Lapping project - trying this out

[Wheelhorse]

I also lap my uppers in the verticle position and would not do them horizontally.

The way I see this is that most uppers are pretty close as shipped and comparing the benefits of lapping one of those will be hard to see.
Considering that the question is, do you assemble the rifle as is and live with it or do you make sure all are lapped during the assembly to remove this possible fault from the equation?

[lazyengineer]

Further assessment thoughts: Overall, I'm not entirely sure the test results aren't within the standard error of the test methodology itself. Which is to say, if I had done nothing at all, is it possible I could have gotten the same results? I actually think so, but can't say for sure, due to the direction of the bias of the results (i.e. none of the results showed a statistical decrease in accuracy: only no increase, or some increase).

To think on that - if the lapping systemically improved accuracy - or more to say, if the mating surfaces were improperly polished, then one would think lapping the upper to remove that defect would have systemically improved the accuracy across the board. I'm not sure these results show that.

But I am seeing potentially some improvement. It's inconsistent, and only largely shows up with the 90 TNTS. Of note there, prior experience with TNT's has shown that some days (pre lapping) they will shoot very well (close to these results), and some days (pre-lapping), they'd only shoot pretty-good. My baseline test with the TNT's was only pretty-good that day.

The fact that 3 of the 5 ammo's showed some improvement, and none showed measurable decline in accuracy, suggests that the measured improvement is probably more than just random noise, since they all bias in the same direction. So I can't conclude the lapping was a complete waste of time. But it's not really that profound of an improvement.

I will say, the PSA upper is actually a pretty darned good upper. It's forged, when mounting optics thewindage tends to be spot on. When lapping, the erosion patterns were quite even, suggesting it was already pretty square. The barrel extension fit into the PSA upper was quite tight. By all metrics, it seems to be a pretty great upper actually. My other Grendel is also a PSA, and I get similar accuracy results with it with the same ammo spectrum. I'm not going to bother pulling that one's barrel to lap it, I don't think this process is really worth the bother.

As the BBL itself, by my own analysis with one of those digital bore-sight rod-things, the machining looks quite good and uniform, throat looks good, and the crowns are good and square, there's nothing obvious amiss with the barrel.

FWIW, my Grendel has A2 FH on it, with a classic crush-washer. For my next check, I'm going to double check the torque on that, as muzzle devices should cap out at 30 ft-lb, or they can start to deform the barrel and affect accuracy (or so I'm told).

[imaguy3]

Returned to the range with lapped and rebuilt Grendel. Purpose of the test was to test as scientifically as one guy casually can; on if lapping really works. To this end, the only variable changed was the upper was lapped. Same gun, same barrel, same ammo, same distance, same targets, same optics, same shooter, and same shooting techniques (which with other guns easily beats 1 MOA results).

(pre lapping accuracy)
2.0 MOA: 129 gr SST w/ 27.8 gr Ramshot TAC: (0.8 MR)
2.2 MOA: 95 gr H Varm. w/ 27.0 gr LT-32: (0.7MR)
2.5 MOA: 90 gr TNT w/ 31.0 gr 8208: (1 MR)
2.6 MOA: Wolf Steel: (0.9 MR)
3.0 MOA: Federal 120 Gr Fusion factory: (1.2 MR)


Todays results (discussion will be in the next post):
90 TNT load: 1.3, 1.0, 1.2, 0.84 MOA (1.1 avg; 0.5 MR)
95 H Varm: 1.9, 1.7 MOA (1.8 avg; 0.7 MR)
100 gr Wolf: 2.2 MOA, 0.9 MR
120 gr Fed Fus: 2.7 MOA, 1.0 MR
123 gr Nosler: 3.1 MOA, 1 MR
129 Gr H SST: 2.3, 1.8, 2.5 (2.2 avg, 0.8 MR)

Was the barrel nut torque the same for both pre lapped and post lapped? What was the barrel nut torque?

[lazyengineer]

Was the barrel nut torque the same for both pre lapped and post lapped? What was the barrel nut torque?

