Okay, I'm new to owning my own AR-15, though I am very familiar with them from my service in the Army.

I was out plinking (with Wolf) and after about 20 rounds, I pulled the trigger and there was a click.  So, I pulled back the bolt and I could see that the conical firing pin housing part (forgive me for not remembering the technical name) had split right at the spot were that larger "T" pin goes in (right under where the gas extraction tube is).  After removing the broken part of the firing pin housing, (which was easy) and the round in the chamber, I could see that the firing pin had hit the primer but the 'dent' wasn't as pronounced as what I am used to seeing from my AR-15 (and several other firearms I own).

The AR-15 I have is an essential arms lower (the serial number is an "EA. XXXXX" model J-15, CAL. .223-5.56) with an unknown upper.  The upper has an upside down "G" right over the where the forward pin to field strip it is.  Also, the windage knob is not like that on a standard M-16, it is a more simple design.

So my question is, what do you think it is, the ammo or the bolt?  Also, how easy will it be to replace this broken part?

My rifle was clean, I know how to maintain it, and I had only fired about 70 rounds of Winchester .223 Varmint ammo through it.

I've only owned it for about a month.

Thanks.

I've seen bolts break from time to time - usually from Professional Ordnance and Oly Arms.  If you can't have the seller replace it, just buy a new bolt and throw it in there.  

The dent in the primer is normal and will happen when you chamber a round.  Since the firing pin is free floating it does strike the primer, just not with enough force to fire.

Okay, where's a good place to start looking for bolts?  Additionally, what is the compatibility between them?  Since I can't really track down who the manufacturer of the upper is, is that going to be a problem?

Like I said, I'm really new to this, I just sort of jumped in.  I've had a few rifles all my life and wanted something more in a military style and I am familiar with M-16's and such, but when we had a problem, we turned it into the unit armorer who took care of it.

New bolts are readily available on the web from
- Fulton Armory
- Olympic Arms
- DPMS
- Bushmaster
- Georgia Precision
- Brownells
- J&T
and probably many others.

All should work; unlike certain fire control parts such as hammer and bolt carrier where there is a difference between AR-15 and M-16 parts to enable auto fire (and, btw, such M-16 parts are illegal to possess), to my knowledge there is no difference in spec between AR-15 and M-16 bolts.

Only issue with using a new bolt is that you would be well advised to have the headspacing checked by a gunsmith.

Thanks for the help everyone (I also got your message Shadowblade).

I took it to the dealer, and even though he sold it to me "as is" with no warranty, he agreed to split the cost of the bolt with me and install it/do any neccessary adjustments (or so he basically said).

I'll take his word for that, living in a small community people who are seen as conducting bad business tend to go out of business fast.  

Retail price from Bushmaster on a complete bolt assy. is $45 so it he tells you that your half is $45 then you are paying the full amount. He's just going to install the part which is fine as there is no adjustment you can make.