I'm fairly new to MP5s, and am looking for some input.  What follows is my dumb ass moment.

Last weekend I was shooting my PTR 9ct using Wolf Ammo (I know, Wolf isn't great and that is how these stories usually start) out of an AC unity 40 round mag.

Inserted the mag and it appeared to lock in place, dropped the bolt, attempted to fire, and nothing. I pulled the magazine and locked the bolt back.  It appeared that no round had been stripped, and nothing ejected when I opened the action.  I reinserted the mag and it appeared to lock in place again.  A round may have been slightly forward of its normal position in the magazine.  I dropped the bolt and bang!  The extractor went flying out of the gun, but the magazine, barrel, and receiver appear to be undamaged.

Based on the pics below, I obviously need to replace the extractor spring.  Should I also replace the extractor?  What should I be looking for regarding potential damage to the bolt.  Is it possible a round was chambered, failed to eject, and was set off when the bolt dropped and a second round tried to chamber? Is it possible that a round was sitting wrong in the mag and the bolt pinched it when it dropped home causing an OOB discharge?

Not sure how it went off but MP5s are built pretty tough

Check the bore for any weird things but Id bet its fine

Im sure the extractor is fine just need a new spring

Heres one that locked up my gun once. It got setback I believe

It actually split the casing along all the flutes. Had to use a table on the charging handle to get it out but was fine after that

fouled up firing pin channel causing a stuck firing pin?

There is NO WAY the firing pin can contact the primer unless the rollers are out.

Make sure the chamber is dry.

damn

Finger on the trigger as you dropped the bolt?

Nope. My finger was straight on the lower, above the trigger.

Thanks, @chase45.

Took the hand guard off and examined the exterior of the barrel and the bore. No changes or damage that I could see.  I'll pick up a few extra springs and test it out.

That is an impressive case split!

Honestly, I cannot even imagine HOW that could have happened.

The only thing I can think of is if you had some piece of hard debris in the action that got caught between the primer and the closing bolt.

I’ve read mixed opinions on using steel cased ammo due to the fluted chambers.  Regardless of the ammo, I wonder if you contact PTR would they want to inspect the gun to make sure there’s no manufacturing defect. Could be a safety issue since somehow a round went off OOB and, as you said, without finger on the trigger.

One round in the chamber with a second getting double fed into the primer of the first to make the first round go boom?

I agree, maybe they will replace parts with new just to be on the safe side (CYA) and you won't be waiting for the other shoe to hit the floor every time you take it out to the range.

In my mind, this is about the only thing that seems somewhat plausible given the facts we have in this thread.

My mind keeps going back to OP’s first attempt to fire the gun.

OP, when you said “nothing ejected” after you removed the first magazine and opened the bolt, did you visually and tactilely examine the chamber?

One of the problems with steel cased ammunition is that the grey cartridge doesn’t really stand out against the grey/black metal of the gun.

My first thought as well.

Are you missing or have a bad firing pin spring? That could cause the firing pin to detonate the primer OOB and blow the extractor off...

I did visual inspection, but did not do a tactile examination.  I think this is what set up the problem.

In the past couple of days I got new extractor springs and installed one with the old extractor to see what would happen.  

When manually cycling the action with snap caps, there are times when a round is stripped and chambered, but the bolt does not fully seat.  This would happen less often with the HK slap, but did still occur at least once post-slap. When pulling back the bolt, there were also occasions where the round was not extracted from the chamber by pulling back the charging handle.  This of occurred when the bolt did not fully seat, but also happened at least once or twice when the bolt was fully seated and the charging handle was pulled back with force (trying to imitate recoil action).

My thought, as Red stated, is that a round was chambered and the bolt did not fully seat (thus no initial bang). When I worked the action and no round ejected, I missed it during the visual inspection and I didn't move the gun enough for it to fall out of the chamber (keeping the gun pointed down range in a safe direction). If that were accurate, then once I replaced the mag and hit the charging handle, a round was stripped and forced into the primer of the chambered round.  Seems plausible?

My plan, at this point, is to thoroughly clean it, and see if the problem persists.  Regardless of whether the problem persists post cleaning, I'll contact PTR and see what they advise.

My Zenith has done something similar twice now with Winchester 115 grain white box.I’ll load and insert a mag on an open bolt.Give it the pimp slap to close the bolt,squeeze the trigger and nothing happens.Then both times I proceeded to pull the charging handle back and the bolt/extractor didn’t grab the round out of the chamber.I had to take a rod down the barrel and knock the round out.It can be difficult to see if there is a round in the chamber of MP5’s without shining a light inside the ejection port.I’ve replaced my extractor spring and stopped shooting the Winchester white box 115 grain stuff in the gun.I can definitely see how it’s possible to have a round in the chamber and not know it and wind up trying to chamber another round and set off the one already in there.

Physically impossible.