Lapping project - trying this out

[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.