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AR15 upper Lapp or not to Lapp?
- Thread starter mahiman
- Start date
I always lap my uppers to make the seating area perpendicular to the bore axis in both the upper receiver and the barrel.
Of course, I'm assuming that the barrel extension is true and the barrel is cut properly.
Of course, I'm assuming that the barrel extension is true and the barrel is cut properly.
It depends on your own personal expectations for accuracy out of the upper assembly.
The 6.5 Grendel barrel that I get from Paul Craddock will go in a BCM upper receiver that will be lapped.
Neither parts would be considered sub par, but I’m not willing to risk tolerance stack up and my expectations are pretty high for this upper assembly.
I have the tools, so the 5 to 10 minutes it takes for me to lap an upper during the initial assembly is worth an hour and a half to 2 hours to tear an upper down, lap the receiver, clean it and then re-assemble it. Then there’s range time, ammo, etc. that I’m out if the upper shoots like crap.
It’s not worth taking a chance. I know it’s done and it’s one less thing to worry about if either I or the rifle is “having a bad day”.
The 6.5 Grendel barrel that I get from Paul Craddock will go in a BCM upper receiver that will be lapped.
Neither parts would be considered sub par, but I’m not willing to risk tolerance stack up and my expectations are pretty high for this upper assembly.
I have the tools, so the 5 to 10 minutes it takes for me to lap an upper during the initial assembly is worth an hour and a half to 2 hours to tear an upper down, lap the receiver, clean it and then re-assemble it. Then there’s range time, ammo, etc. that I’m out if the upper shoots like crap.
It’s not worth taking a chance. I know it’s done and it’s one less thing to worry about if either I or the rifle is “having a bad day”.
Let me see if I have this right. The BBL EXT is exactly (we assume) 1.000" the receiver nose is 1.012 (we assume). We already know the EXT will drop into the nose freely, so why would anyone want to make the hole in the nose bigger by lapping it?
This. I always prefer to lap the upper, because I want my gear to be the best it can be, not just "Eh it shoots, must be good enough" like some guys. Lapping is certainly not always necessary, but it never hurts if done right with good tools and only takes a few minutes extra. There are plenty of upper receivers out there these days with a few thousandths extra clearance at the barrel extension, so squaring up the receiver face helps ensure those hold the barrel extension square with the bolt for even lug contact.
Also - yeah those BCM receivers with the tight fit are great, I really like those.
I've honestly never lapped and AR15, and all of mine have shot between .8-1.2 MOA consistently with the right ammo. Maybe I'm just lucky? I did lap my most recent AR10 build in 6.5 CM because I plan on shooting it further than the typical 100-400 yards. Also, 6.5 CM ammo is more expensive.
But I agree, it depends on your expectations, tools, and time. I do not expect my 223 AR to shoot .5 MOA or better, I simply have no need for that. It's more for fun and it shoots minute of coyote when they get too thick around here.
But I agree, it depends on your expectations, tools, and time. I do not expect my 223 AR to shoot .5 MOA or better, I simply have no need for that. It's more for fun and it shoots minute of coyote when they get too thick around here.
First, I am a Noob here and to AR's.
I find the biggest reasons to lap a receiver is initial confidence and later when things seem to go south.
Eliminate one variable in advance. If your premium upper was trued by the seller then probably not benificial.
If you go cheap or the upper was mishandled sometime before you installed the barrel then the front might benefit from being trued.
Many will complain about the cheap lapping tools but I think done with care they will work.
If your chamber isn't in line with the bore and extension flange, then you have other problems.
Besides,
It's not just squaring the receiver but increasing the contact area, sometimes a thin ridge less than 360 degrees around that will yield later as the rifle is abused through it's life.
Lapping can also be used instead of shimming to get right on your target torque. What ever that is
I find the biggest reasons to lap a receiver is initial confidence and later when things seem to go south.
Eliminate one variable in advance. If your premium upper was trued by the seller then probably not benificial.
If you go cheap or the upper was mishandled sometime before you installed the barrel then the front might benefit from being trued.
Many will complain about the cheap lapping tools but I think done with care they will work.
If your chamber isn't in line with the bore and extension flange, then you have other problems.
Besides,
It's not just squaring the receiver but increasing the contact area, sometimes a thin ridge less than 360 degrees around that will yield later as the rifle is abused through it's life.
Lapping can also be used instead of shimming to get right on your target torque. What ever that is
Never lapped an upper in my life. Never had a problem with accuracy...
+1
Treating an AR build like a bolt gun is a sign of ignorance masquerading as "eliminating variables"
You mostly use BCM uppers though right, with the shrink fit on the barrel extension? Those are the only uppers I don't lap, since the bore of the upper does more for truing than the shoulder.
