Pmags and bolt lock back

mijp5

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Anyone have issues with the bolt locking back using 20 rd pmags? Been happening to me a lot lately
 

flyer

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Have you accidentally dropped the hammer on your lower lately?
 

mijp5

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You mean when the lower is disconnected? Good question. I don’t think so, but is there a wear mark that I should be looking for?
 

flyer

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If it squeezes the BHO slot, the mag follower might not be able to push it up.

Just put in an empty magazine and push the BHO button to see if there are any sticky spots or drag in the travel.
 

mijp5

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seems loose, but I noticed that the older Pmag has a lighter spring than the newer gen3 20 rd pmag. If I insert an empty mag, the bolt catch actually depresses the follower a bit. There is a shiny spot on the top of the catch, so I am assuming the bolt is scraping past it on the last round
 

flyer

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Assuming the followers are the same, the mag with the stronger spring should work the BHO better. If that is not happening, that tells me it's the BHO, the lower receiver or the bolt.

The lower receiver is the least obvious problem to spot which is why I started there.
 

mijp5

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I was wondering if the spring for the BHO was too stout. Would it help to clip a coil off?
 

flyer

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If you want to try that, I suggest you get a couple spares and use the shank of a small drill bit to test instead of putting the roll pin in and out several times.

One thing that might happen some times is a hydrolock on the BHO plunger hole if the fit is tight and you get some lube or water in there. If that is happening, you might try cutting a tiny groove in the hole top to bottom to create a pressure relief, just be sure to deburr it so things move smoothly.
 

mijp5

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It's either that or replace my Pmag 20s. I just hope my 30s are not like this. I haven't used them in a while
 

mtrmn

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I had the same issues with all my EARLIER GENERATION 20 rd pmags. 30 rders have no problems. I have a total of ONE newer generation 20 rd pmag and it does not exhibit this problem. I just stopped expecting flawless performance from the 20 rd mags and moved on.
 

CANTHITSH!T

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Anyone have issues with the bolt locking back using 20 rd pmags? Been happening to me a lot lately
Any more info besides "20 rd pmags"? You sure its a magazine issue? How about worn rifle parts causing "short-stroking"? Have you tried a NEW "pmag"? And are they really "pmags" or just "polymer" magazines?
 

mijp5

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It was a suspicion so I inquired about it to legitimize the claim. If everyone said no, then I would look elsewhere. But it looks like the Pmags are likely an issue.
 

BLDTYLRY

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Does the bolt catch work if pulling the charging handle on a empty mag? Or is this happen only when you fire the last round?
 

mijp5

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Forgot to mention this happens on a few uppers
 

mijp5

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I had 2 new 20 rd pmags and tried them today. Bolt locked back every time no prob