So I got an Aero Precision upper from CDNN sports, and got a decent deal on it. I come home from work today, and everything's on the table waiting for me. Get the upper put together, mate it to my lower...and theres a noticeable gap between the buffer, and end of the upper. I won't say its  a huge gap, but you can see light.

 

Will this affect operation? If so, would there be a way to fix it?

 

I'll get my pics up tomorrow, my phone is charging.

If the rear pin is engaged locking the upper to the lower, and the bolt travels without binding you should be gtg. Pics would help to confirm.

I have a similar gap on my 7.62 with the poly lower. Bolt functions fine and ive never had issues

This usually occurs when both the upper & lower are within mil-spec

but on both ends of the acceptable limits.

 

Example:

 

     let's say Mil-spec is between 1 & 10

Upper =   1........x.10

                                    > Both may be within Mil-Spec

lower =    1.x........10

 

Sometimes you get a gap in between the upper & lower sections............

 

 

If there is any play between the upper & lower you can use 

an  Accu-Wedge behind the rear pin to take up any free play.

O-ring #41 on the front pin does it also.  Check it out on YouTube.

As far as any wobble in the gun goes, there is none. Like zero. But that's because the gap for the recoil lug in the fcg pocket is super tight.

 

That's the gap of the receiver.

 

 

 

 

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Scratch that. After tugging on it with just brute force, it seated into where it's supposed to be. But it's stupid tight. Which tells me that there could be a dimension issue with the take down pins and gaps for the upper.

 

 

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Shoot it as is it will loosen up soon enough.

After playing around with it, and making sure everything locked I tested the charging handle and there was no clearence for it to be pulled back. Disassembled, and ended up shaving the top of the lower down enough to get the charging handle to move freely.

 

If I ever do another build, I'm not going with poly lower again. I've had nothing but problems with assembly.

 

 

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What the hell did you do to your upper finish? I normally don't care about scratches but that is a odd one.   

What the hell did you do to your upper finish? I normally don't care about scratches but that is a odd one.

Missed my punch, and caught the side with a tiny hammer. Don't ask how, cause I don't even know how I managed it. Just my luck I guess.

 

 

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A tiny air hammer? Lol

A tiny air hammer? Lol

Yup.

 

Tiny hammer.

 

 

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As far as any wobble in the gun goes, there is none. Like zero. But that's because the gap for the recoil lug in the fcg pocket is super tight.

 

That's the gap of the receiver.

So when you took this pic, where the pivot or take down pins in place and slid through? I'm guessing not. I just don't get how you were able to slide the upper back some with the pivot pin slid through. If there is a gap his large when firing then your buffer will get chewed up and the buffer retaining pin will break off. You need the BCG to PISH back on the buffer just a hair to releave any tension from the buffer on the retaining pin.

Out of curiosity, which poly lower did you use? I've sold thousands of New Frontier LW15s and haven't seen that issue before. Poly lowers aren't perfect but they certainly are nice for certain builds like a .22 plinker for a kid.

So when you took this pic, where the pivot or take down pins in place and slid through? I'm guessing not. I just don't get how you were able to slide the upper back some with the pivot pin slid through. If there is a gap his large when firing then your buffer will get chewed up and the buffer retaining pin will break off. You need the BCG to PISH back on the buffer just a hair to releave any tension from the buffer on the retaining pin.

The pivot pins weren't fully engaged. It was 50/50 operator error and lower. The problem was the rear pivot pin area was a tad to tight for the upper to fully sit back towards the buffer.

 

 

The area in the picture is what I'm talking about. The difference between lower and upper lugs was .27, so I filed either side evenly until I reached a difference of .1 and everything slides into place but needs a bit of push to seat it to be able to push the pins place.

 

 

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Out of curiosity, which poly lower did you use? I've sold thousands of New Frontier LW15s and haven't seen that issue before. Poly lowers aren't perfect but they certainly are nice for certain builds like a .22 plinker for a kid.

It's an LW15 lower. Bought it from you last year! The problems could just be me, but I have no other reference to know.

 

 

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It's an LW15 lower. Bought it from you last year! The problems could just be me, but I have no other reference to know.

 

 

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Poly lowers are a strange animal that can (but don't always) require fitting though in literally thousands of poly lowers shipped in the last 18 months, I can't say that I've ever seen one stick like that. Usually, it's things like the mag release hole that need a little filing. NFA is really good about customer support so you may want to try emailing them to see if they've seen this before and if so, what they'd suggest as a fix. I know that area was beefed up when they took over the Plum Crazy poly lowers because it was a failure point. Have you tried multiple upper receivers? Or that upper on a metal lower?

I don't have any other uppers to test. I could try and get a buddy of mine to bring his at over and see if I get the same for issues, but since relieving the rear pivot slot it hasn't been AS bad.

 

Oddly the other spot I had a bear of a time with was just behind the charging handle. The flat spot outside the threads on the top near the castle nut. Charging handle couldn't pass over that one spot until I filed it down.

 

I might email them, but since it's been over a year I don't think they'll be much help but I'll give it a shot

 

 

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New Frontier has excellent customer service and a lifetime warranty. I've broken bread with the owner and he puts customers first (both wholesale and retail).

I dealt with a couple of NFA lowers in the past during the panic and made this for others to use as a reference.

 

Looks like I found my project for my day off Monday!

 

Thanks for posting that, makes a lot of sense and makes me feel better that it just isn't me with tight holes (ha!)

 

 

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Hey Lalo, great post.  Why don't you start a new thread with this and ask to have it pinned.  Great info here for others.

I used that same picture to make my nfa stripped lower work correctly.

There are plenty of other threads in this forum that discuss the NFA lower. This is the original one I made with more info, including what NOT ato do...

 

New Frontier LW-15 Builds - http://njgunforums.com/forum/index.php/topic/49819-new-frontier-lw-15-builds/

I just did a build with a GWAKS Cav15 lower...a one-piece poly lower/grip,/stock. With that they purposely set the takedown pin holes wider than standard because the polymer may eventually loosen up a little. You actually have to hammer the pins in and use a punch to break the gun down for awhile 'till it loosens up.