I lap looser fitting uppers, but really don't think there's much difference in accuracy. The difference I see is squaring everything up to even out bolt lug contact. Don't need to do that with BCM uppers.
Lapping is certainly not something that has to be done, but on some uppers it's pretty clear once you start that they weren't actually square to begin with.
It’s also a point of reference that is included in the sum total of the parts that gives you the confidence that the build was done correctly. If it was a redundant step, but it was done correctly, why would anyone object ?
That is the funny part of these threads. The only real objection seems to be "well, I don't do it that way...".
I believe it is this way because there's no proof of lapping to be of much benefit. Especially on an AR15. Maybe if you bought a super budget receiver, but then if you're going to buy the lapping tool, why not just buy a better receiver? Someone looking for extreme precision is probably going to buy a higher end receiver and barrel, where lapping could possibly just make is worse. So it really does just come down to "some do, and some don't".
I see it as you wouldn't buy a Surgeon 591, start jacking with it and possibly mess up clearances. I lapped my Aero M5E1 because it was a blem, and I consider it a "good bang for the buck" receiver. Don't know that it did any good or not. She's shooting 3/4 MOA fairly consistently with match ammo now. Is that because I lapped it? I doubt it, but who knows.
If you want an AR for it's original, as designed purpose, tossing little light bullets toward targets dog sized or bigger, and are happy with "almost a MOA" then lapping is NOT for you.
If you buy "quality" the vendor sends you "quality" then lapping is NOT for you.
If you check squareness and it is off, do you send it back?
If you run your AR hot and hard, Aluminum Vs Steel will change your fit up, then lapping is NOT for you.
If you touch the front of a receiver and you see incomplete 360 contact though, tight or not, lapping just might set the barrel more inline with the bolt.
Bolt gun straight? Not a chance.
I'm willing to guess that there are 100 times more cheap uppers sold than higher priced "quality" uppers. Some folks only shoot factory ammo and are happy with the results.
Some of the new chamber offering for the AR put more load on the bolt lugs (not the original design of course).
Happy with an MOA AR? Great. Got a bolt gun that will outshoot any AR? Great.
Like playing around with assembling your own AR, developing loads, shooting YOUR AR and YOUR ammo, then those little ignorant variables add up.
If you buy "quality" the vendor sends you "quality" then lapping is NOT for you.
If you check squareness and it is off, do you send it back?
If you run your AR hot and hard, Aluminum Vs Steel will change your fit up, then lapping is NOT for you.
If you touch the front of a receiver and you see incomplete 360 contact though, tight or not, lapping just might set the barrel more inline with the bolt.
Bolt gun straight? Not a chance.
I'm willing to guess that there are 100 times more cheap uppers sold than higher priced "quality" uppers. Some folks only shoot factory ammo and are happy with the results.
Some of the new chamber offering for the AR put more load on the bolt lugs (not the original design of course).
Happy with an MOA AR? Great. Got a bolt gun that will outshoot any AR? Great.
Like playing around with assembling your own AR, developing loads, shooting YOUR AR and YOUR ammo, then those little ignorant variables add up.
No, I switched to them 2yrs ago after being unhappy with AeroPrecision's customer service. Ive used hundreds of Aero uppers with extreme accuracy. I still have 4 or 5 Aero based uppers in the safe right now that are some of the most accurate AR's Ive ever built. Now I use MEGA billet uppers since JSE has had those MEGA reinforced uppers so cheap for so long. Stock up
One of the biggest benefits of ensuring that the receiver face is true to the barrel extension is evenly loading the bolt lugs.
This becomes even more important when you venture out of the standard 556 mil spec bolt face into the 65 Grendel, 68spc or Valkyrie bolt faces were more material is removed and there is a greater possibility of breaking a bolt due to the lugs on one side taking more of the bolt thrust as opposed to being evenly distributed.
This becomes even more important when you venture out of the standard 556 mil spec bolt face into the 65 Grendel, 68spc or Valkyrie bolt faces were more material is removed and there is a greater possibility of breaking a bolt due to the lugs on one side taking more of the bolt thrust as opposed to being evenly distributed.
"those MEGA reinforced uppers so cheap for so long. Stock up"
Which one?
Never mind, think I found them.
Which one?
Never mind, think I found them.
