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Posted: 12/17/2018 9:04:34 PM EDT
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for trueing upper receivers? Is this one from Brownells the one most people use?
https://www.brownells.com/gunsmith-tools-supplies/rifle-tools/receiver-tools/ar-15-m16-upper-receiver-lapping-tool-prod20220.aspx |
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Lapping the receiver has its risks.... see the comment from the 2A-Armament rep Ryan in this thread:
https://www.ar15.com/forums/industry/Do-I-need-to-lap-a-Balios-Lite-upper-/493-269790/ "One thing to consider, and BK already hit it in a more technical sense, is this.... If the lapping tool can fit down the bore, it means that it's sitting inside that bore at an angle. even if it's a thousandth out on that bore, it will translate to a thousandth or more! out of square at the face of that receiver. You could be doing damage by lapping that face. EDIT: After looking at these tools on line, I am going to go ahead and say that they are in fact TAKING THE FACE OUT OF SQUARE of the bore! The bore dimension for the extension is tighter than the dimension on the carrier bore. If you look at these tools, the pilot is inserted, and skips past the tightly held extension bore, and goes into the looser dimensioned carrier bore. So as this tool falls into the carrier bore, it is forcing the laping face out of square with the extension bore." |
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There has been several reports of AR’s stringing shots as they warm up and it was traced to the receiver face being way “out of square”.
I’ve used mine on at least a dozen receivers, some were damn near perfect, some were WAY out. IF you consider how tight it is in the receiver AND the length of the shaft, I personally feel that the odds of error are tiny. |
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IF you consider how tight it is in the receiver AND the length of the shaft, I personally feel that the odds of error are tiny. And frankly at 30-40 bucks for a flattop upper, it's not like it's an expensive mistake even IF the tool doesn't correct anything. Though every receiver I've used one on has been corrected, and not worsened. 
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| Ya, I figure I can't hurt too much on a $40 Anderson, but it might help quite a bit. I looked at the one Midway carries but I didn't like the flexible drive that chucks into the drill. And there were quite a few negative reviews. Think I'll go with Brownells. Thanks for the replies. |
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Ya, I figure I can't hurt too much on a $40 Anderson, but it might help quite a bit. I looked at the one Midway carries but I didn't like the flexible drive that chucks into the drill. And there were quite a few negative reviews. Think I'll go with Brownells. Thanks for the replies.  ); I'm tearing down my MATEN this afternoon to start a deep clean on the gas block (it's supposed to be adjustable but at the moment is pretty much in a fixed position cause i got lazy with the cleaning regimen  ) and I'll be pulling the barrel to true the receiver face with it. Given its mediocre performance to date with just about everything other than 168 NBTs (which were surprisingly fantastic) it will hopefully do SOMETHING. probably be back at the range a week from Thursday to find out.
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I have used the Brownells lapping tool on dozens of uppers and had about a dozen more turned on lathes. What I have found from my experience comparing hand lapped uppers to those I have had trued up on a lathe, is that one way is as good as the other at truing up the face of the receiver. IMO those telling you the hand lapping will hurt you is them trying to sell you their services based more on them wanting to get your money than any real facts or evidence. I’m highly skeptical of their so called evidence. Also, I have seen repeatedly that both hand lapping and lathe truing will almost assure you have sights zero in near the center of their adjustment range but in neither method have I seen even a tiny improvement in accuracy. Granted, I’ve only compared accuracy in a half dozen builds I did before I started lapping. But of those I did accuracy testing both before and after truing, I saw zero change in accuracy. I also have not seen any longevity increase because I have never worn out any bolt, carrier, barrel extension or anything else with either method of truing. For me, sights zeroing near their center range of adjustment is enough reason alone for me to continue lapping my uppers. |
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I bought this lapping tool currently $20 and also includes lapping compound. It also came with a flexible driver which I thought was cool. I used this lapping tool on every AR-15 I own (other than my MEGA monolithic receiver set and never had any negative issued that I could detect. |
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Make sure your FSB isn't canted. That was a thing for a little while apparently. Quoted:
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Thanks Big Bore, I have a PSA upper that has the rear sight all the way to the left to zero. Think I'll take that apart and true it to see if it helps. |
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PTG also sells the lapping tool for both AR-15 and AR-308. when I ordered my -308 tool, the turn-around was pretty quick (same week). YMMV.
