Quoted:


You need to verify that the uppers are not short stroking.  If the carrier is short stroking the hammer may not engage the the auto sear and will follow the carrier back.

Quoted:
Welcome to the world of bolt bounce.  Heavier buffer is the usual quick fix.

The issue is that the carrier is recoiling slightly from battery, blocking free access to the firing pin by the hammer, so it is indeed a timing issue, but the way to prevent the carrier from recoiling so much is to decrease the gas impulse, which is a pain to do, or to increase the buffer mass, which is pretty easy to do.

Quoted:
In addition to a heavier buffer, you might consider changing out the action spring (buffer spring) if you don't know its age, as these get worn out.  Life of this spring is generally about 12K max, though we  we have seen them wear out in 5K due to hotter loads and rifles that are cycling quicker.  There is a reference to a carbine length being worn out when it measures shorter than 10-1/6" in length, but from our experience it has more to do with tension than length.

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123  

Quoted:
Maybe a heaver buffer will help to control bolt bounce (carrier bouncing back off the face of the barrel extension as the hammer is striking the FP,  which cause the hammer to shove the FP stop collar into the back of the carrier, instead of the back of the bolt to achieve full protrusion,  since the bolt is partially unlocked) to solve the problem, but since I have mac jacked Kitty Kat's with  buffers that do not have internal weights as all (no dead blow effect at all), lets think about  attacking this problem the other way, hence getting the sear release gap/timing correct to begin with.

You need a two drills bits to use the back ends of them,
#45, which is .082"
#43, which is .089


Rigs unloaded and one upper at time on a lower, tape the trigger back, selector in full auto, pull the charging handle back, then stick the  back end of the .089 drill bit so it will be sister between carrier and barrel extension faces when you lower the charging back down via the charging handle.
http://garyjavo.com/2012/M16/2.JPG

The ideal gap between the carrier front to barrel extension back surfaces should be .083" when the hammer is released from the auto sear.

So with the .089  back of drill bit between the two to block the carrier from going any more forwards, the hammer should not be released since the gap it too big.
With the .082" back of drill bit in place, the hammer should fall as the back of the drill bit is kissed by the two edges of the front of carrier and back of barrel extension.

Right now on your problem children combo's, going to bank the hammer release gap is in the .100" plus range instead (too advanced hammer release release from the auto sear, and the root of the problems).

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Now the fun time to be hand by all to correct such,
Is the game plan to have all the uppers swap out on any of the lower, or to dedicate the upper to a specific lower instead?
As for the B/C's, are they to be dedicated to the specific upper, or do you plan to swap them around between the upper as well?

Give me the low down on the game plan for the upper to lowers/the B/C's  to uppers (what is going to be swapped around), and I can write the needed novel on how extensively this is going to be if you have the talent to do this work yourself.

If carriers are to swaps between uppers, then you will need to unify the distance from the back of the barrel extension, to the front take down pin locations of all the upper's to begin with.


If the B/C are going to stay with the same upper, and only used a specific lower, then it's the auto sear tab on each carrier than can be adjusted per carrier/upper to work with the auto sear for that given lower.

If you want to play complete swap of all parts,
Then we need to first unity all the lower receiver auto sear releases so they are releasing the hammers at the same time/ forward cam position of the sear tab angle to the front take down pin channel center line,

Then unify all the upper receivers from back of barrel extension to center line of front take down pin,

And the lastly, unify all the carrier sear tabs,  ending up with an auto sear release gap of .083" between the front of the carrier to the back of the barrel extension when the hammer is released from the auto sear for all the combo's.

If this is beyond your skill level, then get in touch with someone like Circuits for the needed machining/smiting work,
or if you have a local smith that does all your work, should be old hat to him a well.