Receiver Lapping??

[Double Naught Spy]

This is a generic AR build question, but two of us are putting Grendel ARs together. My buddy swears by this tool for truing up the barrel to the receiver.

Wheeler Engineering Delta Series Upper Receiver Lapping Tool AR-15

http://www.midwayusa.com/product/821295/wheeler-engineering-delta-series-upper-receiver-lapping-tool-ar-15

Apparently, the gizmo indexes off of other surfaces inside the upper receivers to square the surface for the barrel to mate with. While I understand the concept of wanting a properly mated receiver and barrel, I fail to understand how this is necessarily accomplished. The concept seems to be predicated on correcting the tolerances of one aspect of the receiver based on the tolerances of another aspect of the receiver.

My first concern is that how do you know if you need this? What is the test to determine if you need it and if you do, how do you determine that the work you did accomplished the task properly?

Second, since the barrel mates to the upper, shoulder the barrel not also be lapped to assure proper squaring. Otherwise, you are just fixing 1/2 of the potential problem, right?

Third, is there anything detrimental that could result from doing this? If this is supposed to alter the mate surface sufficiently, could this also not produce a change that would have a negative impact?

What I got from my buddy was that he felt that at worst, his receiver would be no better off than had he not done the work and that it would very likely improve things. He did not feel that anything could be screwed up by using it. He knew of no way to verify that the work was needed, - No way to verify that everything was properly aligned to do the work, - No way to verify that the resultant work had produced any actual benefit.

My thoughts on the deal are that you build the upper first. If things don't appear to be true, then take it apart and then maybe see about trying this gizmo to seem if things can be brought into alignment...but I have never built from scratch before, just rebuilt uppers.

Thoughts?

[montana]

I lapped all of my uppers. When you first lap you will see a shiny circle begin to form on the outside of the upper where the barrel mates to the upper. If the circle is even all the way around the front of the receiver your upper is square. If you see only a part of the circle shiny your upper is in need of lapping. My SLR, AA, and Yankee Hill uppers were perfectly squared or needed very little lapping but the others were in need of lapping. The only way to measure the barrel would be to put in a lathe and gauge it that I'm aware of. Lapping is an easy process that I do before any build. Many people have had perfect luck without lapping but my 18" Grendel's accuracy improved immensely after I lapped the upper. I was surprised how much some of my uppers were out of spec after lapping .LRRPF52 had pictures of one of his builds during lapping of one of his builds showing the process on this forum.

[Bill257]

Receiver Lapping??

Disclaimer, I am not a gun smith and did not stay in a Holiday in express last night, just a guy who has built a few and is interested in them being accurate.

1 - Using the tool determines how much is needed, if any at all. Run it for 5 seconds and examine the upper. If there is an even ring of anodizing worn off it is done. If there is a "high" spot, run it in short intervals checking often till you are satisfied it is even.

2. Presumably the barrel and barrel extension were cut on a lathe and through the nature of that process are square... However you have to take that on faith unless you have access to machinery and an experienced machinist to check. This falls into the check/ fix what you can catagory.

3. No way I could imagine this having a negative impact.

I have only used a similar tool sold by Brownells on 4 uppers. Three were confirmed square in a few seconds. One needed 3 or 4 five second cycles to square. Does it make a difference? No way to know unless the upper is tested, then squared and tested again. Will I do it again, yes. It is easy to do the first time and not wonder about what would have happened if I had... I'd be interested in a before and after comparison; the people who make the tool would have to do this as everyone I know puts it together with the intention of it staying together.

I'd recommend being careful to ensure the compound is only on the face of the tool that contacts the receiver. Also, check / clean the tool and inside of the upper when the tool is removed - you wouldn't want any of the grit transferred via finger prints inside where a tight barrel / upper fit is desired. (use a q-tip to apply the compound to the tool. Second tip, a little compound is all that's needed. third tip, if multiple grit compounds are available, start with the finest)

Whatever you did in the past works, as evidenced by your great shooting. I always enjoy your videos. If you have reservations, skip it. But, I do feel it is worthwhile, although I think bedding the barrel and getting the barrel nut tight are more important.

Keep up the good work and best of luck with the build.
Bill

[LRRPF52]

I use the tool to check square on all uppers. Most have been square lately, so no lapping needed, but in the past, I've found many that were way off.

The big problem is bolt lug engagement. If the receiver is out of square, you can have major problems with only 2 bolt lugs taking the load of bolt thrust and shear forces.

Aside from that, it contributes to accuracy like blueprinting a bolt gun action.

[just_john]

I have built about 20 AR's of various persuasions and started lapping after one of 'em was sufficiently off-square that I could see the barrel angle. Most have been"pretty close" but none have been truely square. Also note that I will only use forged uppers.
There is another consideration, I picked up some ceracoated uppers with the coating applied to all external AND internal surfaces. When I did a first pass "do the pieces all fit?" there was some interference between the bolt and the coating inside the upper. Using the rubbing compound with the tool on both the length of the tool shaft and the barrel mating flange it cleaned it up real well. Note, when doing this, I mount the upper in a vice vertically so that there is no lateral force to the process.

[llamaboy]

About 30 years ago, I took up benchrest shooting. My first rifle was based on a Rem 700 action. My gunsmith who had assembled many very accurate rifles, some setting national records, explained that the receiver had to be trued. The theory is that everything must be square/parallel to the bore. He lapped the locking lugs, squared the bolt face, squared the receiver face, trued the receiver threads, and god knows what else. When all was done, I had a shooter!


