i converted my single stack Zastava a month ago to double stack.  the second mag i fired, i had a broken shell.  i ordered a broken shell extractor and it removed it perfectly.  this weekend i fired some more rounds and had another broken shell.  I also purchased the go, no-go gauges and they show the gun is fine.  what would be causing my gun to breaking shells?  

oh yeah....... i bought a Polish bolt for the double stack conversion.
yesterday i bought a romanian bolt........ any ideas if this one will work better?

what ammo are you using? maybe just some bad rounds. try switching up some brands and if you are shooting steel switch to some brass and see what happens