The Mystery of the HK P7M13 and the COR®BON Ammo as of 10-DEC-2001

Here is the saga, a Mystery in five parts, told in chronological order, the mystery of my Heckler & Koch P7M13 pistol and COR®BON 9 mm 115 gr. ammo.   All parties are innocent until proven guilty in the court of public opinion.

Part One -- The Failures Occur

Once upon a time (about 1995) I owned a P7M8 9 mm pistol from Heckler & Koch.  My very favorite ammo was COR®BON's 9 mm 115 gr. round.  It was very powerful, a +P, and had some good results logged by officers who had to try it out.  I shot box after box after box of it through the P7M8 without any problems whatsoever, and many other guns without a single problem -- about 2,000 rounds total.   The P7M8 turned out to be a fantastic pistol.

So I went ahead and bought a used P7M13.  Took it out and shot all sorts of FMJ ammo through it.  Loaded up some COR®BON 115 gr., fired about ten times and the last round did not extract from the chamber.

I took it home, made sure there wasn't any bullet in the bore.  I used a rod and hammer to drive the brass out of the chamber.  P7's have grooves cut lengthwise in the chamber walls to let some gas in from the barrel around the brass and help in extraction.   The round appeared to show all the signs of what at that time appeared to be severe overpressure, the brass being forced fully into the grooves making a full, detailed impression of the chamber walls.  This caused the round to jam in the chamber very solidly.  The primer didn't look all that bad, didn't show the typical signs of violent over pressure.  But the brass sure did.  (Or did it?  If the grooves filled with the same high pressure, the brass would not have deformed.)

So I thought about it, and it did seem to me I might have a recollection that the round before the jam didn't seem to fire correctly.  I began to suspect that the previous round didn't leave the barrel, and when the next was fired it over pressured.

So I got rather angry at COR®BON and posted a note titled "Corbon Ker-Bang"  in a guns news group.  Someone passed this on to the owner (I believe) of COR®BON, who contacted me.  He said he couldn't believe a round had been stuck in the barrel, as there was no barrel bulge, and this just didn't happen without generating a bulge (I now believe him to be correct.)  I told him I didn't have too many more theories.  This ammo was from a new case I had just bought, so I sent back all that was left for them to take a look at.

They looked at all of it, found nothing wrong with anything.  They sent me some replacement ammo.

I looked over the gun very carefully.  Other than over pressure, all I could think was perhaps the groove ends were blocked at the barrel end, stopping them from pressurizing.  I made sure they were clear -- and besides, what's the chance they were all plugged somehow?

So I went back out to the range with the new ammo.  Loaded up a magazine, and on the second shot the brass was jammed solid in the chamber again.  Note that this ammo was from lot #1139 12/21/95, and the brass had the head stamp "C-B  9mm +P."

I went home, put the gun in my safe, and ignored it till about a month ago.

Part Two -- H&K Takes A Look

I recently got sick of looking at it lying in my safe, untrustworthy.  I first corresponded with Chick Blood, a technical expert with The American Gunsmith's Association to which I belong.  He had me take a look at a few things, but I found nothing wrong.  He suggested I send it to Heckler & Koch.

I sent it off to H&K, along with the last piece of brass that had jammed, and the rest of the box of COR®BON ammo it had come from.

They got it and a very nice man Mr. Coons test fired it at their range.  He loaded it with the COR®BON I had sent, fired it, and the very first round had too little or no powder, left a bullet jammed in the barrel.  (Sorry COR®BON, that's what the man told me -- fresh from the box, replacement ammo you sent me, box stored in a humidity controlled gun safe, gun in working order.)  He got the bullet out, and then fired it with a few rounds of COR®BON 9 mm 115 gr. +P from their stock, and also with some Remington +P+, and with a round of Federal.  Nothing jammed and the results are shown below:
Ammo Brass Condition
COR®BON (mine) 1139 12/21/95
Head Stamp C-B  9mm +P
Jammed for me, Squib round for H&K
Head stamp COR-BON 9mm LUGER +P
Significant groove pattern
Remington +P+ Slightest groove pattern
Federal Almost no grooves visible

Note that while the COR®BON brass didn't jam, it has quite a chamber pattern impressed on it.  They show being forced into the grooves far more than the Remington +P+ does.  The Federal almost doesn't show the grooves.  Some images:

I asked Mr. Coon what he thought was going on.  He was most diplomatic for someone who had to pound a bullet out of the bore of my pistol.  He suggested that the problem was soft brass, that an extra-soft case or two in my batch had come along and given me trouble.  The fired cases seemed to suggest to me that all the COR®BON brass is a bit soft for this application.

