While shooting recently, I had a shot that blew the casing at the base next to the rim.  The shot sounded different and the mag follwer was blown down into the mag tube sideways.  The bulged area on the casing made it look like the rifle fired out of battery.  I could see no visible damage to the chamber throat or the bolt and lugs.  This really has me worried.  Is it a headspace problem, an ammo problem, or something else?  I'm shooting some Quatemalan I.M.G. ammunition.  It's only happened one time but then again I've only fired 200 rounds through the rifle.  TIA

Look at technical note 49 on the Armalite web site. Also look at "the dangers of cheap ammo" in the trouble shooting section of the knowledge base on the bushmaster site.

If you examine your bolt/carrier you will notice the firing pin cannot protrude until the bolt is turned to the locked position.

If the bolt/firing pin is normal then the case must have failed. I would not shoot that stuff.

Sounds like a head seperation to me. Out of battery firing is tough to get in AR15's because of the way it is built.

What happens in the firing sequence is that the firing pin shoves the case forward until the case shoulder reaches the chamber shoulder. Then, as powder pressure builds, the neck and shoulder and most of the body sticks to the chamber wall, and the case head is shoved back to the bolt, stretching the case near the head. If you reload it a bunch of times, or if the headspace is excessive, or if an anneal step was missed during the case drawing sequence, this failure will happen. You can either have too long chamber, too short ammo, or both.

You just witnessed that a head seperation can damage mags with leaking gas, endanger your eyes with flying particles, but will not necessarily destroy the rifle.

Check the gun headspace, the ammo headspace, and suspect the cases of being brittle too. Learn from this.

I'm leaning towards, the ammo, I got it for real cheap.  I still have several hundred rounds left of this stuff too!  I will have the headspace checked (again, I had it checked right after I built it, it was fine) by another gunsmith.  Thanks again for the prompt replys.