Upper Receiver Lapping

Rlandry

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is there a benefit to doing it? Also, doesn't lapping affect headspace and how do you know wen enough is enough? Are you just looking for a shiny surface all the way around?
 

OLD308

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Years ago we did this, still have the tool some where. Once I started experiencing comparing receivers that were lapped vs unlapped I stop doing it.

Never had one so out that it effected headspace. Never took much more than a few rounds to show even. That’s all your going for. Even wear mark all the way around.

I think the shop offered it solely as a billable option. If it was a real issue, it would have been preformed on the lathe. Not worth it to me. But if you want to try I might be able to find the tool.

Hope this helps.
 

Merle

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An AR headspace is set in the barrel extension and chamber, not the upper receiver. I could maybe see a problem with the bcg not going forward far enough that the hammer wouldn't hit the firing pin but you would have to probably take a metric shit ton of material off the face of the receiver for that to become a problem.
 

redneckbmxer24

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Using a stiff receiver like the VLTOR MUR and then bedding your barrel extension in the receiver is far more beneficial. On a standard mil spec receiver lapping will get you a little but start with a high quality receiver and no way would I touch it with a cheap lapping tool that who knows how straight it is.

Headspace is set in the barrel extension/bolt. Receiver and lapping or not has nothing to do with it.
 

bfoosh006

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I look for a shiny surface all the way around. Make sure the tool is a good fit... I bought a Wheeler brand ( just to see, on sale ) .. and the fit was so poor, I'd bet it would do more harm the good.

I also use Loctite to fill any voids in a "less" then snug barrel extension and upper.

My Pacific tool & Gauge ones fit very well.
 

Cy Rosenlund

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I look for a shiny surface all the way around. Make sure the tool is a good fit... I bought a Wheeler brand ( just to see, on sale ) .. and the fit was so poor, I'd bet it would do more harm the good.

I also use Loctite to fill any voids in a "less" then snug barrel extension and upper.

My Pacific tool & Gauge ones fit very well.
Do you use red or blue? Or does it not matter?
 

Cayenne6

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I never lap an AR receiver but the know people who do. It’s personal preference. Kinda like lapping a good set of rings lol.
 

ssdrew

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I have in the past. I had an upper reciever that wasn't quite in spec. The feed ramps on the barrel didn't line up with the ones on the upper and used it to even those up. I've used blue loctite to bed the extension to upper. I will say, it's a pain to remove. These days, if the extension is a nice tight fit in the upper, I just torque it down and send it. I've had good luck with Aero uppers lately.
 

346ci

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That tool is a joke, it needs to be chucked in a lathe for a good job.

Doing this can help bolt to extension interface, cure uneven wear on bolt lugs.
 
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XLR308

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Lapping in many cases isn't necessary but if you have the tools allready there's no reason not to do it other than laziness.
For those that have mentioned the tools being a sloppy fit the use of the proper lubricant can make all the difference as in the use of a thick motor oil like a 15w 40 used in diesel engines or a gearblube to create a proper fluid bearing that takes up that space.
You only want to see 85-95% clean up to ensure the lugs on the bolt are making even contact in the barrel extension to ensure even loading and less likelihood of breaking bolts or uneven wear, this is obviously more of an issue in non standard calibers with a larger bolt face and more bolt thrust than the 556 such as the 68spc and 65 Grendel and thier variants.
Of course the use of higher quality receivers makes a difference but is no guarantee that the condition of unsquare surfaces won't exist it just depends how much a person wants to invest into the process of building.
 

Yondering

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Years ago we did this, still have the tool some where. Once I started experiencing comparing receivers that were lapped vs unlapped I stop doing it.

Never had one so out that it effected headspace. Never took much more than a few rounds to show even. That’s all your going for. Even wear mark all the way around.

I think the shop offered it solely as a billable option. If it was a real issue, it would have been preformed on the lathe. Not worth it to me. But if you want to try I might be able to find the tool.

Hope this helps.
It doesn't help. In fact, your info is misleading to those who don't know any better.

- Lapping an upper receiver has zero effect on headspace. It's completely unrelated. If that's your comparison between lapped and unlapped, obviously you never saw a difference.

- Lapping the receiver squares the barrel extension for even bolt lug contact. This is about maximizing bolt life; it's not really about accuracy except in extreme examples.

- Tight fitting receivers and barrel extensions don't benefit much from lapping. Looser fitting receivers do.

- Receivers vary a lot in how square they are. Some are pretty good out of the box, and some aren't. Most I've worked on can use a little truing, some need a lot.

- Truing does NOT have to be done in a lathe to be done right. A lapping tool that correctly fits the upper can do just as good a job or better, and is usually faster to use as well. Speaking as a guy who has a lathe and about 20 years of machining experience, and has done it both ways.
 
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Shifty6BR

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I lapped a receiver post build for two reasons. One was for timing a Noveske barrel nut to a more suitable torque, prior to lapping the bn would only time at the very, very low end or at a very, very high amount of torque, even with playing with shims I could not find a preferred range. Lapping alleviated and also eliminated the need for barrel nut shims.

