I have used the search function on this with very little information gathered other than it is a feel good measure.
I have a BCM4 upper on the way with a Brownells facing tool also.
Thinking facing is a good thing to have receiver face square for proper barrel installation and equal bolt lug engagement.
Am I on the right track ?
Building a precision upper for bench rest and prone shooting.
Taking my time on this build, maybe a year.
Currently looking at barrels, specifically the Lija at this time.
many will tell you it is not necessary. I will tell you it is cheap insurance. Hell I just did a mur upper receiver and lapping the face will tell the tale. It was not square.
For a precision upper I would definitely do it. When it comes time to time the barrel nut and the gas tube and get the barrel nut torque where you want it lapping is your friend.
I’ve wondered what kind of real-world difference this makes. I have an upper where the rear BUS is almost all the way to the left. If I lapped the upper would it make enough difference that the BUS would come back more towards the center?
If the face of the receiver isn’t square then what are the odds the threads are. If the threads aren’t square you may have full contact between the barrel and face of receiver after lapping, but you will still have uneven pressure from the barrel nut.
Honestly I hate to go to the trouble of pulling the upper apart just for that, but at the same time I’m curious what would the results would be from an academic standpoint. I’m thinking of rebarreling this upper before too long so I might lap it then, but putting it back together with a different barrel wouldn’t really tell us anything for sure.
Since BCM started machining their own uppers for a tighter barrel extension fit I wonder if they also machine them so the face is true. Although I don’t know how subsequent anodizing would affect that.
Some of the more experienced shooters here who’ve tried it claim it helps. Like SteveL, I’m not pulling a gun apart to do it, but if I get back into putting uppers together, I’m buying the tool.
Wast of time and money. Will you also true the mounted barrel extension flange? You gonna true the nut? Wiggle the bolt inside the carrier and tell me if it is a tight fit? Wiggle the carrier inside of the receiver tell me how tight it is. There is slack everywhere. If there wasn’t the rifle wouldn’t run very long. Trying to true a receiver with a hand tool is a waste of time. You might even make it worse.
Personally I like my barrel pointed straight and fully supported all the way around at the receiver/barrel extension interface.
I think it has a lot to do with what you are trying to accomplish with the gun. If it is just your typical SHTF AR then who cares. But if you are using heavy match barrels and trying to shoot tiny groups then every little bit helps.
Read my post again and tell me what it stands a chance of doing other than f—ing something up. I like Brownells but a lot of they stuff they sell is a joke. The lapping tool is one of them. Did you believe all of the claims for HP gain printed in the JC Whitney catalog?? Truth is… If you blow enough smoke it will find it’s way up someones ass. It’s called marketing!
well from your reply I assume you have absolutely no experience lapping a receiver face. You have nothing to base your opinion on. Probably something you heard somewhere on the internet.
and I would assume from your reply that you have no intention of ever trying it.
The problem is that truing just one surface of a two surface union doesn’t guarantee perfect mating - there is interface between the threads on the barrel nut and receiver, the threads on the barrel and barrel extension, and the shoulder of the barrel extension and the lapped front of the receiver.
To me it would make more sense to remove the index pin from the barrel extension and lap the two surfaces together then replace the pin. The problem is that it would be hard to keep lapping compound out of the throat of the receiver and the movement of the lapping process would probably increase the tolerances in the extension receiver throat area.
I don’t think the Brownell’s tool would hurt much, it may help some.
Something Sully shows at his armorer courses is the difference in dimension of different receivers (upper and lower) - often dependent on how the person doing the machining clamped the blank into the machine. The engineering feat of mass produced weapons is making parts (lowers and uppers, lpk’s, bolts, bolt carriers, etc.) within +/- tolerances so that random parts taken from a bin fit together to make a functioning weapon with acceptable accuracy and function. For that reason big gains in accuracy don’t often come from one simple operation.
The collection of “random parts taken from a bin” seldom leads to really accurate results, however. Isn’t that why a really good 1911 will be full of hand-fitted parts? Sometimes costing as much as or more than the original gun? “Acceptable accuracy and function” mean different things to different people.
And while if may be true that “big gains in accuracy don’t often come from one simple operation”, those big gains can come from a series of small operations, all working to the same end. Any hand loader can tell you how large a difference can be had from a small
.2 grain change in loading, or a small change in bullet depth, or changing primers or brass. It can and does make the difference between .5 MOA and 1.5 MOA.
In light of these things, why would you not do something so easy and simple that can nudge you toward your goal?
I recently completed an upper built with a BCM receiver. They certainly nailed the tighter fit for the barrel extension. But the front of the receiver was not perpendicular to the bore. It is now…
I had the same experience with my first precision build. A top of the line receiver set. Gun wouldn’t hold groups. I noticed my windage was well off dead center. Tried a lot of things and then finally read about the brownells lapping tool. Took it apart and lapped the face and things straightened right up. It also makes timing barrel nut torque and gas tubes a breeze.
I wonder when someone is going to do an AR like Tony Kidd has done the 10/22. He did away with the slip fit and actually threaded the barrel into the receiver. Bet that would make a difference in precision.
The one potential problem that no one has mentioned is that if you take too much material off the receiver face, the barrel extension could sit back too far into the receiver when assembled.
If you have M4 style feed ramps where the extension feed ramps and receiver feed ramps match up perfectly, lapping a receiver face could create a ledge that could affect how well the cartridge (bullet tip) feeds into the chamber. If you already have a little bit of a ledge before lapping, lapping could make it worse.
Do I lap my receiver faces? Sometimes I do and sometimes I don’t. It depends on my mood, the gun I am building, how much time I have, etc. But before I lap a receiver, I always slide the barrel into the upper to see if the fee ramps match up OK or if there will be a problem.
I take very little off in reality. Lap a little and look. if part is shiny and part is still black I lap a little more. I just do it until the entire face is shiny. I have yet to do one that was shiny all the way around the first time I looked.
Now when I am lapping to time the barrel nut torque and gas tube I might take off a little more but it is still very little.
I wouldn’t lap if I was assembling a normal AR with a standard barrel. Not worth the trouble.
but if I am installing a heavy match barrel and expect tiny groups then I will lap.
I will certainly check the feed ramps in the future.
I don’t think threading a barrel extension into an upper would have an appreciable effect, particularly if it’s a similar issue with the receiver not being square/perpendicular to the bore.
There are plenty of slip-fit configurations that are very accurate such as the DTA rifle. And, as you know, the 10/22 is not exactly the same analog. It’s a slip fit with only the two lightly torqued screws in the v-bock securing the barrel to the receiver. An AR barrel extension does slip into the upper (better if it’s a snug fit), but torqued and held in place to a secure fit with the barrel nut.
Changes in the design of the barrel extension, bolt and chamber would have a better effect resulting in tighter bolt tolerances, a shorter throat, and tighter neck dimensions. Of course, that ain’t going to cut it with anything but a precision rig and not conducive to a fighting rifle that has to feed and extract all sorts of ammo varieties at variable sustained rates of fire.