AR guys: NFA poly lower help? Can't install mag catch


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1KPerDay
March 25, 2013, 11:01 PM
I've never "built" an AR before, and I can't even get the first part in. New NFA lower, PSA LPK, mag catch won't enter receiver. The rounded end of the channel near the post seems to be too tight, and the width of the channel appears to be inadequate. I measured another PSA catch I have and they appear identical. It's almost as though the hole for the post and the button is not in the right place.

Ideas? :uhoh:

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taliv
March 25, 2013, 11:32 PM
probably going to need some pictures on this one

1KPerDay
March 25, 2013, 11:52 PM
dunno how much a pic would tell you... the tolerances are really close. The long oval shaped channel for the mag latch just seems to be too tight. The mag catch won't slide freely into the receiver so I can thread the button on. I don't really want to get the dremel out..... :uhoh:

holdencm9
March 26, 2013, 12:05 AM
Don't some of those polymer lowers require some reaming and such.

MachIVshooter
March 26, 2013, 12:06 AM
Ideas?

I'd use my vertical mill to open it up.

Barring that, very careful hand filing.

Just curious, but why SBR a poly lower? To me, it doesn't seem like a great idea to tax stamp a budget gun.....

Dr.Rob
March 26, 2013, 12:14 AM
In this case I think NFA = New Frontier Armory not National Firearms Act.

I bought mine complete, rather than as a build.

DO NOT DREMEL. THE DREMEL IS NOT YOUR FRIEND. HAND FITTING IS NOT DONE WITH A DREMEL.

Calm down and set the power tools aside. Search multiple forums for others doing builds on a poly lower. Read read and re-read before you start filing or sanding anything.

Girodin
March 26, 2013, 12:17 AM
I don't know if this is your problem, but sometimes lowers, particularly cheaper ones, are out of spec. It sounds as if it simply is out of spec.

I will note that some polymer lowers that I have experience with like the cav arms lowers tend to be a little tight in places initially and wear in a bit with use. that is different than simply not fitting at all though.

As to ideas, I'd say there are a couple of possibilities.

1) try a different catch, although it sounds like this wont make any difference because it sounds like it is not a matter of the catch being out of spec. (you might also check your mag catch in another lower).

2) Contact the seller and/or manufacture.

3) Carefully remove material (in theory from the mag catch or the lower) until it can fit. I agree with the above advisos about 1. not using a dremel and 2. researching it more before you do something you cannot undo.

4) Leave it un-built, keep it or sell it and learn a lesson about buying cheap plastic AR lowers. I have learned my own lesson about cheap lowers, having previously gotten two forged lowers that were out of spec, one very seriously so, I've learned a bit of that myself.

A quick google search turns up other instances of parts not fitting in real well. See e.g. http://www.ar15.com/forums/t_3_4/601444_NFA_lower__mag_release_button_will_not_go_in_.html&page=1

daveit
March 26, 2013, 12:21 AM
I was researching these lowers last week and found this issue. There are lots of posts on ar15.com from folks with the same issue with this lower. Search over there for some tips...I believe there are some YouTube videos that address it too.

Good luck & post up pics when you get that baby built!

MachIVshooter
March 26, 2013, 12:21 AM
In this case I think NFA = New Frontier Armory not National Firearms Act.

Ah. Should have thought of that. Duh!

dhracer
March 26, 2013, 12:24 AM
I have one of the those lowers and they definitely require hand fitting of most of the lower parts kit. Use a fine small file set and take your time.

Sent from my SAMSUNG-SGH-I747 using Tapatalk 2

1KPerDay
March 26, 2013, 01:07 AM
Okay, thanks guys. I don't even own a dremel, BTW. :D
I'll go slow.

It was relatively cheap and available, and I thought it'd make a good lightweight build with something like a 6520 or 6720 upper. If it fails I've learned a lesson cheap. But I've read generally favorable things about new frontier armory; although I agree the NFA thing wasn't thought out properly. :)

taliv
March 26, 2013, 08:30 PM
I wouldn't modify it myself. It's defective. Send it back and let them fix it

Girodin
March 26, 2013, 08:35 PM
I would be more inclined to send it back if they are paying shipping both ways. If it was going to cost me any money I'd probably do it myself. It's not like it is particularly challenging work. Shipping that thing there and back would cost almost as much those poly lowers will be selling for again soon.

wally
March 26, 2013, 08:57 PM
The mag catch won't slide freely into the receiver so I can thread the button on

This sounds backwards! You push the spring and button all the way in and thread the catch onto the button, once the thread starts, you pull on the catch as much as you can and rotate it to screw it the proper depth into the button, letting it slip into the slot on the last revolution.

