This is kind of long but here are the facts.

I already posted a story last week on the pistol caliber board about my new upper test report. In that post I mentioned that I found some split cases and all of the fired brass was smoked up or burnt looking.
This ammo was brand new UMC 115 gr. FMJ.

After that range trip I inspected and cleaned up the chamber area, forcing cone in front of chamber and the cone area of the bolt. All these surfaces had a real heavy and gritty phosphate coating and I was concerned that it was holding the bolt back from closing completely.

I polished these surfaces by using some fine
or mild 3M Scotch-Brite wrapped around a cleaning jag and chucked in a drill to spin it.
This polished it real nice and smooth with no metal removal at all.

Also checked headspace best I could by taking a loaded round and measuring it to verify length and painted it with a black Sharpie.
Chambered with bolt (firing pin removed) and then checked that case mouth was making contact with front of chamber evenly and that bolt was not holding off seating. All looked OK.

Back out for a shoot and still had a couple split cases! Tried a differant brand of ammo, Olympic 124 gr. FMJ, for comparison.
After 20- 30 rounds I checked spent brass and found some with blown out primers! Shot couple more and POW!! had a round blow the base off and stuck the shell casing in chamber!! Funny thing was that I was holding it at waist level and watching bolt on these rounds to see if it was closing completely and it looked like it was. Talk about a scary face full of fire and metal bits! Good thing I wear glasses or I could have lost a eye!

That Olympic Greek ammo must have a thinner case or differant dimension than UMC.

OK, now its back to the house because the stuck shell casing is not coming out easily. Futher inspection shows I also now have broken firing pin! And face of bolt has primer pocket imprint!
I don't think I will be shooting this baby again until it goes back to OLYMPIC ARMS.

On a bottleneck rifle case I would say I had a oversized headspace problem from the evidence so far?

Not long at all...  We gotta fix this bro.

I read your post in the other topic with great interest, as  I have an Olympic 9mm upper on order.

It sounds like something Olympic should definitely look at.  My investigation tells me their support is excellent.   If the Diameter of the chamber or portion of the barrel where the cartridge head enters is a fraction too small off caliber, it creates more pressure which would apparently drive the force backwards, creating the damage you describe.  The fractured shell casings were only a harbinger.

Let us all know how this works out.  I am personally interested.

Thanks,

-- Fargo007

Semi or full auto?
Dimension on the chamber sidewalls, not the headspace?
Buffer and spring used?
Hammer used, or if the hammer spring was lightened?
Were you shooting lead bullets?
SOCOM block with sten mags or Oly welded mags?
Suppressor?

Bottom line is the rifle is a blow back design.  The hammer/carrier/buffer/spring all have mass to slow the blow back as the round is being fired.  If you opened up the chamber sidewalls when you polished the chamber or the chamber was reamed too wide, this could case too much of a gap between the case/chamber side walls, and when the round is fired.  The loose fitting round will expand sideways in the chamber, which stretches/budges the case too much and it splits the case.

As for the rounds being burnt/smoked up, welcome to blow back guns.  Once you pull the trigger, the bolt starts moving (no locking device) and the barrel gas is what drives the spent case back, not transferred threw a gas port to slow the unlock.

If semi, it sounds like is the chamber was reamed too wide, and/or the rifle lacks the mass to hold the bolt closed until the barrel pressure drops.  Also, Oly uses the standard hammer on their 9mm, so if you lightened the hammer, spring, or used the wrong buffer, you just lightened the resistance of the bolt to stay closed under pressure.  

That sounds bizarre!  I've owned an Oly 9mm upper for quite a while and the only other possible thing that I can think of off of the top of my head is that the firing pin return spring is missing, broken, or weak and you are actually getting slightly out of battery slam fires.

The other question I have is what are you using for a buffer?

To provide some more info:

Buffer and spring are standard CAR-15 stock set I have been using on 5.56 upper with no problems
A heavier spring or buffer may be a good idea to try and delay blowback movement. Would a standard rifle size spring or buffer be any stiffer/heavier? And will they fit in four position CAR stock?

Hammer and trigger group are same unit used with the above parts from my 5.56 CAR-15.

Semi-auto only

No suppressor

Pre-ban flashhider on 16" barrel

Ammo has all been factory loaded FMJ, 115 gr. and 124 gr.

Same results using the Oly supplied modified Sten mag and SOCOM block with Sten mags.

No changes made to lighten bolt/carrier assy or hammer spring, etc.  

Closer inspection of cone area in front of chamber shows an irregular machined step kind of like a burr or non-clean up at radius area where cone turns into chamber. Looks like it could be holding bolt back from full closure.

My cleanup and polishing efforts didn't open the chamber up any as I stayed out in the cone area only where it had heavy, gritty phosphate coating.

I have not measured the chamber diameter yet but that is a good question.

I will be on the phone to Olympic Arms Monday morning for certain. I'll let you folks know what happens.

I really enjoy firing this unit, I hope I get its problems solved.

The full size buffer will not work in the car stock. As for the standard car buffer, it is too light for a blow back action (the 223 uses a locking bolt to slow the unlock, the 9mm does not).

For a buffer, you should be using at least a M-4 "H" buffer, but a 9mm SMG buffer is even heavier and would be better.  For the New buffer spring, use a Wolff Car extra power spring (Stock No. 16500). By using the heavier buffer, spring, you will slow the bolt down, ejecting the spent case, and should solve your problems, as long as the chamber sidewalls are in spec.

[url]http://www.biggerhammer.net/ar15/buffers/[/url]

P.S. The 9mm SMG buffer has a plastic bumper.  If you use a suppressor or hot loads, it tends to chip.  To solve this problem, order a Buffer technology replacement bumper, which is made out of urethane (like the standard buffer bumper).