bedding the barrel.

 

[Halfnutz]

I was hoping to refrain from putting the brand out there, but it's an Ultimate Upper Kit. LaRue.

 

[Halfnutz]

I will say it's not a 2moa gun,but it's not 1 moa or better like I hoped for.

 

[padom]

What trigger? Have you shot large frame gassers before? Scope mount/rings installed and torqued properly?

 

[Halfnutz]

Gieselle, and I use a torque wrench on everything, rings, mount.
This is my first large frame AR.

 

[whatsupdoc]

With gas guns especially, before you go in for surgery always let an experienced shooter shoot it.

 

[buffybuster]

Gas guns always more complicated to get to really shoot tight. If you want to consistently shoot 1/2MOA without much fuss..... you know what I'm going to say.... bolt action.

 

[CaptNemo]

What optic do you have on the rifle ?
Are you shooting from a sturdy bench off of sand bags or balancing off the magazine on the hood of a truck ?
I’m going to echo the fact that an AR-10 pattern rifle is a different animal in terms of technique and follow thru.
You really need to be steady as a statue shooting a large frame rifle to get best accuracy.

 

[elmuzzlebreak]

If you are looking to lap the receiver X-caliber BART 2.0 is what you need. You can use the tool hooked to a drill but what I have found to work best is put the receiver in a vise with the barrel part up, and let gravity help. Do a few turns at a time and don't rush it. If you need to bed it check out the video I've pasted here. It uses a gasket compound that is good with heating and cooling cycles.

One more thing about the BART, you can also use it like you would a geissele reaction rod.





With regards to actually shooting as some posts here have said I will just add that shooting AR rifles is an entirely different ball game since they magnify every bit of movement or influence you put into them.

Happy building!

 

[LawnMM]

This is my first large frame AR.

There's your problem. As suggested, let somebody who knows how to shoot one try it. Trigger control and follow through will murder your accuracy if you slack off in an AR10.

They're every bit as accurate as a bolt gun but getting that accuracy out of them takes a shooter that knows their shit. It's not as simple as lay down and put up a half minute group like you can with a bolt gun. The lock time is longer and you're dealing with moving parts and multiple recoil impulses instead of one.