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On the 100 yard range last weekend I was running a new Colt 20" Bull bbl upper (5.56 chamber, 1:9 twist) on an existing Colt LE6920 (carbine) lower. When "cease fire" called and upon clearing the live round from the chamber, I noticed that there was a firing pin strike on the chambered round (Hornady 55 gr BTHP 5.56).

When the line went hot I chambered and cleared again and saw the same strike on primer marking. I immediately ceased use of the firearm. I suspect that there could be a headspace or bbl seating issue? I did pull BCG and visually inspected, saw nothing apparently out of ordinary. Upper had about 40 rounds through it before I noticed this.

Any opnions on what I should look for would be welcome! Any recommendations for qualified gunsmiths in DFW area (I live in N. Dallas, work in Irving) that could help me?

THANKS!

No worries. That's normal, in my experience.
The AR-15 has a light "floating" firing pin that will contact the primer when chambering a round.

If it troubles you, google "ar-15 firing pin inertia dimple" to see more images that look exactly like yours.

Perfectly normal. As jimd says, the AR has a floating firing pin which will strike the primer when the bolt closes. There isn't enough inertia in the pin to set off the round, so a slam fire is nearly impossible. Just keep on shooting

When I reload 5.56 ( which I rarely do now that ammo is available) I use CCI #41 primers just for this reason.

Thanks for all the feedback! Very useful and much appreciated!