Upper Receiver Lapping

[jadam]

The upper receiver is only a case; it doesn't dictate or affect accuracy of the barrel.

Take a look at the JP LRI-20vSemi-Monolithic Long Range Precision Rifle - they added a heck of a lot of material at the barrel junction because this "counters the weak point of standard AR design by adding an abundance of material to stabilize the barrel mount. Supporting this, we’ve also added extra material to the front pivot pin joint. Paired with our MicroFit™ Takedown Pins, the LRI-20™ offers a 100% unified chassis system that we’ve seen tighten groups on even the “problems children” of our barrel sample library."

JP is well regarded, and their rifles are widely accepted as shooters. Perhaps they are right about the idea that the upper can affect accuracy?

The bolt carrier group has, essentially, no bearing on accuracy. The bolt is the business end of the bullet fire-y bits (which for all intents and purposes is floating) locks into the barrel extension and voila - centered and repeatable - every time. The only (three) things that truly matter on the AR is the 1) barrel (complete with extension) and 2) the bolt and 3) the round being fired. The rest of it is superfluous nonsense that makes those three things work.

Some very well regarded gunsmiths would thoroughly disagree with that statement. Bolt carrier tilt is something folks will go to great lengths to address.

I highly doubt either you or I are sufficiently versed in the details of the remarkably complex physics involved in the chambering and firing of an AR to be able to authoritatively state one way or another what is nonsense.

You insist others are wrong, I'm merely pointing out there are varied opinions. You do you, I'll do me.

 

[thespen]

I haven't seen anyone mention the issues of torquing a Steel barrel extension/flange onto an aluminum receiver yet.

Just an aside, a lot of this is speculation. You could indeed line everything up perfectly in an ideal world where you are a machinist, and have access to a set of lathes and mills.
Doing this at home with a lapping tool is not realistically going to result in any more exact a tolerance than it was manufactured to.
I've used and own the standard metal bar lapping tool, have built some elaborate jig, and have more tools than the average person. Even so, I'm never going to get it lined up at a perfect 90 degree to the receiver face with any certainty or repeatability.

Back to my first point, ideally you're torquing the barrel onto the receiver correctly, repeating the process 3 or 4 times. Aluminum is not a hard metal, part of this process is intended to deform the face to match the barrel extension's flange. This results in the exact flatness of the receiver face being somewhat irrelevant in relation to the orientation of the threading and how the barrel nut pulls the flange into the receiver. Having it a PERFECT mate up before torquing still isn't a guarantee that it will be perfectly straight after. Any number of variances in metal composition could cause an uneven deformation.

So the point of it all is obviously chasing that microscopic bit of accuracy improvement. This is a fun goal in and of itself for sure. When you're diving that deep and considering what matters, it's easy to put blinders on. You have to realize that EVERYTHING matters, and it's easy for you to become the weak link in that chain. The precision you get out is the precision that's put in. After all that, the question is, can you accurately lap that face to a 90 degree angle with a certainty that it has been improved? In the end what someone does to their build is up to them. Do as you see fit and enjoy. With how many variables exist in this equation it's impossible to A-B it anyways and a properly built AR will be decently accurate regardless.

That's my two cents, take it however you will.

 

[beltfed]

Take a look at the JP LRI-20vSemi-Monolithic Long Range Precision Rifle...

That's as far as I need to read your post to know you haven't a clue.
JP Enterprises stuff is pure ### candy.

Apples to Apples comparison, John Paul's stuff doesn't run any better or more accurate than the next gun.

Carrier tilt is a mechanical issue with piston driven guns.
It doesn't impart accuracy issues with an impingement run rifle.

Yeah, agreed: you be you and I'll be me.

 

[jadam]

I've said my piece. I'll leave it to others to decide if they believe a random poster on the internet who cannot be bothered to read a reasoned reply to his opinion or the designers and engineers at JP Rifles. I know who I believe.

 

[Epoxy7]

I've said my piece. I'll leave it to others to decide if they believe a random poster on the internet who cannot be bothered to read a reasoned reply to his opinion or the designers and engineers at JP Rifles. I know who I believe.

Except Beltfed has been right with his opinion on what is required to get an AR to shoot. The JP rifles are nice but they are boutique guns. You don't need any of that to get these rifles to shoot sub .5 moa. Those JP rifles have unique things such as side charging handles and are billet. Perhaps they need the extra etc due to that charging handle design? Looks to make a less rigid upper due to that design and might require thickening it up. A standard AR rifle just needs a very good barrel/chamber/extension along with the right twist rate/length for the ammo being used. Add in a quality good trigger with a low lock time. That's it. No lapping of anything. No extra thickness, which you often see with billet vs forged. Not required for pure out accuracy.

