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JD failed with Magpul LR-20


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I just slammed a brand new LR-20 mag into my rifle for the first time. The whole point of this build was to be as lightweight as possible. I built my dpms compatible 308 with the concept of using the polymer magazine. Needless to say, I was more than upset to see my magazine wobbled front to back in the magwell. Not just a little wiggle, but almost a half inch range of movement from front to back.

I ran to the Internet and typed in my problem. I found a guy on CalGuns.net who had the same problem and already did the follow up with JD Machine regarding the problem. He mentioned how his rifle failed to feed unless he was holding the mag in place, or resting his mag on a bench.

JD says their 308 is NOT designed to work with the Magpul LR20, and they actually suggest C-product mags. Unless I can figure a way to remedy this, I will be researching a new build soon.

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We'll continue to see things like this in the .308 AR platform, simply because there is no real standard to follow for the design.  That sucks, but it's true and it'll keep happening.  Also, tolerance stacking - dimensions for certain parts will have a few thousandths of an inch tolerance one way or the other, and still be within spec.  Combine some parts that are a few thou one way (large side or small side of the spec), and you'll have parts that won't work together.  That happens throughout the industry. 

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It's not really anything against JD - it can happen to any manufacturers that are making CNC guns and parts.  A few thousandths too wide across or lengthwise on the magwell is probably all it would take.  I still wonder if it was tolerance stacking as the primary issue with what we saw here on the Iron Ridge gun that was eating it's own buffer.  <dontknow>

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My CMMG lower will not work very good with the Pmags. Works fine with Cproducts & DPMS.

As has been said there is no std in 308's like there is with the M16/15 family & that should be expected because the M16/15 family of rifles are all set up as milspec ,so they will all adhere to a set standard of spec's .

The 308 versions are not set to any milspec standard. That is changing a little as these rifles evolve.

I guess I can't expect the CMMG lower to except a mag. that was not available when it was designed .

Although I also had to modify the Bolt Catch for it to work also & that as far as 308 Bolt Catch's, is a standardized part ,even if you used one from the M16/15 rifles .

That, I can blame on out of spec lower or since they did not make uppers or complete rifles in 308 at the time ,there was no way of them proof testing there lowers with them .

When assembling a rifle you have to fit certain parts, that's what gunsmithing is all about .

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  • 4 weeks later...

I don't know if this is of any use to anyone, and it's not complete because I don't have a C products mag to measure, or a JD receiver to compare to my other receivers.

I took some measurements and found that of the 3 mags I have available to measure, the DPMS measures 2.93" front to back not counting the center rib 3.085"with the rib, and 1.08" wide. The KAC mag measures 2.93", 3.08", and 1.065" wide, while the Pmag I measured is 2.940 front to back without the rib, 3.085 front to back over the center rib,and 1.07" wide. Although there is some variation here, on the mags I checked, the Pmag was actually bigger front to back than the others. Now I didn't check multiple mags of each type to get an average and I don't have a JD receiver to try my mags in, but all my receivers took the three types of mags snugly (although I never assembled the POF and have yet to fire the Kaiser) Robocop, Have you tried the mag that the mfg recommends? If that's also sloppy you might possibly have a bad lower and maybe JD would replace it?

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I read a post on CalGuns.net that stated JD was aware of the issue and they recommend C-Product mags.

Since JD is a Cali company, a lot of CA gun owners buy JD rifles. They are in every gun shop in my area, 5.56 lowers and .308 sets. CalGuns had group buy last year for JD matched sets, and people jumped all over it.

Since then a handful sporatic of post have been made from people asking about how to fix this issue with LR20 mags in the JD 308. One writer explained that he was part of the testing process for JD and the mag issue was recognized early on. There has been no mention of how to fix or remedy the problem... Except for pushing your mag in harder and having to hold it there by hand, or posting your rifle off the mag when sitting at a bench/table.

I have since sold my JD receiver and am patiently awaiting the arrival of my SI Defense custom order. I really like JD, but I feel they may have fell behind the curve when new builders like Mega and SI came out. Now with Colt's new design everyone should be scratching their heads.

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Wish i had seen this thread before i bought my pmags. I was able to find a reasonable source for Cmags on the net.


unfortunately i have also found out that C-mag is out of business so what is out there is out there.

I do hope these work well. or i will be contemplating that paper clip option mentioned above.

depending on how things go i may have a story for this group but i hope it will have a good ending.

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  • 5 months later...

Just started building on a JD Machine receiver set, bought a couple of Magpul LR-20s and with the JP Rifles Bolt set everything looks good so far, very little wobble in the mag , with or without the bolt (max deflection at the base with the bolt in place and mag empty is about 3/16 on an inch forced from one end to the other). Still have to install the barrel and put some real rounds through it but it seems great so far.

She sure is purty pops...

Using Fulton armories lower parts kit with a Magpul MIAD Grip and PRS stock, (from Fulton with their buffer and tube but using a Tubb precision flat wire spring). Waiting for delivery on the Geissele SSA-E. Going to have Kreiger install a 6.5 mm (either Creedmoor or 47Lapua), just have to figure out which compensator/flash reducer to use.

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  • 1 month later...
  • 3 months later...

Well @#$% $@#$!%!!!!! pmag was what I wanted to use I mean of coarse they were. I guess I have to call JDM and get the lowdown from them, like what mags will work in my new JDM PR3 lower. A few days ago i tried one of my brother in law's mags he uses in his SR25 and the thing wont drop freely, have to pull out with hand.

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I read semi-recently that the current generation of JD 308's will work with the LR-20 PMAG.... But I can't find it again, and I can't confirm that 100%. All I know it that MY JD did not work with PMAG's, thus I no longer own it.

Yeah? oh man I sure hope so. robocop my lower s# is 007xx, is yours close to that?

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