I've had a MP5 conversion (registered sear/SBR) for quite some time, and yesterday was the second time it's began firing when closing the bolt on a loaded mag.  It only does it when it's REALLY dirty, and works fine when I clean her up.  Is this something I can write off to (admittedly) poor maintenance, or do I have a more serious problem that I should investigate?  

TIA.  I've gotten a good understanding of several weapon platforms, but have never really had time to dig into my H&K gear....

    Black Fox

Possibly a weak firing pin spring, or just a lot of gunk holding the firing pin forward, or a combo of the two.

Thank you, sir.  I'll clean and check the firing pin spring....

Quoted: Possibly a weak firing pin spring, or just a lot of gunk holding the firing pin forward, or a combo of the two.

+1

Probably something holding the firing pin in the extended position. Check that first.
Disassemble the bolt/bolt carrier COMPLETELY, and clean well. Also check for broken parts.

HKO

"MP5 fires when closing bolt - ideas?"  

It only does it when it's REALLY dirty, and works fine when I clean her up.

No flame intended, but...  

All - thanks for the input.  I was finally able to disassemble her and found what was probably the problem - the locking roller holder was broken in half.  I also have the standard firing pin spring, and although it still feels resistant, may be worth replacing.  So, it looks like it's time for a new locking roller holder and firing pin spring (just to be safe).  It's a shame, because I had an all-women's CHP class this weekend, and they were all looking forward to firing an MP5 .  

I also noticed in reading the H&K documentation that they call for a major cleaning every 500 rounds when suppressed.  I always fire my MP5 suppressed, so that probably contributed to the problem....

osprey21 - no worries.  I'm talking about an entire day of shooting here (don't normally break down and clean guns every few hours while shooting) - not weeks or months.  Now that I know there are broken parts, I'm not sure cleaning had a whole lot to do with it anyway?

For all those interested, there is some information on the MP5 at www.hkpro.com/mp5.htm.  There is also a copy of the MP5 manual at www.biggerhammer.net/manuals/ for those that don't have one.  

    Black Fox

The part that broke is inexpensive.

If you want to shoot it still, you can.  It will fuction just fine without the roller holder.   All the holder does is keep the rollers from falling out when disassembled.

I have seen lots of people shoot with "loose" rollers without an issue... It's just a little tricky to get the bolt assembly into the gun... you need an extra set of hands.

By all means, you should get a replacemsnt, but it will function, and you will not damage anything if you fire the weapon without the retainer.  Just be careful to not lose the rollers when you disassemble the gun.

It sounds like the broken piece was keeping the firing pin pushed forward.  This may have caused undue stress on the firing pin, and I'd order a new one of those while you're at it.

Damn that sounds dangerous.  

Quoted: Damn that sounds dangerous.

Yep!  That's why - Rule #1: ALWAYS point your firearm in a safe direction

When chambering, reloading, etc., I try to keep the firearm on target.  It also helps that I have a range on my property, so I don't have an angry range owner looking over my shoulder....

Dangerous for 2 reasons...

1) Unexpected discharge, and pointing the muzzle in a safe direction.
2) Firing out of battery.


The roller holder is not essential to safe operation. But when broken, can cause problems with two pieces of metal floating around inside the bolt.

CZ - Completely agreed - it's dangerous, which is why I asked.  I didn't mean to dismiss it....  (sorry, I guess the instructor in me was looking at this as a "teachable moment?")

With that said, did have a great class today and everyone shot the MP5.  She functioned flawlessly, and had no problems.  Thanks again for your help, all.  Some first-time female shooters had a great time this afternoon with an MP5 thanks to your help.

    Black Fox

I think a question you need to be asking is why did it break.  That's not a normal failure on a lightly used gun.

Quoted: I think a question you need to be asking is why did it break.  That's not a normal failure on a lightly used gun.

Really?  I was going off the bottom of www.hkpro.com/mp5.htm where they displayed a "standard holder that breaks frequently" (which is what I have, albeit in 2 pieces now).  

The MP5 I have has not been abused by any means - the parts showed no wear when I got it (the owner claimed he never shot it?), but I've put several thousand rounds downrange.  If there's something specific I should look for, I'll certainly do so.  

The roller holder IS the most common part to fail on a MP5.   Extractor Springs wear out a lot also

I'm really glad you were able to put a smile on some new shooters' faces!

You might want to check with Phil Flak, I hear he has the best prices.   There's also Adam Webber and Gordon Miller.   Between these three people, you should be able to get all the parts you need.