SDI GRAD ADVICE - Lapping Your AR-15

On this week's SDI Grad Advice, Caleb shows you had to make the connection between your upper receiver and barrel sure and true!
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Sonoran Desert Institute (SDI) is an online school that helps students learn the skills and techniques they’ll need to be successful in the firearms and unmanned technology industries. SDI is accredited by the Distance Education Accrediting Commission (DEAC).
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www.sdi.edu/ytlearnmore/
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Sonoran Desert Institute, 1555 W. University Dr., Ste. 103, Tempe, AZ 85281
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[@Gun4All]

Some dykem or sharpie marker will help you to see the high and low spots a lot easier.

[@WHATAGIRLWANTS2009]

I would strongly suggest a light oil to lubricate the guide inside the receiver and double-check that you haven't left a burr on the ID and OD of the lapped surface. Also, going a little slower RPM helps to not throw the lapping compound and to reduce heat build-up.

[@taposgunsmithing]

I would highly highly suggest watching Brownells' video on lapping your upper receiver. It is also important to make sure that if you do this to check that the shoulder on the threads is perpendicular to the bore of the receiver.

[@drake0074]

This is basically want you want to do for a legit SPR build. It's a basic part of accurizing the rifle.

[@demomanmatt]

After the first run, you had lapping compound on the tool where it contacts the bore of the receiver. An effective way to increase the diameter of the receiver and wear out the anodizing. Oil works better on that part of the tool.

[@sorryociffer]

Suggestion… use a sharply marker to color the portion of the part you are truing so that you can have a much better visual indicator of your progress.

[@joncross4537]

You should put some oil on the mandroll when you put it in the receiver.

[@alesanaalesana5430]

Thank you for the video; going to help a lot. May 22nd, looking forward to school and learning.

[@codyjenn8785]

Thank you for the awesome video I love sdi

[@sklmarksmanhd7560]

Can't wait to start my school, I start June 19

[@tommyjlink03]

I am an SDI grad myself! Something I have found that helps accuracy after the lapping process, is using a little green loctight on the inside of the receiver where the barrel extension slides in. I tried this on my LR build during school and it improved the accuracy very well.

What about the barrels shoulder mating surface how to we know its perpendicular to the barrels center line?

[@5jjt]

Do you grind until all of the color is gone from the face of the receiver?

[@zevgoldman6769]

How do you address an uneven surface on the mating face of the barrel ring?

[@MassholeMachinist]

Those barrel extensions are machined as such that it is nearly impossible for that face to not be perpendicular to the cylinder.

[@briankennedy7040]

Is the lapping compound aluminum oxide ? That's what I use in my shop ...just curious

[@pwn3426]

i use valve lapping compound .

[@---td4ff]

Sooooo your gonna check headspace now right

[@JJGuccione]

Bingo • I knew if I scrolled down far enough someone else had to ask. Purchasing cheap uppers isn’t recommended. I saw the bulge in the mating surface. That’s the foundation of cascading failure. A drill press and a method of guaranteeing perpendicularity ± .0001” isn’t unreasonable. If you can’t establish a control, your best bet is to purchase an upper receiver from a reputable manufacturer. And, you still have to inspect it.

[@SkyShupe]

Made me cringe, no oil on the guide and to high of speed.

[@kennethdennis40]

AI and machines can do this much more efficiently. We should take advantage of it now.

[@longbow6416]

I think he's going more for the 'experienced' home builder. Like he said, this upper ended up getting through QA that came off multi-million dollar CNCs. I might build 1 or 2 rifles a year, I'd just end up sending a bad part back to the manufacturer. Most manufacturers are more than happy to take back a bad part, one, it alerts them to a possible bad batch and two, it shows good customer service. If I was a major hobbyist or a gunsmith, I'd probably have more than half the tools he has..

[@nathanhuxtable5758]

The number of people who do things just to do them and feel good about them without understanding what is going on... Trying to get a precision anything using an electric drill motor by hand in a plastic block you tossed in a vice? Don't do this.Theres no way you're going to achieve concentricity with this setup You could be just as accurate with a sanding block and 800 grit sandpaper to flatten the face evenly.

[@benpolishchuk209]

So how should be done then? What is the closest to perfect method in your mind?

[@kissitnow898]

Can't recommend this method. Please look elsewhere folks

[@pwn3426]

nothign wrong with it. I have done dozens.