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80% Lower Problem

richc

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I've built a number of AR's so I know the drill. Not my first rodeo.

I'm making some "demo" AR-15's for a product I'm working on. I figured I'd use 80% lowers to reduce the likelihood I'd need to bring a fully functioning AR into a business or potential customer location. Or to make it ease to mail it to someone.

I've made a demo AR-15 in the past with no issues. Now I'm finding that the lower and upper receivers don't mate properly. The rear takedown pin appendage on the upper receiver can't fit in the mating hole in the lower. Peeking in there with a flashlight it's obvious there's a difference in size.

Has something changed with AR lowers? I've tried two different mfr's and have the same problem. But not so with my older 80% lower.

I suspect I'll just dremel out the lower receiver mating hole some. But I'm wondering why I'm having this new problem. And for the record all the uppers work fine in a regular AR-15 lower receiver. The problem is definitely with the 80% receiver.

Thanks

Rich
 
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same manufacturer? Different?

Change to the process making it so the end user has to machine that pin hole to size so that they could machine another section and still be able to meet the legal requirements of being considered 80% or less complete?
 
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richc

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Completely different manufacturers. Something changed. Old 80% lowers didn't have this issue.

Anderson is one of the mfr's. Don't recall the other.
 

chris_1001

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Bad 80% jig or worn out?

these are standard dimensions. Except to the large or small colt pins, the placement should be spot on.
 

richc

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I've not milled it at all. Never had to. The 80% lower and upper receiver mated properly. Now with 2 different types of of 80% lowers they don't fit together.

I can certainly dremel out the rear takedown pin channel. But it seems something has changed.
 

greencobra

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i'm confused. the "pin appendage" doesn't fit down into the machined out lower? or it fits but the pin hole on the upper is a different diameter on the lower?
this is the "appendage" correct? aren't these specs suppose to be standard? just asking is all.
upper4.jpg
 

GM-GUY

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Check for a burr or lump of crud. Check that the upper and lower are mating correctly (I had to dremel the buffer tube area on an AR-10 (DPMS format). Mil spec vs Commercial?
 

richc

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The lug that you have circled in red does not fit into the cutout in the AR 80% lower. However the same lug fits perfectly on other functioning AR's I own.

@92G the 80% lower is precut for the rear shelf as shown below but not big enough for the lug. An older 80% lower did not have this issue.

Below are two pics showing the demo build. First pic shows the upper lug and lower rear shelf area milled out. Second shows what happens when I try to close the upper and lower. That's where they stop. The lug does not fit in the provided milled lower shelf area.

1579443230255.png

Screen Shot 2020-01-19 at 9.15.36 AM.png

i'm confused. the "pin appendage" doesn't fit down into the machined out lower? or it fits but the pin hole on the upper is a different diameter on the lower?
this is the "appendage" correct? aren't these specs suppose to be standard? just asking is all.
View attachment 325514
 

richc

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In summary, your honor... if Anderson went to the trouble of milling out most of the rear shelf unit why didn't they go all the way and make it mil spec?

Anderson didn't make a mistake here IMHO. There's some reason they shorted that milling job. Part of me wonders if they want to make absolutely sure they are below the "80%" rules. A little more milling requirement makes it less likely to be challenged by the ATF.

I'm just curious is all. I have a dremel and can shave this down a bit. But if we investigate differences and change we can learn something. I figured someone here on NES would have run into this!

:)
 

richc

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Intellectual curiosity... why have they changed? If Anderson goes to the trouble of milling out most of that hole, why not finish it like they used to?

My name is Rich and I'm OCD...

:)

@richc this is normal. again when the lower is finished the rear lug of the upper will fit fine
 

Golddiggie

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IIRC they only have that area machined so that you don't go too far towards the buffer retainer and bugger that. Most (if not all) of the lowers I've finished haven't had that area already milled. Just means a bit more care needs to be taken to get it right. Then again, with the plans and such, it's not that hard to make sure you have enough milled, without going too far.
 

allen-1

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IIRC they only have that area machined so that you don't go too far towards the buffer retainer and bugger that. Most (if not all) of the lowers I've finished haven't had that area already milled. Just means a bit more care needs to be taken to get it right. Then again, with the plans and such, it's not that hard to make sure you have enough milled, without going too far.

Yup. None of the 80% lowers I've done had any of that area, (rear shelf), removed before I started. Anderson is just giving you a starting point, more milling needs to be done.
 

Golddiggie

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You need a longer bit to machine that area. Otherwise you will be running into the buffer tube mount.
End mill holders can make a big difference for that. As well as for regular machining. I picked up a good set from Shars that got delivered on the 10th.
 

Tinkermatic

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Oddly, I was on the Anderson website early this morning (couldn't sleep) and saw this: (copy and pasted from the website itself.)

***Please note that the rear lug pocket for the upper must be finished in the machining process in order to seat an upper***

 

richc

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Thank you! I did look at their site but didn't see that notice.

Oddly, I was on the Anderson website early this morning (couldn't sleep) and saw this: (copy and pasted from the website itself.)

***Please note that the rear lug pocket for the upper must be finished in the machining process in order to seat an upper***
 
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Oddly, I was on the Anderson website early this morning (couldn't sleep) and saw this: (copy and pasted from the website itself.)

***Please note that the rear lug pocket for the upper must be finished in the machining process in order to seat an upper***

From what I've seen most are the Gen 2 style with the rear lug already cut. I thought that was pretty much the standard now. Even more surprised Anderson hasn't performed that step.
 

Tinkermatic

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From what I've seen most are the Gen 2 style with the rear lug already cut. I thought that was pretty much the standard now. Even more surprised Anderson hasn't performed that step.

One would think that this step would be taken care of. Having never bought or attempted anything 80%, I have no experience with this step. Suffice it to say if I ever get in to 80%, I'll be looking for something other than the $40 Anderson lowers.
 

Boris

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One would think that this step would be taken care of. Having never bought or attempted anything 80%, I have no experience with this step. Suffice it to say if I ever get in to 80%, I'll be looking for something other than the $40 Anderson lowers.

I've no idea why they changed the lower's finish. Every operation is additional effort though and for these guys, they are going for the cheapest of the cheap, I can expect it.

They had some Andreson's for sale for something like $29. That's very hard to beat in terms of price. If takedown pins and magwell are GTG, everything else will finish just fine with a mill.
 
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