Weird feed-malf.... any ideas?

[bgary]

Have a home-built PCC that has run fine for years.  F1/QC10 receiver set, JP everything else (barrel, trigger, bolt, SCS buffer)

 

Built a new PCC a couple of weeks ago, and it ran great in testing, which of course meant it barfed when it heard a "beep".  3 times over the course of a dozen stages, it fed more than one round - which resulted in a mess in the chamber.  One round nosed into the chamber with another round wedged above it against the ceiling of the upper.

 

New PCC is an R&R barrel (ramped 8" barrel with shroud to make it legal length), SI bolt, Blitzkrieg buffer... and except for those things, identical to the above (same receiver set, same buffer tube, etc).  Also, same ammo.  And same mags (MBX tubes with an assortment of MBX extensions)

One thing I have in my head is that - I think - each of the three malfs occurred at a port with low/close targets behind it, which means the PCC was pointed down 45 degrees or so.  I'm wondering if somehow the weight of the mag is rocking it forward, changing the relationship between the top round and the bolt.  But... it's an identical receiver and the same mags, so I'm not sure why it would happen with this one and not the other.

 

Any ideas?

[JM_]

31 minutes ago, bgary said:

Have a home-built PCC that has run fine for years.  F1/QC10 receiver set, JP everything else (barrel, trigger, bolt, SCS buffer)

 

Built a new PCC a couple of weeks ago, and it ran great in testing, which of course meant it barfed when it heard a "beep".  3 times over the course of a dozen stages, it fed more than one round - which resulted in a mess in the chamber.  One round nosed into the chamber with another round wedged above it against the ceiling of the upper.

 

New PCC is an R&R barrel (ramped 8" barrel with shroud to make it legal length), SI bolt, Blitzkrieg buffer... and except for those things, identical to the above (same receiver set, same buffer tube, etc).  Also, same ammo.  And same mags (MBX tubes with an assortment of MBX extensions)

One thing I have in my head is that - I think - each of the three malfs occurred at a port with low/close targets behind it, which means the PCC was pointed down 45 degrees or so.  I'm wondering if somehow the weight of the mag is rocking it forward, changing the relationship between the top round and the bolt.  But... it's an identical receiver and the same mags, so I'm not sure why it would happen with this one and not the other.

 

Any ideas?

If you still have the old PCC, change lowers see if that changes anything. Change bolt to the JP and see. Just start eliminating components.

[Orlandoech]

On 9/8/2022 at 2:16 PM, JM_ said:

If you still have the old PCC, change lowers see if that changes anything. Change bolt to the JP and see. Just start eliminating components.

what he said. Process of elimination but starting with the lower as the angle for mag seating may change the angle the round gets inserted is the best starting point. 

[TRUBL]

If you had TRUE double feeds......your mags are junk. I suspect that you either had a failure to chamber or failure to eject.....pulled the charge handle and fed another round....making a mess.