Mac-11 bolt closes and traps spent casing trapping it between bolt and receiver.  Tried different ammo and mags.  Spring too strong?  Bolt not going back far enough?

sounds mag related.Id check the feed lips on the mags..if they are spread too far apart,the mag will dump several rounds into the chamber..1 or many  rounds depending how bad it is or the bolt will ride  the next loose round and when the bolt comes back from firing and the extractor grabs the rim of the spent case.. it will fail to eject and dump the brass anywhere inside.


you might not notice the feedlips issue but the recoil from the gun firing will jolt the rounds up from spring pressure in the mags. alot of how long your mags last depends on the fit between the upper and lower,the more slop there is,the quicker the feedlips wear. I have a SWD m11/9mm that has no slop,it still reliably uses the original mag that was made for it back in 85.Ive had other m11s that were sloppy,the mags didnt last over 200 rnds because the bottom of the bolt was spreading the feedlips apart unevenly.

other issues could be a weak extractor spring or extractor..dirty chamber or the mag catch. but most commonly its the mags because the original zytel mags were made to be disposable,back then they were very inexpensive. nowdays, you can buy a steel insert to fit inside the mag that has steel feedlips,restroring the reliability of the zytel mags.