A week after my A-2 Ka-Boomed, I'm still researching the possible causes, and I think I found something.  I had downloaded the PDF file with the Oly Owner's Manual, but hadn't looked at it for quite awhile.  Silly me, I thought all AR parts were pretty much interchangeable between makes, but apparently that's not so!  On the FIRST page of the manual, Oly tells me NOT to swap bolts or carriers between guns.  "To do so may cause damage or injury to you or your rifle."  Well, guess what, that's exactly what I did.  

My Oly is a Match-Target heavy barrel model, stainless barrel close to an inch in diameter, but I could never get it to shoot up to my expectations.  I also got tired of the weight, and decided to get a different upper, something lighter and handier.  After shopping around, I bought the A-2 upper from Del-Ton ten months ago, and have been delighted with its performance.  The chamber was cut to .223 specs, whereas the Oly was a 5.56.  I quickly found that ammo that would feed flawlessly in the Oly wouldn't even chamber in the Del-Ton half the time, until I began sizing it down to minimum specs.  But in discovering that, I found that if an oversize round was fed, it would not let the bolt close, yet it still went CLICK when I pulled the trigger.  Were the gun unable to fire out of battery, it shouldn't have done that, but I never made the connection.  It only cost me about $500 to learn what that CLICK meant.

I was trying to save $85 by not buying Del-Ton's bolt and carrier, figuring that all AR parts are made to the same specs and were totally interchangeable.  Well fuck me running backwards, I guess they ain't.  Experience can be expensive.  I just hope somebody else reads this before they make the same mistake.  I spend a lot of time lurking on forums like this one, and I've learned a lot.  I'm close to fifty, but I'm still alive and have all my fingers and eyeballs, simple proof that the Lord loves fools and drunks.  Sometimes I've qualified on both counts.  They say what doesn't kill us makes us stronger.  I'm not sure about the stronger bit, but I'm just a hair smarter than I was last week.  

My remaining question is why it took close to 2,000 rounds for this situation to rise up and bite me.  Guess I was just lucky, eh?

Papajohn the Enlightened

swapping uppers with your bolt shouldnt have kaboomed unless it was way out of spec I have swapped the same bolt into oother uppers many times as have many others here its not a big deal.

You went from a 5.56 chamber to a  .223 chamber. You also mentioned possible long brass in the other thread.

It appears you are closing in on your overpressure source.

5.56 is a hotter load compared to .223... When using 5.56 ammo in a .223 chamber you increase chamber pressure which can lead to KABOOM...

Swapping bolts had nothing to do with your kaboom. I'd venture to guess that 99% of the time, it is an ammo problem. Perhaps you overloaded or underloaded a case, or had too long of a COL (which allowed the bullet to engage the rifling before firing.)

The click was the hammer dropping, but the rifle can't fire when out of battery because the firing pin can't reach the primer until the bolt is locked.  The one exception is a high primer, which can receive enough energy from the bolt going forward to ignite.  That's why I clean my primer pockets every time with a uniformer; it does a much better job than a brush, especially in the corners, and ensures that the pocket is deep enough.  I also inspect every primed case to be sure that the primer is seated below the case head.  If your brass was longer than SAAMI spec and you had a high primer it's a recipe for disaster, as you found out.

"The bolt was apparently driven back hard enough into the carrier to peel away the bottom end of the bolt, bulge the carrier, and consequently the receiver."

What was the condition of the cam pin?  Could it have been sheared or missing causing the bolt to not rotate to lock?

Not buying the bolt and carrier for $85 was certainly not a savings, since you could have sold it new for $110 on the EE.  

"They say what doesn't kill us makes us stronger. "

Getting stronger is killing me!

Swapping bolt and bolt carriers?

At least get a field headspace gauge....

Any time bolts are swapped, you MUST check headspace.

Also, 5.56 chambers are LONGER than .223 overall, with the same bullet weights. Chamber pressures are equal enough to not be significant. The real problem between the two ammo types, is 5.56 has a longer leade cut, enabling any NATO ammo to be fired. .223 leades are NOT cut as deep, resulting in 5.56 ammo pushing into the rifling (your situation with difficult chambering), and possibly pushing the bullet down into the case.

You had two errors that resulted in your KB: Ammo not fully chambered, and a bolt not fully locked. The explosion blew the bolt/carrier violently to the rear, resulting in the damage you see.

M16/AR15 have one thing in common, the bolt may not be fully in battery before hammer drop. The last "nth" of bolt travel is chased by the falling hammer, to result in both meeting at the same time right at lock-up. Obviously more prevelent in the M16 with the sear releasing the hammer, impossible to duplicate with an AR.