quote:

Is the barrel tight with the upper itself?

This is probably the key to 90% or more of the Accuracy issues with an AR. Because not only dues the barrel nut have to be secure the Barrel Extension needs to be tight AND a good snug fit into the upper receiver. In an ideal situation when you try and slide the barrel extension into the receiver you want a fit that requires 10-15 lbs of force to get it to move and when it's 3/4 of the way home you need to give it some "help" with a rubber mallet. Bad news is that there is enough variation in the lower cost receivers you need to take the barrel you plan on using with the extension mounted and test fit into various receivers until you find a good fit.

Some good news is that it is possible to use a single wrap of shim stock with some Locktite to "improve" the fit but be aware that when using this method you'll need to heat the barrel extension and receiver to something around 160 degrees if you ever need to separate barrel and receiver. I had to do this with my 300 Blackout due to purchasing an upper well ahead of getting my barrel. It required using a wrap of 0.0015 inch thick shim stock to get a good fit. The benefit of taking the time to do this is my Blackout shoots at 3/4 MOA with a cheap 9 power scope and loads featuring a 125 grain Nosler Ballistic tip.

What type of sights are you using, and how are they attached?

I once spent a considerable amount of time chasing down an accuracy issue that turned out to be a removable carry handle. The issue was that the mounting screws that secured the handle to the receiver needed to be torqued down so that they were properly seated in the carry handle itself, before they could be properly torqued to the upper.

I've been working with this upper for almost 9 months, and I've used Magpul BUIS, a Holosun red dot, a Redfield scope, and a Leupold scope, all mounted properly and torqued correctly. The same optics work great on my other ARs.

Then barrel to upper torque is not right, barrel crown is not right, flash hider is affecting trajectory.

Or the barrel itself could have defects.

Chamber reamed too deep, chatter in the lands, etc.....

quote:

Originally posted by Maestro:
I've been working with this upper for almost 9 months

Was this barrel new when you bought it?

Roughly how many rounds do you think have been fired with this barrel?

One of the possible things inside the barrel that could cause this problem is an improperly drilled gas port. The hole for the gas port may be coming down through one of the lands in the rifling. As rounds are progressively fired over time, they can erode that affected part of the land, causing a loss of accuracy and blowing out the gas port.

This can be checked with a borescope at the gunsmith's shop.

I take it you have sandbagged or bench rested the rifle in order to minimize the shooter as a cause?

If you have an erosion gauge you can check to see if the barrel is possible worn out.

you could slug the chamber and measure to see if it's in spec.

You and also use a headspace gauge set to find out what the actual headspace is. this could also effect accuracy.

Have you tried it on a different lower?

This message has been edited. Last edited by: CD228,

quote:

Originally posted by Maestro:
Nitrite finish, only marking is the 1/8 twist, and it's an M4 profile. There's no CHF marking on this barrel like my PSA.

Don't shoot me for asking this question, but are you certain it's a .223 or 5.56 barrel? Just asking to confirm the obvious.

I've never tried to shoot 5.56 ammo out of a 300BO, but always wondered how big the groupings might be.

quote:

Originally posted by myrottiety:
I can only guess that maybe the barrel nut is loose and letting the barrel wiggle some?

It would need to be real loose in the upper, not just the nut. I shot a rifle for years with a loose barrel nut (it rocked left and right held only by the gas tube), it was one of the most accurate AR's I've owned.