Anderson upper receiver pivot lug too large.

[Disciple]

I bought an Anderson slick-side upper receiver to use with a Mega Machine lower receiver. Checking them for fit I found a problem: the Anderson upper pivot lug is too large, and when I try to hinge the receivers open it hits the front of the Mega lower between the pivot tabs. I can hinge it far enough to get the BCG out but no more.

Does this kind of thing happen when mixing brands or is the Anderson receiver out of spec? Should I try to return it? Can the lug be ground down some to relieve the interference?

[Klem]

Yes, sometimes with tight fitting specs. Mega has tight specs. Seekins is another brand that I've seen this happen with; their pivot holes were too tight to fit any other lower than their own.

Yes you could machine it down but if you can get the BCG in and out and pull the two sections apart why do you need it to pivot any more?

[Disciple]

I guess I could use it but if I accidentally let it swing open in an uncontrolled way it could damage the front of the magazine well. The upper pivot lug is not as round as other lugs, but has a bit of a corner to it. That corner is what I would try to grind off. I would rather grind the Upper than damage my MEGA lower.

Are there any other interference points to check for?

[Klem]

Looking at photos of the Anderson uppers the bottom of the lug is evenly rounded like all other uppers so I am not sure what you mean.

Can you post a photo of this 'corner'.

[Disciple]

Yes, here. (view large)

Anderson lug corner.jpg

[Klem]

Disciple,

Yes, I see what you mean. That hole is not centered in the lug like all others on Google Images; there's more meat to the rear than the front. Might just be an Anderson design or it might be a mistake. Is the upper is being pushed rearward more than it should? Does this make closing the upper a tight squeeze at the buffer end?

You could file a bit off the forward edge to free it up a bit. I can;t see how that will change how tightly the two halves are held, or increase any slop, or reduce any meaningful strength in the bind Mega lowers have that screw in the pistol grip section you can use to tighten and stop the slop anyway.

Or, if a mistake you could send it back.

[Disciple]

I don't think my lower has the tension screw. That sounds like a billet feature and this is a forged receiver.

The hole seems to be in the right place as both pins push through by hand easily. Do you think this should justify a return in the eyes of the retailer? I worry that something else might be off too.

[Klem]

No doubt their cheaper MILSPEC version doesn't have the extra fruit but it still will be precise and good quality given their excellent reputation. Is this a recent purchase since Zev bought them out?

The lower can be out of spec or a sloppy fit and this will have little impact on anything. Obviously the trigger is a big influencer on your ability to shoot but it's the upper where all the accuracy is. Everything else just holds it together and makes it work.

Up to you whether you want to exchange it. Not being able to fully open the upper is not the worst thing that be wrong but it would irk me.

[Wethepeople]

Sounds like one of three things or a combination of the three.
1- Wall thickness between the two forward lugs on your lower. Measured a Colt lower, it measures .098.
2- The height of the front lug on your upper. My Colt upper is .477.
3- The hole location on the front upper receiver lug. Without a proper sized plug gauge or an optical comparator, I can't get an accurate measurement. You could measure the wall thickness from the hole edge to the front, rear and bottom of the lug and get a fair idea compared to a known good milspec upper for comparison.
I'd give Mega a call and ask what compatibility issues you might have with your particular upper before I sent anything back for replacement.

[Disciple]

I tried the receiver on a different lower and I could pivot it all the way. I also tried a different upper on the Mega Gator and again I could pivot it all the way. I counted this as unfavorable tolerance stacking and as I found nothing else wrong I continued assembly. Eventually I'll grind down that lug so I don't damage the Mega Gator.

[Croak]

Get a #6 or #7 rubber o-ring from a hardware store and wrap it around the upper lug. It will act as a tiny little rubber bumper when hinging open, and will also tighten the fit between the upper and lower if there's any play.

[FLshooter]

[Lastrites]

I vote to contact Anderson and get them to replace it. If you start filing or grinding on it all warranty goes out the window.