Part Three -- More Testing is Performed

I got the P7M13 back and took it out to the range along with a P7M8.  I brought along some cheap FMJ from Romania, some Winchester 115 gr. +P+, and some COR®BON 115 gr. +P.  The COR®BON was from two batches.  One was interesting that it had a head stamp of ¤---¤ 9mm +P.  This means this brass was from Star Brass, a well known company.

I shot about 20 of rounds of the FMJ from each gun to "warm them up."  This brass showed some minor groove patterns from each gun, hard to judge if either looked worse -- perhaps the P7M13 had a bit more.

I shot 2 rounds of the Winchester +P+ from each gun.  Note that this ammo is loaded 20 - 25% higher (approximately 40,000 C.U.P.) than SAAMI specifications of chamber pressure levels for this round. COR®BON does not exceed the SAAMI specification (they say on the box).  Just like at H&K, there was only the slightest groove pattern on the brass from both guns.

I then shot 30 rounds of the COR®BON (Star) from the P7M8.  Had no problems, and the brass showed minor groove patterns on each round.

Then I shot 5 rounds of the COR®BON (Star)  from the P7M13.  Only 5, because the gun failed to extract the fifth round, the brass jammed tight in the chamber.  The first three rounds showed about the same amount of grooves as the P7M8 rounds.  The fourth had a heavy pattern.  The fifth gave the full chamber impression after it was removed with a hammer.

I switched the P7M8 over to the COR®BON (COR®BON) batch of ammo.  I had no jams, but the groove pattern seemed deeper from this batch.

I had a theory that the cheap Romanian FMJ may have had brass that was a bit thin, which may have caused the minor patterns.  I shot a few more FMJ round of various makes from my "junk" box in the P7M8 and they all had almost no groove patterns.
Ammo P7M13 Brass Condition P7M8 Brass Condition
COR®BON S1199 6/18/97
Head Stamp ¤----¤ 9 m m + P
Minor to major groove pattern, then jammed solid Minor groove pattern
COR®BON RC1203 6/24/97
Head Stamp COR-BON 9mm LUGER +P
(Not tested) Significant groove pattern
Winchester +P+ Slightest groove pattern Slightest groove pattern
Romanian FMJ Minor groove pattern Minor groove pattern
Misc. FMJ (not tested) Minor to no groove pattern

Part Four -- COR®BON Responds?

Here is everything I have written to them and have received by 11/11/99, over a month since the first letter.

 -----Original Message-----
 From:   Chris []
 Sent:   Saturday, October 02, 1999 1:03 PM
 Subject:        A Request
 Importance:     High


 I would appreciate it if someone at Cor-Bon could read my article at, and would welcome (and will publish) your

 I will also appreciate any help in solving this mystery, and will
assist  in any way I can.


 Chris Cooney

 Peter wrote:
 We have read your article and have passed it on to our research and
 development, and technical services departments for comments and
 evaluations. We will notify you of our findings.

-----Original Message-----
From:   Chris []
Sent:   Wednesday, October 06, 1999 8:22 PM
To:     Peter
Subject:        Re: A Request
Thanks.  I'm trying very hard to not jump to any more conclusions.  Most
of all, I'd just love to find out exactly what is going on, and would
welcome any assistance from Cor-Bon.


Peter wrote:

And you will get it just as soon as we hear from them. On a side note I
carried a P7M8 for 3 years while I was a member of our Sheriff Dept's
Mounted Division and fired over 5,000 rounds through that gun without a
single failure to feed or extract the 9MM+P 115gr Cor-Bon load that was
dept issue.

-----Original Message-----
From:   Chris []
Sent:   Thursday, October 07, 1999 7:25 PM
To:     Peter
Subject:        Re: A Request

Yes, it has been flawless in my P7M8 too.  That's why this problem has
driven me nuts.  I'm not exactly an amateur when it comes to being a Pistolsmith
and I just can't see anything, and H&K gave it a careful examination too.  I'm
just about ready to switch barrels between the two guns.  And if you have
any desire, I'd be happy to loan the P7M13 to you folks.

Just for my curiosity, what Sheriff's Dept. was cool enough to use that

Peter wrote:

Livingston County Sheriff in Michigan.  It was not issue, but we could
carry what we wanted.