I think so but didnt write it down and don't remember. In my experience, barrel nut torque hasn't historically matered a lot. In the classic world of high power service rifle and traditional barrel nuts, one would be forced to torque all over the incredibly wide range of appropriate torques from build to build, to get the gas tube lined up. And it never seemed to matter much.

[mtnlvr]

Now you should check the crown of your barrel and consider a recrown if it's weak, damaged or non existent.

[Mad Charlie]

I also lap my uppers in the verticle position and would not do them horizontally.

The way I see this is that most uppers are pretty close as shipped and comparing the benefits of lapping one of those will be hard to see.
Considering that the question is, do you assemble the rifle as is and live with it or do you make sure all are lapped during the assembly to remove this possible fault from the equation?

As easy as lapping and bedding is to do, and relatively inexpensive. Why leave potential accuracy on the table?

[just_john]

There is another consideration for lapping, it not only trues the barrel to the receiver, but it also helps to insure that all of the bolt lugs have the same ( or nearly so ) bearing surface area in contact with the extension lugs. This will go a long way to mitigate premature bolt failure.

[lazyengineer]

There is another consideration for lapping, it not only trues the barrel to the receiver, but it also helps to insure that all of the bolt lugs have the same ( or nearly so ) bearing surface area in contact with the extension lugs. This will go a long way to mitigate premature bolt failure.

That wasn't really my experience as being a need. With the PSA upper at least, it came from the factory quite square.

As to lapping being "easy", it's doable to an enthusiast who has all the right tools and gumption. I don't consider it a reasonable expectation for this to be a normal part of ownership for everyone to do. And that's one of the challenges of Grendel; it's pushing some engineering boundaries for the purpose of delivering impressive performance. But no other round has people encouraging lapping over fear of breaking bolts if you don't.

[Growler]

Lazy (the irony is strong with this one):

Sincere thanks for a detailed and objective report on your findings.

I think I’ll start by spending time and money on lower hanging fruit. That’s not to say I won’t lap, I just don’t plan to do it until I spend more $ on a barrel and the artist running the lathe.

[montana]

My 18" Grendel's accuracy improved from 1 1/2" groups to sub MOA after lapping. My YHM upper receivers were spot on and needed no lapping. My Anderson uppers always needed lapping. It is easy to see which manufacturers have tighter tolerances after lapping.

[LRRPF52]

That wasn't really my experience as being a need. With the PSA upper at least, it came from the factory quite square.

As to lapping being "easy", it's doable to an enthusiast who has all the right tools and gumption. I don't consider it a reasonable expectation for this to be a normal part of ownership for everyone to do. And that's one of the challenges of Grendel; it's pushing some engineering boundaries for the purpose of delivering impressive performance. But no other round has people encouraging lapping over fear of breaking bolts if you don't.

The first uppers I heard about out-of-square uppers breaking bolts were 5.56 and 7.62x39.

The principles are the same regardless of caliber/chambering, so I wouldn't call it cartridge-specific condition at all.

[ARErector]

That wasn't really my experience as being a need. With the PSA upper at least, it came from the factory quite square.

As to lapping being "easy", it's doable to an enthusiast who has all the right tools and gumption. I don't consider it a reasonable expectation for this to be a normal part of ownership for everyone to do. And that's one of the challenges of Grendel; it's pushing some engineering boundaries for the purpose of delivering impressive performance. But no other round has people encouraging lapping over fear of breaking bolts if you don't.

The first uppers I heard about out-of-square uppers breaking bolts were 5.56 and 7.62x39.

The principles are the same regardless of caliber/chambering, so I wouldn't call it cartridge-specific condition at all.

I hadn't heard of a connection between bolts breaking and the upper not being square on other calibers, just here on the 6.5 Grendel site. I think that has more to do with the case rim size, since there's less meat on the bolt lugs than on 5.56. That's not to say it doesn't happen on other calibers, I just think it applies more to cases with a larger diameter rim like the Grendel.