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Tight tolerances if your pairing them with MEGA's Billet AMBI lower and these uppers are THICK. You will know what I mean when you hold one in your hand. Perfect for heavy contour or long barrels.
https://www.jsesurplus.com/AR-15SBUBilletUpperwithSideChargingHandleSlotM0300JSEEXCLUSIVE.aspx
https://www.jsesurplus.com/AR-15SBUBilletUpperwithoutSideChargingHandleSlotJSEEXCLUSIVE.aspx
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Cant beat those prices for those SBU MEGA uppers. I've been buying 1 or 2 almost weekly cause I know when they are gone they will be for good
Here's what I can't get my head around about lapping the receiver face. If the receiver bore somehow isn't "square" with the face, lapping ain't gonna fix that. If the face isn't square with the threads, lapping ain't gonna fix that either. The only thing I can see lapping doing is flattening the receiver face if it was not machined or coated properly. In that case, are we expecting lapping to fix a receiver that wasn't built properly?
The other thing that has steered me away from it is that I feel like the potential for making the system worse is higher than making it better. Is the lapping tool a perfect fit for the receiver bore? By definition it can't be, it has to be a little smaller. So then you're introducing your own wiggle and wobble as you spin the thing around with a hand held electric drill.
I dunno, power to anyone who's gotten use of doing it. All my uppers are Vltor/Noveske or MEGA.
-Stooxie
The other thing that has steered me away from it is that I feel like the potential for making the system worse is higher than making it better. Is the lapping tool a perfect fit for the receiver bore? By definition it can't be, it has to be a little smaller. So then you're introducing your own wiggle and wobble as you spin the thing around with a hand held electric drill.
I dunno, power to anyone who's gotten use of doing it. All my uppers are Vltor/Noveske or MEGA.
-Stooxie
"If the receiver bore somehow isn't "square" with the face, lapping ain't gonna fix that. "
The HOPE is that is exactly what lapping should do.
So the barrel extension flange seats in line with the receiver bore (and maybe the carrier and bolt).
Other than lapping, the final finisher could set the receiver in a fixture indexed to the receiver bore and indicate off the face and turn the face square.
Which is what every finisher should do long before it gets to the assembler.
That includes every factory bolt gun making truing totally unnecessary.
EDIT: The MEGA upper (and it's little brother the BCA side charger ) have lots of strength behind the threads.
7075 helps. Big heavy barrels seat against a thin flange at the very front of the receiver.
A tight fit helps but the flange contact surface sets things straight. Or not.
Won't fix a bent barrel though.
I'll keep typing
Isn't this similar to checking headspace when asked to supply YOUR bolt to the barrel maker?
Some won't give you that 'accuracy guarantee' unless you do.
You have got to be kidding me. How can a bolt manufacturer mess up on lug length and face depth?
Is there really a problem (5.56) with bolts out of spec enough to affect headspace?
The HOPE is that is exactly what lapping should do.
So the barrel extension flange seats in line with the receiver bore (and maybe the carrier and bolt).
Other than lapping, the final finisher could set the receiver in a fixture indexed to the receiver bore and indicate off the face and turn the face square.
Which is what every finisher should do long before it gets to the assembler.
That includes every factory bolt gun making truing totally unnecessary.
EDIT: The MEGA upper (and it's little brother the BCA side charger
) have lots of strength behind the threads.7075 helps. Big heavy barrels seat against a thin flange at the very front of the receiver.
A tight fit helps but the flange contact surface sets things straight. Or not.
Won't fix a bent barrel though.
I'll keep typing
Isn't this similar to checking headspace when asked to supply YOUR bolt to the barrel maker?
Some won't give you that 'accuracy guarantee' unless you do.
You have got to be kidding me. How can a bolt manufacturer mess up on lug length and face depth?
Is there really a problem (5.56) with bolts out of spec enough to affect headspace?
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It might make them consistent relative to each other but it doesn't necessarily make them square to the whole system. What if the receiver face was actually square but the bore was not?
I think much of what we're talking about is low probability. Personally, I think it sounds good in theory but doesn't do much in reality. If the receiver is cattywampus then 1) it might shoot just fine anyway or 2) lapping really isn't going to change much.
Again, that's just my thoughts, I'm open to seeing it differently. I've often pondered this topic.
-Stooxie
That doesn't make any sense. Square to what? Think about what "square" means in this context; one feature by itself can't be square, that has to be measured relative to some other feature. Look at a geometrical square - the individual lines are square to each other. No one line by itself (just like the receiver bore) is "square" without at least one other line for reference.
In this case, the entire point is to make the receiver face square to the bore. The tools used to do that ensure that's what happens, either correctly setup in a lathe or with a good lapping tool. The result of making the receiver face square is to make the barrel parallel to the receiver bore, because the shoulder of the barrel extension is supposed to be square to the barrel bore. It's like butting two squares against each other, which makes the top and bottom lines parallel to each other. If the side of one square was not square, but at some angle other than 90*, then the top and bottom of the two squares would not be parallel, which means uneven bolt lug contact.
