It's an AR15 with the factory gizzards. Granted it's a bit old (20 yrs) , but...everything inside is original and has been working just fine...

Using Britich XM193, Wolf 62 gr FMY, and handloads (55 Gr Rem fmj)

It just started to do this @ the range. Pull the trigger, no joy. Let the trigger go, BANG! It keeps doing this...

WEIRD. WTF, Over?

A quick look in the lower FC parts showed no broken pieces, Bolt, BC, CH, all functions OK.

Anyone run into this B4? any ideas?

Time to pull the FCG and do a good cleaning.

Sounds like either the disconnector is hanging up and retaining the hammer (check the disco spring to make sure it's in place) or the hammer J spring is walking out and binding the hammer on the front of the receiver cavity.

Laying odds, I wouldn't be suprised if you found a piece of a blown primer stuck under the trigger, and it alone is causing the problem.

I would pull all the lower parts out and take a look at the trigger and the safety selector....

Sounds like the trigger is travelled lower than it should (for semi) and the disconnector is engaged.  When triggler is let go, the disconnector releases but because the trigger is not in the right position it will not catch the hammer and round is discharged.

Make sure you don't have a mix bag of parts or mod parts.  When the safety selector is in the wrong position, mix parts may cause this problem.