Even with other calibers I have heard positive things about lapping the upper before assembly. Mostly it was for accuracy. I didn't get around to buying the tool 'til I built my Grendel, but I plan on lapping my uppers on all my future builds now that I have the tool. I will probably tear down some of the old builds as well, starting with the least accurate, but it is not a high priority. I'm curious to see what difference it makes on them.

Thanks for posting your experience with it.

[LRRPF52]

ArmaLite Inc. (Mark Westrom ArmaLite-formerly Eagle Arms) and LMT even made 5.56 AR15 bolts with balanced lug designs to relieve asymmetric loading on the lug opposite of the extractor before Grendel was even born.

The TDP calls for a square receiver face on the AR15. I might have mentioned it already but anodizing after machining is the culprit for out-of-square receiver faces most of the time. You would really have to louse up the machining process to cut the face out-of-square, since a lot of shops do the boring of the bolt carrier raceway and extension tunnel plus face in the same operation for optimum tool time.

[Tm1293]

Lapping is a good start to improve accuracy as bore alignment to the bolt face is more crucial than most realize.

HOWEVER, what most dont realize it that no matter how much you tighten the barrel nut to a perfectly square receiver face, if the barrel extension isnt a perfect (down to as much as .001in) fit to the receiver's barrel port, there WILL be movement and degradation of accuracy. No barrel extension mass produced is the exact size and fitment to any receivers barrel port. Check it with a micrometer next time.

In the many builds I have done for match and varmint, the best and easiest method of accuracy gain (while building the upper) comes by shimming the barrel extension into the receiver. Brownells, Midway, etc sell sheets of shim metal ranging from .001 to .05 thickness.

Start with a sliver about 2mm wide and the length of the extension from the lip (where it fits the receiver face) to the edge where it is just past the inside of the receiver. Insert the barrel with the shim attached to the extension (preferably at the top side away from feeding ramps) and take a small bit off the width of the shim (2mm and slowly work down to 1mm wide) until it fits. When it is a proper fit, you will have to tap the barrel in with a deadblow (thread a junk A2 on the end to protect the crown) and have a decently hard time removing the barrel. I have commonly used green loctite (press fit, works like a tacky glue) on the bearing surface of the extension to reduce any chance of movement further. Careful to not get it inside the action or lug area. Then just tighten barrel nut to factory torque specs.

I have made m16-a2 Vietnam era rifles shoot sub-moa with this trick.

[A5BLASTER]

Lapping is a good start to improve accuracy as bore alignment to the bolt face is more crucial than most realize.

HOWEVER, what most dont realize it that no matter how much you tighten the barrel nut to a perfectly square receiver face, if the barrel extension isnt a perfect (down to as much as .001in) fit to the receiver's barrel port, there WILL be movement and degradation of accuracy. No barrel extension mass produced is the exact size and fitment to any receivers barrel port. Check it with a micrometer next time.

In the many builds I have done for match and varmint, the best and easiest method of accuracy gain (while building the upper) comes by shimming the barrel extension into the receiver. Brownells, Midway, etc sell sheets of shim metal ranging from .001 to .05 thickness.

Start with a sliver about 2mm wide and the length of the extension from the lip (where it fits the receiver face) to the edge where it is just past the inside of the receiver. Insert the barrel with the shim attached to the extension (preferably at the top side away from feeding ramps) and take a small bit off the width of the shim (2mm and slowly work down to 1mm wide) until it fits. When it is a proper fit, you will have to tap the barrel in with a deadblow (thread a junk A2 on the end to protect the crown) and have a decently hard time removing the barrel. I have commonly used green loctite (press fit, works like a tacky glue) on the bearing surface of the extension to reduce any chance of movement further. Careful to not get it inside the action or lug area. Then just tighten barrel nut to factory torque specs.

I have made m16-a2 Vietnam era rifles shoot sub-moa with this trick.

I use the shim stock myself. But I cover the whole extention and then do a freeze and heat method. So when I install the barrel with shim it's a very tight compression fit.