Lapping AR Uppers

[Frankly]

So I never encountered anything about truing uppers by lapping for a better barrel fit until I was hanging out on Typers Hide (Snipers Hide). I take a lot of their anal retentive stuff with a grain of salt but now that I've watched a few YouTube videos and read various threads I'm thinking about buying tools and doing it.

 

Any experience and advice? 

 

Has anyone ever actually done an A-B test before and after? The skeptical part of me still wants to say you adjust your sights slightly to compensate for any off-centerness and as long as the barrel is tight all is well. 

[xtian999]

Lapping is done for a better fit/less movement between receiver and barrel, I assume. Makes sense if the optics are attached to one and the bullet exits the other. Would silver solder work even better? 

[RAINY0DAYS]

Lapping is done for a better fit/less movement between receiver and barrel, I assume. Makes sense if the optics are attached to one and the bullet exits the other. Would silver solder work even better? 

Lapping is done to square the receiver. It's not necessary with a quality piece, but it's a little extra step that you can do if you want. I do it because it takes less than 10 minutes of my time and I have the stuff to do it already.

You'll see more improvement by having a tight receiver to barrel extension fit. It doesn't make a huge difference that most people will notice, but it's just another extra step you can take to accurize an AR. This is why JP shims and thermofit their barrels. You can use shims or sleeve retaining compound, both work. If your extension and receiver already have a tight fit, I'd skip this step. My last build I had to tap in the barrel with a rubber mallet, so I called it good.

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[chicoredneck]

What Rainydays says is true, but squaring the reciever face will help with precision even with a tight fitting extension. If the face isn’t square, the bolt lugs don’t engage the extension evenly and the round isn’t held in the chamber perfectly square.

 

Also, if you don’t have a tight fitting barrel extension, you can bed it to the reciver extension. I just use blue locktite. That way I can still remove it, but even broken down under heat, it still provides structure to fill in the voids.

[RAINY0DAYS]

What Rainydays says is true, but squaring the reciever face will help with precision even with a tight fitting extension. If the face isn’t square, the bolt lugs don’t engage the extension evenly and the round isn’t held in the chamber perfectly square.
 
Also, if you don’t have a tight fitting barrel extension, you can bed it to the reciver extension. I just use blue locktite. That way I can still remove it, but even broken down under heat, it still provides structure to fill in the voids.

Try using loctite sleeve retaining compound. It's a bit pricey, but it's actually made to fill in gaps and is perfect for the application.

And you're right, didn't mean to make it sound like an either or kinda thing. Squaring a receiver and ensuring a tight fit do two different things. Doing both is the prudent thing to do if you're really trying to go through the extra steps to accurize a rifle.

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[Chip3]

Here is a decent video series that covers lapping, sleeve compound etc. 

 

 

It is a 4 part series of brief videos that go over the aspects of building an upper. I am sure there some things that someone can find fault with, but I found them helpful when I decided to to put together a more accurate upper.

[12glocks]

I followed the steps in the series above.  Interestingly, the first barrel i got my wife for 3 gun was a Odin Works 18" 3 gun barrel.  We put a couple hundred rounds and one match through it.  It was too heavy for her and we changed it out for a lighter barrel.  Later when I shot out a barrel on my match rifle, I took her original barrel (which shot about .75 MOA) and mounted on my rifle with a new tight BCM upper which I lapped and bedded with green loctite.  It shot 0.38 MOA after that, a real hammer.  I just rebarreled another rifle for my son and used the same technique.  I have not shot it off a rest yet though, just got it kinda zeroed and adjusted the gas block Saturday.   

 

I am convinced this technique is a benefit, not a detriment.  I am not a member at Snipers Hide, but my impression is there are some posters over there that are pretty sophisticated as far as reloading and longer range shooting goes.

[MRF]

Only can speak anecdotally, but lapping the receiver and bedding the extension appeared to tighten my groups by a modest amount when using a rainier super match bbl and an SSA-E.

[AustinT]

I’ve been having trouble with a wandering zero for years. This time, when I rebarreled, I lapped the front face of the receiver with the brownells tool and loctited in with sleeve retaining compound. No more wandering zero. I also switched to a Criterion barrel.

 

Now which of those was it that fixed it? No idea, but I’ll definitely be lapping and using sleeve compound from now on, and I haven’t found a reason to use anything but criterion.