I also wanted to see if it would help with my first round flyer, which was extreme, always 3" high and 2" right, not good. This would always happen after cycling up a new mag, even after warming up the tube. It also alleviated this problem. Now was this flyer issue related to the original torque value of 31 ft lbs prior to lapping or laid elsewhere in the system, I can not say for certain, but I can say that whatever it was it no longer exists post lapping.

I have a few other uppers that have not been lapped and did not exhibit the odd quirks of the upper described above. The tool I used is a GG and it was a tight fit.
 

Longshot85

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I followed Padom's advise

This was a somewhat "budget SPR style" build....

Got a BCM upper that is thermo fit and also used above mentioned GREEN loctite on barrel extension

Result is an upper that shoots 0.5-0.75 MOA with factory IMI 77 grain Razorcore

Hand loads coming soon
 
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padom

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I followed Padom's advise

This was a somewhat "budget SPR style" build....

Got a BCM upper that is thermo fit and also used above mentioned GREEN loctite on barrel extension

Result is an upper that shoots 0.5-0.75 MOA with factory IMI 77 grain Razorcore

Hand loads coming soon
There you go. Best forged upper out there.. I cant count how many I've built off the BCM upper.
 
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bfoosh006

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Mike_in_FL

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I followed Padom's advise

This was a somewhat "budget SPR style" build....

Got a BCM upper that is thermo fit and also used above mentioned GREEN loctite on barrel extension

Result is an upper that shoots 0.5-0.75 MOA with factory IMI 77 grain Razorcore

Hand loads coming soon
You don't need to reinvent the wheel if you're getting good advice. Try 8208XBR if you want your own 77 gr. loads.
 

Longshot85

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I have some, in the middle of a move....had some initial stellar results with 8208 and TMK 77 grain....was too hot though....I'll back it down to lower node
 

Yondering

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I followed Padom's advise

This was a somewhat "budget SPR style" build....

Got a BCM upper that is thermo fit and also used above mentioned GREEN loctite on barrel extension

Result is an upper that shoots 0.5-0.75 MOA with factory IMI 77 grain Razorcore

Hand loads coming soon
BCM is great for that, and don't need to be lapped since the alignment comes from the tight fit of the receiver to the barrel extension.

However - if you used one of the green Loctite products (there are several, just saying "green" doesn't mean much) and heated the receiver to install the barrel - you got very lucky to get the barrel fully installed before the Loctite set up.

Don't use Loctite sleeve retaining compounds on heat fit installations; you don't need it, and it can set up with the parts only partially assembled.
 

Yondering

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Good links, especially the third one.

For gun parts assembly - just use #680 for every "green Loctite" application. It handles up to .015" gap and is as strong as any of the others. If you have 680, you don't need any other green sleeve retaining product (for guns). It's also good for installing handguards and bedding scope bases to receivers. 620 handles another 100° F higher than 680, but in practice on gun parts like AR upper assemblies it doesn't matter. What does matter is that 680 is oil resistant. Both 620 and 680 are good choices though if used appropriately (clean surfaces really well for 620).

To those guys just referring to "green loctite" - keep in mind green is more than one thing. There is also a green wicking grade for example that's totally different than 620, 680, etc; it's really thin and is intended to wick into assembled threaded fasteners. Not what you want to use on a barrel installation.
 
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Constructor

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Do you use red or blue? Or does it not matter?
It doesn't really matter, we have been using blue since the late 80s. When you want to change barrels we just tap the barrel out of the receiver with a wooden dowel. We started doing it so the connection was more rigid and bolt lugs square with extension lugs since the receivers were so sloppy back then. Same thought as lapping the lugs on a bolt gun.
 
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Cy Rosenlund

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Well, Saturday I got to hang out with grandpa and he spun up a mandril for lapping the reciever face. Ended up using some valve lapping compound (not sure if thats socially accepted here) but its what we had on hand. Went alot smoother than I thought!20191123_114134.jpg20191123_124001.jpg
 

Longshot85

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As far as the green loctite, I followed the specific number Padom referred me to.....he did specify which one

I am very appreciative of those that share their wisdom

But as far as lapping from of receiver......back tracking to the bolt.......if there is ANY "play" in the bolt as it slides into extension.....I fail to see how lapping does anything at all

Unless the rationale is it lines up bolt more with extension in a "straight line"..........
 

Yondering

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But as far as lapping from of receiver......back tracking to the bolt.......if there is ANY "play" in the bolt as it slides into extension.....I fail to see how lapping does anything at all

Unless the rationale is it lines up bolt more with extension in a "straight line"..........
It's all about angles, eliminating them that is. If the front of the receiver is crooked and a loose fit, the barrel extension will sit in the receiver at a slight angle. The bolt can move around radially, but can't make up for an angle except a very tiny amount. If the barrel extension is crooked by just a few thousandths at the receiver face, the back of the bolt carrier would have to move many times that much to allow the bolt to sit at an angle, and it can't. The mass of the carrier also prevents that from happening even if it had enough clearance.