Rifle: (all new parts, I built the lower, but the upper was built by someone with whom I only had email communications)
- Armalite upper (without M4 feedramps) w midlenght barrel (with M4 feedramps)
- No name B/BC group (rough finish type, no stamping, possibly DPMS or ASA)
- Bushy lower with DPMS LPK
- RRA std buffer & mainspring

Ammo:
- only American Eagle 55 gr FMJ, so far.
- Half of it was stamped "Federal" (that's normal), while the other half had the NATO symbol and  LC 04 (Lake City), with red sealant around primer

I've shot a total of about 250 rds and in the last 25 (the NATO LC04 casing), i experience about 3 FTF.
The primers were struck, but too softly. When putting back the round in the rifle a second time it finally went off. Bolt locks back on last round everytime.

Do i just need to relax & carry-on w the break-in  ?

Here's two possible issues I can think of:
1) The no-name B/BC was checked, staked & extractor system replaced by a pro armorer... Armalite's official retailer in Canada, in fact. He and his friend (the guy who assembled my upper), said that there was a lot of internal friction between that BC & the upper, but I just needed to shoot a few hundred rds to see if it would work out. So I put alot of CLP & fired away.
Since the bolt is new and could be slightly out-of-spec, can it be sometimes not fully locking on the lugs, thus making more distance between the bolt face and primer ?

2) is it known whether Am Eagle 55 gr, especially the NATO stamped stuff, have very hard primers ?

Thanks

I've never had American Eagle ammo fail to fire. I've never had a rifle fail to fire because it was new. By your own admission it sounds like your B/BC is junk. Do you have another B/BC you could stick in and try out? If it were me I would get a CMT B/BC.

Any possibility the feet of the hammer spring are resting on the floor of the lower and not on the trigger pin?
FWIW
YMMV

Quoted: Any possibility the feet of the hammer spring are resting on the floor of the lower and not on the trigger pin? FWIW YMMV

Good thing to ckeck indeed... but nope...they're on the trigger pin alright.

When I unscientifically check hammer power (by pulling the trigger & releasing the hammer on the palm of my hand) I feel a good stroke. Been handling Diemaco C7s for a dozen years now and it feels about the same regarding hammer spring tension.

With stripped bolt in hand, insert/bottom out the firing pin and check protrusion.  It should be .032 to .038.  If needed, mill/roll sand the back of the bolt (entry point section where the firiring pin collar bottoms out) to increase the protrusion to spec.

Another item to check is to make sure that the bolt is fully locking up.  You have to remember that the firing pin of the bolt floats, so the pin will slighty indent the primer on loading.  What you may think is a first hit light strike, may just be the firiring pin indentation on load, and the bolt is not locked up, hence the carrier blocking the firing pins full travel and the firiring pin never reaching the primer at all during ignition (first try, with the first try impact actual locking the bolt, and the second try allowing the firing pin to actual reach the primer).

Armalite tends to run there chambers on tad on the tight side, so if it's a bolt locking problem, the rifle/chamber will polish out threw live fire, and the problem should go away in a few hundred rounds.  Until this happens, just rememeber to keep all the upper bearing parts lubed with a light coat of CLP (including the buffer and inside of the receiver extension).

I have now fired at least 250 more rds without any problems.
A few Am Eagle 55 gr fmj and mostly Win 45gr jhp.

After cleaning up my AR this morning, I noticed that when holding the upper with barrel pointing upwards, the BC group (M16 weight) would fall off just by gravity... even with ejection port door closed.

Is this a sign that the "break in" period is finished ?
Or could it be the bolt is out-of-spec (lugs are too short) ?
This could explain both events (the FTFs, 'cause firing pin is not protuding enough, and the unlocking of bolt by gravity).

When I put just the bolt in barrel extension, holding it with my fingers (no carrier), I can feel some play and it rotates quite freely.

The gas rings don't seem too eroded as the bolt stays in the extended (unlocked) position when inserting in the carrier (placed vertically, even without FP, FP ret. pin or cam pin).
A normal amount of force is needed to push down the bolt into it's locked position in the carrier.

Quoted: You're having Failures To Fire? have the firing pin protrusion and headspace been checked?

No, not yet.
will do that shortly... when i'll find someone with the right tools