PTG AR-15 upper receiver lapping tool PTG AR-10 upper receiver lapping tool |
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PTG also sells the lapping tool for both AR-15 and AR-308. when I ordered my -308 tool, the turn-around was pretty quick (same week). YMMV. PTG AR-15 upper receiver lapping tool PTG AR-10 upper receiver lapping tool |
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thread which actually discussed the math:
https://www.ar15.com/forums/AR-15/Lapped-upper-receiver/118-732745/?page=1&anc=bottom#bottom |
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Having not done one  , I believe you'd be better served and minimize error if using either a cutter or lapping tool, if the procedure was performed vertically and let gravity do it's thing.my time is valuable to me. |
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having done many, if you're going to rely on gravity, it's going to be a while. leaning into it with steady "medium" pressure, it already takes about 5-10 minutes with fine grit at slow speed, since you have to stop and check progress every minute or two. my time is valuable to me.
I should have said a little more. Yes apply moderate pressure but let "gravity" effect the tool much the same way as it does a plumb bob. Assuming the rcvr is secured vertically, the facing should be better while minimizing error. |
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lapping tool tip (wall of text):
I like to check my progress every minute or so. to do this, you retract the tool about a 1/2" or so, the main point being that you do not retract the tool too far such that the tool journal surface (that rides in the receiver bore) comes in contact with the lapping compound. If the journal surface does come into contact with lapping compound, then when you push it back in to resume lapping, you end up with swirl marks in your receiver bore. not performance affecting, and probably covered up by the barrel extension, but still highly annoying because you know they're there and you have OCD. slow speed, about 30 rpm to 60 rpm. you dont' want to burn up the surface from heat build up, lapping compound chunking. anyways, with the tool retracted the 1/2" or so, you disconnect the drill from the lapping tool. get a couple of Q-tips, and wipe a couple of spot areas clean on the upper receiver so you can see how you're progressing with the anodizing, aluminum. check at least 3 areas opposite of each other, or at least distributed around the circular area. you want to get to at least 2/3 lapped surface area to get the barrel extension flange to seat flat. you don't really need 100%, 3/4 of the area is more than sufficient. if you remember from high school geometry, 3 points determine a plane, and you want those 3 points to surround the bore center. and lapped anodizing counts as lapped area, they don't call it hard coat anodizing for nothing. just cause you can't see bare aluminum doesn't mean it's not lapped. with the lapping tool retracted, now is the time you use the small 1/8" blade screwdriver to reapply more lapping compound to the lapping surface, small dabs. I have the upper receiver mounted in a clam-shell vise block, in a bench vise, to support this effort. When doing the progress check, I un-chuck the drill motor from the lapping tool, because if you leave the drill attached to the tool, the weight of the drill motor will cause the clam-shell to flip up, the tool and drill suddenly slide out of the receiver onto your shop floor with no warning, maybe something gets damaged (tool, drill, toe), and then you're really pissed at yourself for your stupidity. at the same time, you don't want to clamp the vise down harder, because then you'll squash your receiver out-of-round. now, for the pièce de résistance: i have the receiver mounted in a clam shell block HORIZONTALLY, because when you check your work, with the lapping tool pulled out 1/2" or so, you will need all your hands for doing the check, so there's no 3rd hand to keep the lapping tool suspended 1/2" if you have it mounted vertically while you do your checking and reapply compound. when you're finished with this upper receiver, you pull the tool completely out, and give it a complete wipe down before starting on the next upper, because when you pull the tool out, the journal surface gets contaminated with lapping compound. If you don't completely wipe it down, you end up with those haunting swirl marks on the next upper. another tip: I use the cheapest wheel bearing grease to lubricate the lapping tool in the receiver bore. Before inserting my tool, I apply a very light coat of grease inside the receiver bore as far back as the lapping tool will reach, and I also apply a very light coat of wheel bearing grease to the whole length of the lapping tool journal area (the part that rides on the upper receiver surface). process improvement suggestions are welcome. |
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