Don't "open" anything up! the catch slot prevents the spring from pulling the catch all the way into the mag well, which would interfere with magazine insertion if it did.

WinThePennant
March 26, 2013, 10:53 PM
I have an ATI poly lower (dedicated 22 LR build), and I had no problems with the mag catch. I had a heckuva time getting the bolt catch installed. Every time I put one of these in, I always tell myself that it's my LAST AR build!!! Until the next one, of course...

justice06rr
March 26, 2013, 11:02 PM
I wouldn't modify it myself. It's defective. Send it back and let them fix it

Probably not defective, more like out of spec.

I bought one of the poly lowers from ATI/Omni for a lightweight AR build last year. Luckily I bought a complete lower instead of a stripped receiver.

I can tell you for sure that some areas are out of spec. The holes for the front pivot pin and rear takedown pin are very tight almost to the point where you have to kinda hammer the pins in and out of place when assembling/separating the receivers.

The magazine well is also tight; magazines don't fall freely like they do with aluminum lowers. I bought this poly lower to T&E and build a lightweight AR pistol, not as a primary rifle. Other than that, it has functioned fine with 5.56 and 300BLK uppers.

1KPerDay
March 27, 2013, 02:15 AM
This sounds backwards! You push the spring and button all the way in and thread the catch onto the button, once the thread starts, you pull on the catch as much as you can and rotate it to screw it the proper depth into the button, letting it slip into the slot on the last revolution.

Don't "open" anything up! the catch slot prevents the spring from pulling the catch all the way into the mag well, which would interfere with magazine insertion if it did.
Wally, the mag catch won't slide into its slot in the receiver. Regardless of how you thread the button on. The slot is too narrow and too short.

justice06rr
March 27, 2013, 02:26 AM
So you're saying that the long piece of the mag catch (that goes on the left side I believe) won't go in at all? If thats the case, then its definitely out of spec.

I'm not sure if you feel comfortable filing it out just a bit, or just send it back for warranty. I would recommend to at least call NFA and see what they say about your issue

1KPerDay
March 27, 2013, 12:45 PM
Yeh, maybe I will. I don't have a problem sanding/filing/fitting stuff but I want it to actually work when I'm done. :D

rule303
March 27, 2013, 05:11 PM
The last batch of NFA lowers seem to be a little tight in the mag release area. I built several for customers last week, and had to remove a little bit of material for smooth operation. A utility knife works great, just shave a hair off both sides little by little until you have enough clearance.

ewlyon
March 27, 2013, 09:02 PM
I just put an NFA lower together, had the same problem with the mag catch. I used the tip of a knife to slowly scrape away shavings of polymer from the tight area and kept checking the fit until the catch moved freely. A round file would probably be easier/faster. After you get it too fit it is no different from any other polymer lower.

CowardlyHero
March 28, 2013, 06:07 AM
Had to do the same thing on mine, and could only get the button threaded on about halfway.

col_temp
March 28, 2013, 02:25 PM
Interesting reading.
Thanks for all the comments by others. I was looking at wet5her to go with a poly lower for a build. Think I will shy away and spend a bit more.

strange246
March 28, 2013, 07:45 PM
I've built 3 NFA lowers and am about to do #4 & 5, either a small file or exacto knife to open it up slightly, on their unfinished lowers there is always a small amount of fitting right there...Its not out of spec per se, its just excess poly from the mold...

1KPerDay
March 28, 2013, 07:55 PM
Thanks. With an xacto knife or whatever, are you scraping perpendicular to the surface or are you slicing material off parallel with the opening?

1KPerDay
April 16, 2013, 12:50 PM
Update as requested: got the mag catch fitted... it's still a little funky depending on how you press the button. But using the first finger in the usual way it works fine. Pressing more from the front will sometimes move the opposite side (the latch) into the wall of the opening and it will stick a bit, but it still works either way. The rearward curve of the opening needed the most work. Got the bolt stop in, with just a little filing of the inside edges of the notch. Pmags fit fine and drop free; an Adventure Line USGI fits and locks but is a bit snug. I find that a bit weird. I don't really want to file the magwell so I guess I may just use Pmags in this rifle (or fit it for .22 conversion mags...).

