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Posted: 1/19/2011 4:29:26 PM EDT
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What would cause a 308 BCG to slip out of the Hold Open position, with & w/o the mag in?
Tried 2 different latches, my brain says springs. |
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It is a built gun, but not by me. CMMG Lower/DPMS upper LR308 SS heavy barrel.
The first catch is DPMS, the 2nd is from SI & in stainless, I was always told not to mix upper & lower receivers when doing a build. I guess this holds some validity as the BCG will not stay in the BHO after last round out, nor will it stay back for clearing & inspection of the chamber. On a closer look of the bolt I've notice the edges have begun to mar & roll, this tells me the latch is not engaging enough to hold the BCG in the locked position. |
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I also have a CMMG lower , but with a SIDefence upper & pretty much does the same thing .
I have taken measurements of how high the different mags push up the Bolt catch & there is a difference between them (DPMS,Cproducts &Pmag ). The Pmag putting the Bolt catch the highest . I have also taken some material off the back of the BS ,where it stops against the receiver . I have tried two AR15's & two 308 Bolt catch's & by far the 308's work out much better . Might try fitting a 9MM BS in there this year. |
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I took off some more material from the stop area of the bolt catch. The part of the bolt catch that is against the bolt catch spring buffer. I will have to take a measurement of approx. how much I took off , but it works real nice now .
I will take some photo's & will put range time in Monday , but what a difference than before . I was apprehensive about taking material off from back there ,but as long as you don't over do it ,it should be OK . I can see taking too much off & have not enough return spring tension or travel .I took measurements of three different mag. manufacturers ,of how high they pushed the bolt catch up & I can compare the new measurements to the first ones . I think CMMG have there own spec's on the area where the catch pivots , or some are out of spec's enough to cause this kind of problem. |
I took some photo's of the DPMS 308 Bolt catch & a AR 15/16 catch . There are differences between them ,but you can see I took material off the 308 version. I will have to take the one off my DPMS to compare it to the one from the CMMG (both 308's )
And after bluing with 44/40 
The height the Bolt catch is raised by the mag has also been increased , of course . Below is a comparison with a non altered Bolt catch in a DPMS lower & the CMMG lower, with different mags. DPMS (not altered ) ––––CMMG (cleaned up ) ––––CMMG (altered) Pmag–– 0.449" –––––––––––––0.424 " ––––––––––––––0.4665 Cprod––0.399"–––––––––––––– 0.378" ––––––––––––––– 0.4015" DPMS––0.456" –––––––––––––– 0.431" ––––––––––––––– 0.475" Will be taking the CMMG lower'ed 308 out this week for testing ,but at the same time I'm working new loads with the Chrono & making them up now ,so it will be this week some time . |
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Update , mags of all manufacturers worked properly with alteration of Bolt Catch.
I think the spec's on some of these CMMG lowers are not completely correct. This CMMG lowered build also has minor FF problems with Pmags, of all things. All DPMS & Cproducts mags work fine. |
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Quoted:
I also have a CMMG lower , but with a SIDefence upper & pretty much does the same thing . I have taken measurements of how high the different mags push up the Bolt catch & there is a difference between them (DPMS,Cproducts &Pmag ). The Pmag putting the Bolt catch the highest . I have also taken some material off the back of the BS ,where it stops against the receiver . I have tried two AR15's & two 308 Bolt catch's & by far the 308's work out much better . Might try fitting a 9MM BS in there this year. Have you tried the redesigned ArmaLite AR10 bolt catch? Works fine with the M14 mags, don't know about how it would work with Pmags. |
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Bolt catch is working as per spec's now .
The failure to feed may be the tension of the Pmags springs , they do feel a little heaver than the others . I am going to email Magpul to see if they have any idea's , these same Pmags work fine in the DPMS. Can't blame the mags . I have seen rifles that did not like certain brand mags before .I know there is some thing to it ,but is it worth it ,to find out , when other manufacturers mags work ? |
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