Do you have any significant experience working with metal, machining, or even gunsmithing? Because you're talking to people who do, and know what they're talking about here.
And I thought only lefties resorted to name calling when they got confused...
I do remember my 8th grade geometry, doing all those theorems and proofs. I'm well aware of how angles work. I was just saying, just the same as you have, that this is all based on the assumption that the bore itself is square to the entire upper. If I take a perfectly square block of wood and drill a random hole though it, it won't be square to the sides. Lapping the receiver face can/will make the receiver face square to the bore, but both may not necessarily be square to the sides of the receiver. Take your average cheap ass upper receiver forging. Hole has to get there somehow, right? Error can be introduced at any step.
It's low probability, as I said, but then, we're talking about sloppy machining to begin with. Now, if you have the whole thing set up on a lathe where you can check centers, etc, I would have vastly more confidence in the processing being worthwhile than someone with a hand drill and a drop-in lapping tool.
Now, if you tell me that as long as the receiver face is square to the bore then it doesn't actually matter if they are not perfectly square to the sides/top/bottom of the receiver, then fine, but that's not a question that I have seen answered.
-Stooxie
I do remember my 8th grade geometry, doing all those theorems and proofs. I'm well aware of how angles work. I was just saying, just the same as you have, that this is all based on the assumption that the bore itself is square to the entire upper. If I take a perfectly square block of wood and drill a random hole though it, it won't be square to the sides. Lapping the receiver face can/will make the receiver face square to the bore, but both may not necessarily be square to the sides of the receiver. Take your average cheap ass upper receiver forging. Hole has to get there somehow, right? Error can be introduced at any step.
It's low probability, as I said, but then, we're talking about sloppy machining to begin with. Now, if you have the whole thing set up on a lathe where you can check centers, etc, I would have vastly more confidence in the processing being worthwhile than someone with a hand drill and a drop-in lapping tool.
Now, if you tell me that as long as the receiver face is square to the bore then it doesn't actually matter if they are not perfectly square to the sides/top/bottom of the receiver, then fine, but that's not a question that I have seen answered.
-Stooxie
Of the top/side/bottom I guess the top is the next important after the bore. Pic rails need to somewhat in line. At least within the adjustment range of your scope/sights. Bottom needs to mate with the lower but that's nor really a precision dimension is it? Some mate up some don't.
Sides shouldn't matter, not with all the different thickness and profiles available.
I think the issue here is the price range we are dealing with.
I got a Free upper with a BA barrel (supposedly a $100 upper) and just touching the face showed it did NOT need anything done to it. Wide flat full circle contact.
Used mag polish with the lapping tool to mark it, aluminum black and done.
The drill method just scares me. I grease the bore, pad and hold the upper pointed toward the sky and hand lap slowly and lightly.
Let gravity set the load.
A couple Anderson blems obviously need squaring but for $40 what would you expect. Same with a PSA finished by who knows.
3 of the 4 BCA side chargers (like the one in the earlier pic) probably benefited from lapping.
One upper I lapped enough to get the nut to line up rather than pull harder or use shims.
And pulling harder wasn't really an option with a 8 position nut.
I keep looking at those MEGA uppers, and know that they will be gone forever, but just don't have the extra right now
Sides shouldn't matter, not with all the different thickness and profiles available.
I think the issue here is the price range we are dealing with.
I got a Free upper with a BA barrel (supposedly a $100 upper) and just touching the face showed it did NOT need anything done to it. Wide flat full circle contact.
Used mag polish with the lapping tool to mark it, aluminum black and done.
The drill method just scares me. I grease the bore, pad and hold the upper pointed toward the sky and hand lap slowly and lightly.
Let gravity set the load.
A couple Anderson blems obviously need squaring but for $40 what would you expect. Same with a PSA finished by who knows.
3 of the 4 BCA side chargers (like the one in the earlier pic) probably benefited from lapping.
One upper I lapped enough to get the nut to line up rather than pull harder or use shims.
And pulling harder wasn't really an option with a 8 position nut.
I keep looking at those MEGA uppers, and know that they will be gone forever, but just don't have the extra right now
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I'm not interested in side chargers, so I'll just stick to Cross & Vltor MUR uppers & continue to lap them.
BCM's are Ok, but I see no reason not to use a thicker walled upper unless you can't live with the weight; if i want lighter, the V-Seven is also an option.
MM
BCM's are Ok, but I see no reason not to use a thicker walled upper unless you can't live with the weight; if i want lighter, the V-Seven is also an option.
MM
I ordered one and it showed up today. I blame you for the amount of money that I am about to spend! The coating is so nice and smooth and amazing! I’m sad that I didn’t start with these. At least for my “nicer” bastard AR’s. Good find and thanks for the recommendation!
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