Advance now to the present, and I'm into ARs. But I feel the same principles apply. I've bought a RRA in 223, and a PF in Grendel. Col Townsend Whelen's remark about "only accurate rifles are interesting" still guides my behavior. I am completely amazed that a semiauto, especially with all those moving parts can be so accurate. As I see it, the weak spots in an AR are the barrel to receiver fit, locking lugs, fit of bolt in carrier, and the carrier itself rattling down the receiver. Most of these areas require faith that there is a proper fit. The one area that you can control is the barrel to receiver fit.


Enter the lapping tool. Remember, the tool guides on the same raceway as the bolt carrier. So any adjustment to the receiver face will be square to the carrier, which in turn, will be parallel to the bore.


If you are assembling a new rifle from parts, it doesn't hurt to check it for square. If an existing rifle shoots to your expectations/satisfaction, then it ain't broke. If not, break out the lapping tool.

[LRRPF52]

I have built about 20 AR's of various persuasions and started lapping after one of 'em was sufficiently off-square that I could see the barrel angle. Most have been"pretty close" but none have been truely square. Also note that I will only use forged uppers.
There is another consideration, I picked up some ceracoated uppers with the coating applied to all external AND internal surfaces. When I did a first pass "do the pieces all fit?" there was some interference between the bolt and the coating inside the upper. Using the rubbing compound with the tool on both the length of the tool shaft and the barrel mating flange it cleaned it up real well. Note, when doing this, I mount the upper in a vice vertically so that there is no lateral force to the process.

You bring up some good points that I have seen over the years.

First off, Cerakote H Series is not meant to have metal running along it, and it will gall as well as cause significant coefficient of friction on the carrier rails if applied inside the carrier raceway. A true Mil-spec AR15 will have dry film lubricant baked on the inside of the upper, which helps suspend lubricants.

I personally prefer not to coat the inside of the upper when Cerakoting. The lapping tool, as you say, is a great way to take that inner layer of Cerakote down if it's there.

I also do the work in the vertical axis, as to not let gravity bias the tool in one direction, after consulting with an engineer/gunsmith friend who is one of the most exceptional smiths in this industry.

[Dugan324]

Are there better lapping tools than others?

[customcutter]

DNS, there are ways to determine whether or not your upper needs to be trued. However, unless you're a machinist and already have the tools and know how to use them, they would cost more and take more time to set up and determine if your upper was square, than using the tool that you linked. There is no way that the tool can be made in-correctly if it was made on a lathe, unless it has an obvious nick or ding in it.

It is cheap insurance to insure the squareness of the barrel & extension to the upper, and therefore accuracy.

Here are a few more tips you might want to consider.

http://www.65grendel.com/forum/showthread.php?10796-What-makes-an-AR-accurate-by-Robert-Whitley

[devildogandboy]

Are there better lapping tools than others?

don't know if i would say it's better, but it's what i have. http://www.brownells.com/gunsmith-to...prod20220.aspx

Bruce

[Wheelhorse]

I have an AR on which the windage adjustment had to be almost maxed out to zero the rifle.
When checked with a lapping tool, the receiver was clearly high on one side. After it was cleaned up all the way around, the windage adjustment is now very near the center when zeroed.
All of mine get checked now, some are OK and some aren't.
After using the tool a few times, it may need to be put in a lathe and cleaned up, because the lapping compound will quickly wear on the face.

[montana]

I have an AR on which the windage adjustment had to be almost maxed out to zero the rifle.
When checked with a lapping tool, the receiver was clearly high on one side. After it was cleaned up all the way around, the windage adjustment is now very near the center when zeroed.
All of mine get checked now, some are OK and some aren't.
After using the tool a few times, it may need to be put in a lathe and cleaned up, because the lapping compound will quickly wear on the face.

I have put my lapping tool on my lathe 3 times so far. The lapping compound will cut a groove in the face of the tool after some use. Good point Wheel horse!

[Dugan324]

Thanks Bruce. Just threw one in the cart

[customcutter]

I have put my lapping tool on my lathe 3 times so far. The lapping compound will cut a groove in the face of the tool after some use. Good point Wheel horse!

The lapping compound should be cutting a groove of equal depth all the way around the tool. If it isn't, there is something wrong with the flux modulation in the gravitational forces caused by the squared speed of light. :-)

[sneaky one]

Oh, man! Just check them all before a build.

I handed my so-so R Guns rec. to 6.5 Whelon last March to check for square- we were doing some trading of parts- Glad he checked it, it was Way out of close to being square.
The assembled rifle is a shooter! I did ink blot testing on the bolt lugs to contact the Ext. All looked perfect. Damn kids!

Bedding the 2 mating surfaces with Permatex- 24300- blue glue - we call it that- maybe helped a bit also. Confidence helps also, in all you did properly in and upon a build.

Guys with Awesome lathes , - they could offer up the service of a - rec. square. for decent price ? RS? Are you listening? 6.5 Whelon? Montana? Custom cutter, anyone else? LRRFP 52? Guys with the trueing tools can hop in too ! Check for Square dudes!

Remember, this is Team Grendel . We are all in this together. We all help each other. No beatching, no whining. Forward is the only motion. What a team!