Well here's how the saga began.  I was shooting some reloads using some Surplus WCC844 that turned out to be shit.  Half the expected velocity, only about half the powder actually seemed to burn, the rest just served to make my rifle filthy.

The rifle.

Factory M4 Colt upper and barrel, Colt bolt and carrier, preban Colt lower, Jp trigger, stock colt factory hammer,spring, and disconector, Aimpoint, ggg buis, PRI front sight, Giles tactical sling, BMAS, KAC grip, Surefire light, Redi-mag, ZM weapons comp.  About 2K rounds thru it with the only probs being mag related.

First problem.  Carrier jammed (while firing)so bad over the hammer, I couldnt get it unstuck with anything I had with me at the range.  THe carrier was half way into the buffer tube and stuck there.  Took it home pulled the stock and buffer tube off and used a small screw driver to push the hammer down and got everything apart.  Other than being filthy, I could find nothing wrong.  Disconnector works fine, saftey works, firing pin looks good. Hammer and trigger pins are straight.  So I clean, lube (molyslide) and reassemble.  I also cleaned the remainder of the rifle.

Second experience.

Back to the range today. Same rifle.  SA ammo that has run fine thru this AR and every other I have.  Yes, its not the cleanest burning stuff, but good velocities and accurate. I load up the first mag with 20 rounds. Shoot about 15 rounds - everythings fine.  Unload rifle to help a buddy set up some targets.  Insert mag with 5 rounds.  While holding the foregrip left handed. I pull back the charging handle, with my right hand and release to chamber a round - BANG.  No finger on the trigger. Selector set on Fire.

I then take another mag with 20 rounds and cycle them thru the action trying to get it to fire using the same procedure as above - Nothing.  I then tried shoot 2 rounds, pull the charging handle back (ejecting a live round) and release - Nothing.

Shoot 100 more rounds slow, fast.  Everything works fine.  An inspection and function check of the disconnector, saftey -- Normal.

So do you guys think I just had some (too much) grease on the engagement between the hammer and trigger? Or dirt/unburnt powder somewhere? Or What?  I did check the JP adjustment screws and they havent moved since I glued them 1K rounds ago.

This things really starting to spook me since its my house gun.  Im baffled as to the problem.  I can let the first episode go to filthy, crappy powder, but this is getting to be a bit much.

Thanks for your help.

Disconnector is bad.

On cycling, the hammer wasn't grabbed by disconnector and followed the carrier forward.  Then it bound up on the firing pin since your rifle has an exposed pin and an "AR-15" style carrier.  

I've seen that before.   In most cases you should be able to clear the malfunction with the charging handle, but I have seen one where the thing binds up so bad that it can't be moved easily.  

Best bet is to pull the FCG and clean it.  Sounds like you had a un burn flake of powder in the FCG and it held something a little open.

Make sure that you get everything is the receiver cavity out and clean before you re-install the cleaned FCG.

P.S. I know the feeling on using surplus powder.  Got my hands on some old powder for shotguns a long time ago.  You would pull the trigger and hear the primer pop and the primer charge would push the shot and cup into the barrel, then a little rattle of the shot rolling down the barrel, and then a whoosh as the powder started to burn.  Nothing like cutting up hulls just to save shot, wads and primers.  My guess is that your a little less that happy have it pull down freshly loaded rounds.

Thanks for the advice guys.

I did totally clean the FCG between the first problem and the second problem.  In fact I totally stripped and cleaned the entire lower and all parts.  I think it may be the disconnector as well, but why did it work fine for the rest of the rounds - ie, no more hammer following?  The disconnector works as its supposed to by hand and it worked fine for about 100 rounds.

How can I tell if its bad [b]in this case?[/b]

Shouldnt it have continued to malfunction?

When I have seen bad disconnectors before, the rifles would double and triple fire with regularity, not just once and quit.

Im still open to any thoughts or suggestions. Ive never had an AR act this way.  I'm not sure if its gunk or the disconnector or something else.  I own 5 AR's and have built or helped build another 10 or so and I've never seen this set of problems before.  So send me your thoughts..

Best guess is that you have the trigger set tight, and you got a piece of fouling that caused the problem.