Got the trigger group in (I'm using a PSA LPK); the holes for the pins were quite a bit too small (quite a bit meaning I needed to sand them a tad larger before I could drive the pins through). I left them pretty snug as I figure tighter is better than loose; you still need a punch to drive them in and out. At first I thought the disconnector wasn't going to work properly (with trigger held back, I couldn't press the hammer back to the disconnector hook without releasing the trigger pressure slightly; but once safety was fitted and installed, the trigger/disconnector feel and work fine. Dunno if that's normal for ARs; this is my first "build".

The holes for the safety were very tight; I sanded them until the safety rotates with a little thumb pressure (without the detent installed). I would keep going until it flops about under gravity but I imagine it will wear in a bit and I don't want it to get too loose.

The front takedown pin is snug, and I left it that way. You have to drive it out of the left side hole (it slides freely after that). Rear takedown pin fits a bit better but still a bit snug. I figure that's okay.

Overall the only real difficult bit so far was the mag catch.

The notches to allow the upper (where the takedown pins go through) are very snug; particularly the rear one. I can get my PSA upper onto the lower (haven't tried with pins yet) but I have to knock/pry it off out of the rear slot. I filed a bit of flash off, but it still may need more. I imagine tighter is better than floppy/loose so I don't want to overdo it with making space for the upper. I'll try a couple other uppers and see if it's too tight across the board, and then remove a small amount.

I'll post an update and maybe a video when I get it finished to my satisfaction, and a live-fire test. Anyone got a lightweight barrel for sale?:)

1KPerDay
April 17, 2013, 12:21 PM
One more major problem: the threads for the grip screw seem to be either the wrong thread pitch (unlikely) or the entrance to the hole is buggered so the threads of the screw don't enter properly (likely). I messed with it for an hour and finally, in frustration installed the screw and grip using overmuch elbow grease. I'm sure it's cross-threaded and messed up now. But it's tight. If I ever have to remove the grip I'll have to tap it for a larger screw I guess, or maybe install a metal insert.

The Magpul MOE grip I intended to install would not fit; the tab on the receiver was at least 1/16" too thick. The ERGO and USGI grips I tried fit fine, though the ergo's safety detent spring hole wasn't deep enough and the spring prevented full seating of the grip. I installed the USGI type grip.

That and the magazine catch are the major problems... fitting the bolt catch, trigger group pins, and safety are minor issues.

Video discussion here for any interested
https://www.youtube.com/watch?v=HsCbm4ANry4

taliv
April 17, 2013, 12:33 PM
good grief. i know the feeling though. i had a lower that i paid $$$ to get painted and then found out it was out of spec and it took a while to get it working. very, very frustrating

1KPerDay
April 17, 2013, 02:39 PM
Yeah, this was pretty much out of curiosity and for my education. I wouldn't do it again, and I won't use the rifle for "serious" purposes.

If it turns out that it actually works and is reasonably durable, It'll be worth it.

One thing I can't believe is that they expect a steel bolt to reliably hold in plastic threads. That's just plain ignernt. :rolleyes:

justice06rr
April 18, 2013, 02:39 AM
Well, you live and learn. Good to hear you got it assembled anyway.

Buy an aluminum lower for your next build and you will see how easy it is for parts to be installed. PSA seems to be getting stripped lowers regularly now for $120, and I also saw the LPK in stock for $60.

Good luck with the rest of your build.

1KPerDay
April 18, 2013, 04:07 PM
PSA seems to be getting stripped lowers regularly now for $120, and I also saw the LPK in stock for $60.

That's good to hear. :cool:

I completed the initial live-fire evaluation this morning and it functioned perfectly with both .22 and 5.56 uppers. Trigger feels quite good (it is a PSA LPK as noted above), bolt locks open properly, no malfunctions, no issues. One upper was a bit more snug in the rear pin hole but it installed and removed fine. That one also had a bit tighter tolerances on the top of the receiver near the charging handle, and the first couple of times I opereated the bolt one handed it seemed to rub a bit, but after a couple of times I guess the extra poly wore down and it worked fine.

I can't really feel a significant difference in weight between an aluminum lower with receiver extension/buffer tube, spring, and buffer installed, and the poly lower with the same. I need to get a postal scale. But I imagine a poly lower with a poly LPK would save a bit of weight.

So far, I'm liking it quite a bit. The other USGI mags I tried fit fine and drop free, as well as the CMMG .22 mags.

I'll post a video update soon for those interested.

1KPerDay
April 19, 2013, 03:10 PM
http://www.youtube.com/watch?v=4F7BOdrYasM

Works fine! Started with .22 conversion in case something was obviously wrong; no issues. .223 upper worked perfectly as well. Yay! Hopefully it proves reasonably durable.

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