Honestly I have a POS DPMS gen1 with a 24" bull barrel that I had the trigger changed out to a RRA NM since the original was so horrible. Put 167 Lapua in it and it's easily a sub .5 moa rifle for 5 rounds at 100. Complete garbage. 6061 extruded upper/lower. Bolt is mediocre but does the job. This is really the least quality you can get. Even their magazines are garbage. Yet.. with that bull barrel, upgraded trigger and the right ammo it shoots as a target rifle should. It won't give up anything to a JP etc in terms of accuracy. Everything else about the rifle is complete and utter garbage but it shoots. Which is to the accuracy point rather than being the better built, longer lasting and nicer to own/feature rich rifle. That the JP has hands down over the DPMS Gen1 target rifle.

BCL various generations are further examples of this. Right barrel, trigger and ammo. I prefer the Stag 10 greatly, but reality is the much lesser rifles can shoot just as well, providing those key components are good. No lapping etc needed. If you want to lap stuff etc and it makes you feel good. Then go for it. It's your firearm. I just don't see the point. I also believe you are risking doing more harm than good. But the good thing is if your rifle gets screwed up it's easy to get new parts.

Below quoted because it's correct.

OK - lets for a second say you buy an upper and a barrel and you take some measurements to see if everything isn't "100% perfectly true". What are you lapping?
The upper receiver face where the barrel nut tightens the barrel to the upper so that it is perfectly 90 degrees?
Tell me - why does that matter? The upper receiver is only a case; it doesn't dictate or affect accuracy of the barrel

The bolt carrier group has, essentially, no bearing on accuracy. The bolt is the business end of the bullet fire-y bits (which for all intents and purposes is floating) locks into the barrel extension and voila - centered and repeatable - every time. The only (three) things that truly matter on the AR is the 1) barrel (complete with extension) and 2) the bolt and 3) the round being fired. The rest of it is superfluous nonsense that makes those three things work.

I think you guys are buying into the whole snake oil - and there appears to be a lot of it out there for the AR and a lot of people gobbling it up.

 

[jadam]

Except Beltfed has been right with his opinion on what is required to get an AR to shoot. ...

Below quoted because it's correct.

In your OPINION, sure. Not mine, not many others. You do you, I'll do me.

I don't understand why you must represent your opinion as fact. Facts are immutable and objectively correct in all cases. You have presented a couple of examples, in statistics that is not proof. By and large there are a large number of happy JP rifle owners, that is also not proof. Yes, they paid a lot for their guns, so they may be biased (so might you) but there MAY just be something to improving the accuracy of the AR system to sub 0.5 MOA with some extra steps.

You are right that mucking with things could make them worse, for sure. That is pretty reasonable and I have no issue with that statement.

Unless we can back up our opinions with facts we ought to just admit they are opinions. As I said earlier, I suspect none of us is qualified to get into a thorough, fact-based discussion on this matter. We are sharing opinions, not facts. Please stop pretending otherwise.

 

[beltfed]

I've posted this in a number of build / accuracy discussions that invariably go off the rails:

1) Read and dig into some work that isn't a sales brochure.
2) About the best, unbiased info is coming from the world of competitive matches - After you separate out the sponsor's chaff that is - is where you want to look.
3) There isn't a competition out there that has run the AR more through the ringer than US NRA High Power, and the largest player there is the USAMU.
4) If proof is what you are looking for, it is in this world's pudding that you will find it.

If a shooter wants to 'accurize' an AR; before they start buying parts or lapping tools or whatever - I recommend they read up on the subject and see what has been tried, what works, what is debatable and what is just pure nonsense. Compare info and don't believe everything you read so go test and validate for yourself.

If you read one book on this subject, I suggest it is Glen Zediker's "The Competitive AR-15, The Mouse that Roared".
Yes, it is dated now, and yes it goes into far more detail on NRA High Power shooting than most who don't shoot that competition want to know, but it is a really good baseline book on the AR.
It contains frank discussion and material from other students of the AR, goes over what makes them shoot, what doesn't and is an easy read. He has done some follow on work that is worth looking into as well, but that is the place to start, imho.

 

[Epoxy7]

In your OPINION, sure. Not mine, not many others. You do you, I'll do me.

I don't understand why you must represent your opinion as fact. Facts are immutable and objectively correct in all cases. You have presented a couple of examples, in statistics that is not proof. By and large there are a large number of happy JP rifle owners, that is also not proof. Yes, they paid a lot for their guns, so they may be biased (so might you) but there MAY just be something to improving the accuracy of the AR system to sub 0.5 MOA with some extra steps.

You are right that mucking with things could make them worse, for sure. That is pretty reasonable and I have no issue with that statement.

Unless we can back up our opinions with facts we ought to just admit they are opinions. As I said earlier, I suspect none of us is qualified to get into a thorough, fact-based discussion on this matter. We are sharing opinions, not facts. Please stop pretending otherwise.

