What causes double feed jam

[BSA1]

I was shooting a P-64 9 x 18 Brown Bear FMJ'S when I had two separate mother of all jams.

The fired round ejected normally, the next round fed completely into the chamber but the next round in the magazine pushed forward just enough to lock the slide open. It was impossible to remove the magazine so I disassembled the slide from the frame.

In other words I had a live round in the chamber with the slide jammed open from the top round in the magazine creeping forward.

New recoil and hammer spring in gun.

I was using two mags but I did not think about trying to identify if it was the same one.

Likely cause?

 

[rbernie]

The magazine is failing to control the top round in the magazine; look for spread lips, weak springs, or both.

 

[BSA1]

Thank you. I just wanted to verify I was on the right track for correcting this problem.

 

[Sunray]

"...look for spread lips..." That's where I'd start. Springs do not lose temper from being compressed, but extended use will.

 

[BSA1]

I disassembled and cleaned both of the magazines I have. Feed lips look ok and big difference in the mags springs.