And that's all they ever wrote.

What can be concluded?  I'm not sure -- what do you think?

I do know one thing -- don't use Corbon in my P7M13.

Is there something "wrong" with my P7M13?  HK says no.  But it does fail where my P7M8 does not, where Peter's does not fail.

Is there something "wrong" with the Corbon 115 gr 9mm?  It is the only ammo that fails in my P7M13, fails quite often.

What about the squib load, the one round that they left the powder out of?  I guess it has to happen sometime to everyone -- or does it?  What do you think?

Am I safe sticking with this round in my P7M8?  Works great so far, Peter shot 5,000 rounds in his.

Part Five -- Epilogue FEB-2001

Well, it's been two years since all the above, Corbon never did want to reply.  I've had a chance to hear from readers, run some more tests, and give all the evidence a fresh review.  First, the new data.

I took the P7M13 out with a box of every brand of 9mm ammo I could find, everything from cheapest FMJ to Silvertips, even both Winchester +P+ and Federal +P+.  Put almost 500 rounds through it, had to let it cool quite a bit -- the P7's sure get hot!  Not a single failure.  Then I tried 3 boxes of Corbon, each with a different lot number.  All three jammed up after a few rounds.

I've had a few other people write to me with similar problems, not as severe as mine, but with some troubling heavy marking on the brass with Corbon.  None of the other rounds, including the +P+ , deform the brass into the grooves to such an extent.  Even my P7M8, which likes the Corbon, puts heavy marks on the brass with Corbon.

Now, the deductions.  First off, it's not just chamber pressure that forces brass into the grooves -- it's chamber pressure with a lack of pressure in the grooves.  That is the whole point of the grooves, they are connected to the barrel and should pressurize.  If they were at the same pressure as the chamber, there would be no marks on the brass.  The brass is forced into the grooves when the chamber pressure is much higher than the groove pressure.

When a round fires, it may happen very quickly but it is a very dynamic event with many stages.  The powder starts to burn and the case pressure starts to come up.  It rises as more powder burns, but the pressure is trapped in the case.  At this point the brass is being forced into the grooves to some extent.  The pressure finally rises to the point it forces the bullet out of the case and into the barrel.  The barrel starts to pressurize and this pressure starts to flow into the grooves.  As the bullet starts down the barrel the pressure between the inside of the case, the barrel, and the grooves will all come close to being equal.

So, for some reason the Corbon in this P7M13 seems to be building up enough case pressure to force the brass into the grooves before the bullet leaves the case and the pressure flows into the grooves.   Perhaps Corbon has an atypical pressure peak before the bullet leaves due to powder selection?  Perhaps Corbon is using very soft brass?   Perhaps my P7M13 has an especially tight barrel or some other feature that delays the bullet leaving the case or the pressure reaching the grooves?

Here is some interesting data.  Check out the manufacturer's data for these rounds:
Round Muzzle Velocity Muzzle Energy
Winchester Std. 115 gr.
Winchester +P+ 115 gr.
Corbon 115 gr.

That's all very interesting.  Winchester says they have to run case pressures around 20-25% higher than SAAMI standards to get that 1335 FPS.  Corbon goes even faster -- 1350 FPS -- but claims "Cor-Bon follows established SAAMI specifications."

Now come my conclusions and opinions. I believe there is nothing wrong with the P7M13.  It just has no problems with any other ammo -- no other ammo has anywhere near the amount of brass "grooving" that the Corbon does.  (The same is true in my P7M8 -- the Corbon works in it, but has much more grooving.)  So I've concluded that the Corbon round is a marginally engineered round that just might not always work.  Yes, people (including me) have put thousands of rounds of it through P7M8's w/o trouble -- but luck shouldn't be part of handguns.

It's hard for me to believe Corbon does not exceed SAAMI specifications for case pressure.   They may pass a test where they crush a copper test cup, but they have to have some big pressure peak somewhere.   Perhaps they have found a way around F=MA , some way around the laws of physics?  Or perhaps they are running a way high case pressure peak?  Triple base powder?  Quadruple? Their only explanation is "Q: How can you be so much faster than your competitors?   A: We work harder at it."

This is really a shame -- it is a fine performer ballistically.  But with the marginal performance I think it is unwise to use it any gun with a grooved chamber.  And when I look at the big picture which includes the round found by the H&K armorer that had no powder in it -- I'm just not going to use any more Corbon.  Used to be their biggest fan, but the romance is over.


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