The proof is in the pudding. Been there done that. You're where I was 15 years ago. I started with precision AR308 rifles. Only there were few choices and the AR308 rifles weren't the fad, so I didn't have all the BS accuracy voodoo to content with. Instead I had the myths that 1 moa was the best these rifles could do. Meanwhile I was shooting sub .5 moa with ease. Same thing with the far less popular precision AR15 rifles while everyone else was mostly interested in the carbines. If you want to spend the money and time to reinvent the wheel, then go for it. Yup, you do you.

The information is there. Do with it what you will. You can lead a horse to water but you can't make them drink.

Good luck.

 

[PinaKaleada]

You can benefit tremendously from the following before even thinking about lapping:

1. Good trigger
2. Good barrel with appropriate profile
3. Properly torqued barrel nut
4. Anti-cant bubble level(horizontal stringing)
5. GOOD AMMO
7. Good bipod
8. Good and stable scope mount properly torqued
9. Sand bag
10. Muzzle brake if you’re recoil shy
11. Jam nut your muzzle device instead of using crush washers
12. Good shoulder support(helps with vertical stringing)
...
...
...
999. MOST importantly **The shooter**

Those who actually considers lapping their uppers are either in the top 1% or the bottom 10% in terms of skills. No offense.

Please go shoot your gun to quickly realize there are many other bigger factors in improving accuracy before lapping your receivers.

 

[Agent Law]

Sounds like a waste of time and money. The bolt locks into the barrel extension, not the upper receiver, which is really just a box that sort-of, kind-of holds everything together and in-line. That's the brilliance of the design.

 

[Sqr]

Lapping is necessary for assisting Barrel nut timing and fitment on some upper receivers and hand guards. Lapping is done all the time along with Barrel nut shims being used for timing.

As for accuracy I have not found any evidence to suggest it helps.

 

[greentips]

Barrel nut shim is a solution to solve an out of spec under length upper barrel extension.

If lapping is needed, the upper is out of spec. When the upper barrel extension is grossly overly long, there will be headspace problem and hence the accuracy will go down the drain. This is from personal experience.

Normally the upper doesn't affect headspace, unless it is grossly out of spec.

If shimming or lapping a perfectly good in spec upper, check headspace to make sure it is not inadvertently screwed up.

The Torque spec of the barrel nut ranges from 30 ft-lb to 60 ft-lb, so the barrel extension really has no need to be toyed with.

 

[albertacoyotecaller]

I have a lapping tool. If working on a precision AR it gets used. Do what you want.

 

[beltfed]

Lapping is necessary for assisting Barrel nut timing and fitment on some upper receivers and hand guards. Lapping is done all the time along with Barrel nut shims being used for timing.

Please explain this "Barrel Nut Timing" you speak of.

 

[PinaKaleada]

Please explain this "Barrel Nut Timing" you speak of.

That handguard(made by aero and uses the BAR barrel nut), requires the barrel nut to be at a certain position for the gas tube to pass through. Unlike many milspec barrel nut, this type of barrel nut has fewer locations where the gas tube can pass through, therefore you cannot always rely on 30-80 ft/lb range to align the hole.

 

[beltfed]

hmmm...that barrel nut sure looks like it has the same or similar amount of holes in it to allow the gas tube to pass through.
Keep in mind that 30-80 ft/lbs that you are claiming as a requirement is a rough range. It has been proven that the barrel nut torque range can vary quite widely and still perform just fine.

 

[Sqr]

That handguard(made by aero and uses the BAR barrel nut), requires the barrel nut to be at a certain position for the gas tube to pass through. Unlike many milspec barrel nut, this type of barrel nut has fewer locations where the gas tube can pass through, therefore you cannot always rely on 30-80 ft/lb range to align the hole.

Hence the need for lapping or using shims. The mil spec Stag Arms 10 OEM handguard is also in need with most builds because you can observe the gap between the uppers pic rail and the pic rail on the handguard. They should be nearly flush.

Even mil spec m4 barrel nuts need to be timed with the gas tube and the many openings.

 

[Sqr]

Please explain this "Barrel Nut Timing" you speak of.

PinaKaleada has explained it perfectly.

My MS did not require any reworking when using a very forgiving barrel nut.

 

[Sqr]

hmmm...that barrel nut sure looks like it has the same or similar amount of holes in it to allow the gas tube to pass through.
Keep in mind that 30-80 ft/lbs that you are claiming as a requirement is a rough range. It has been proven that the barrel nut torque range can vary quite widely and still perform just fine.

PinaKaleada barrel nut photo has only 4 gas holes, one at every 1/4" (12-3-6-9) position.

 

[beltfed]

PinaKaleada barrel nut photo has only 4 gas holes, one at every 1/4" (12-3-6-9) position.

You sure about that?

It looks to me like it is functioning more like a locating collar that threads onto the receiver threads, then the barrel nut comes in after, torques on threads in the collar and jambs the shoulder of the barrel extension. Not an uncommon method of floating the barrel these days with many rail manufacturers.
But.
The barrel nut is the one highlighted:

D6950833-BAF5-428C-8E8B-F326418124F7_1_201_a by M J, on Flickr