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FTF and other issue with new AR build


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Old 03-02-2016, 10:27 PM   #1
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Default FTF and other issue with new AR build

I had two issues with my second AR build on it's first firing. I built both the upper and lower a few months ago, but just now got a chance to fire it.

First I tried to use a full P-Mag, and the mag would not go all the way in and lock in placed when the bolt was closed. The top of the mag was hitting the bolt, keeping it from locking in place. I pulled the charging handle back far enough so that the bolt cleared the mag well, then tapped the mag in place. I then had nothing but failure to loads from that mag. At times it would strip a cartridge, but it jammed on it.

I moved to a Colt brand mag, and had the same problem with it not locking in place in the mag well unless I pulled the bolt back and then tapped it in place. Once I did that, everything loaded and fired from the Colt mag. No FTF, or FTE with that one.

That tells me the P-mag brand mag needs to go to the trash, but what do I need to do about the mags not locking in? When I load an unloaded mag, they lock in just fine, but not when they're loaded with cartridges.


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Old 03-02-2016, 10:57 PM   #2
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Try loading the magazine with one less cartridge - 20 round mag,only load 19 in it and see if you still have a problem.
Having a new home built or factory built AR can have problems since the components of the rifle haven't had a chance to mate themselves into complete working order or other wise "Broken In".
Loading a new magazine completely full can add to the problems since the spring in the magazine hasn't been used,and retains full strength of upward pressure on the feed lips,and installing the full magazine into the lower with the bolt closed only compresses the magazine spring even more.

Once you shoot a few hundred rounds out of your rifle,it should start operating smoother. They don't always run smooth right out of the gate!


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Old 03-02-2016, 11:04 PM   #3
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^^^Solid advice. Old habit but I always download my AR mags by 2 rounds. No problems.


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Old 03-03-2016, 12:30 AM   #4
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Ok, thank you to you both for the help. I figured it was the P-Mag, but I'll continue to work with it and run a few more rounds through the gun.
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Old 03-03-2016, 12:52 AM   #5
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You'll find that some parts aren't quite as mil-spec as the company says.

With this, you'll find that some things just doesn't work as planned.

I would do as the other members have said. Don't load the mags full until that spring relaxes.

I have the same problem with some new(last Sept) Mako E-lander mags(steel) where the spring is so stiff, I can't load it fully. I've now got 5 partial loaded mags sitting for last 6 months to get the springs to relax some.

One possible remedy that may help with plastic mags is put the magazines in the warm sun for a day to help relax the plastic, then load up to see it will insert/drop properly and get more usable. Can't hurt and if it works, I'll take my royality in ammo of my choice.
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Old 03-03-2016, 01:01 AM   #6
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Ok, thank you. I was leaning on the fact that the P-mag was cheap, and maybe causing the issue. The FTF only happened with the P-Mag and not the Colt mag, but neither mags would lock in place with the bolt closed. The P-mag had never been used, but the Colt mag came from someone at the range that had used it extensively.

So I get to run a couple hundred rounds through it, to let all the parts start working together? Sounds fun!

Thank you
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Old 03-03-2016, 02:26 AM   #7
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P-Mags are inexpensive, but do tend to work pretty well. Nearly every new rifle/mag combination can cause the described condition. Remember the AR is designed to lock the bolt carrier open after firing the last shot. Locking back the BCG before inserting a magazine is proper.
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Old 03-03-2016, 08:37 PM   #8
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All the pmags I have had are 30 rounds, and I usually only fill them with 20 rounds. Also, like Robo said, lock bolt to the rear before you insert mag. Do this, and I think your problem will resolve itself.


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