I finally took my m16 and all the ar15 uppers I have to the range. my 22lr, m16 and 2 ar15 uppers worked flawlessly.

I knew my 300blk shorty was the one that may have some issues considering the round I shoot are subsonic. However, the problems came before I expected. Nothing worked in full auto! (Yes, it is a full auto BCG). Occasionally I could get it to fire in semi auto. Didn't figure out any pattern why. But in full auto, not even one round would fire. Everything was just a very light primer strike.

I put the 300blk upper back on any of my other sbr lowers and it fires flawlessly. Could this be just a bad luck of tolerance stacking leading to the firing pin not being hit hard enough?

Any ideas what the deal could be and solutions to try?

Make sure your upper has the sear relief cut in the bottom.

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Too heavy of a buffer in your M16 lower to reliably cycle the subsonic rounds?

So in full auto, the hammer would drop but round would not go off?  Or hammer would not drop at all in full?

Swap carriers, had a similar problem with a friends 5.56 upper and the carrier was out of spec.

Pretty sure that's the correct cutout right?

There was times when the hammer would not drop and the trigger would just be hitting a wall. Cycling would fix this. I'm not 100% sure but I think this just has to do with things not resetting properly while I am diagnosing. So this wasn't the issue.

The issue is hammer would drop but nothing goes off. It's been a few weeks since and I have been out of town, but if I remember correctly, there was a few rounds that had very light primer strikes. But I know it's neither pieces of equipment that are the issue in and of themselves, since the m16 lower works with all my other uppers and the 300blk upper works with my other lowers. So I can only assume bad luck of tolerance stacking problem?



Hmmm. OK, I will try a different carrier next time. Luckily 300blk is completely interchangeable with 5.56

Did you check this? It's about the only variable which carries across between your upper/lower swapping, which could cause this problem.

There is the issue of bolt bounce. What are you using for a stock and buffer for the M16? So in AUTO the first round fires and subsequent rounds don't fire but have a light primer strike? If the other two uppers run, I would suggest trying one of the carriers from the uppers that run and using it in the BLK OUT upper. Maybe the trip surface of the BLK OUT upper is out of spec and doesn't release the hammer at the right time.

Scott

I was pleasantly surprised when my 300 BO upper functioned 100% in FA on my DIAS lower setup with an A5 tube, Tubb flatwire spring and Kynshot RB5007 hydraulic buffer. I thought this buffer configuration would prove to be too stiff & heavy but she chugged right along.

I'm not quite sure I understand. To be clear:

When the lower is put into semi-auto, it would fire rounds. (if member serves me right, it wasn't full reliable, but I can't remember now)

When the lower is put into full auto, it wouldn't even fire the first round. So at this point, the buffer or bolt bounce wouldn't play any role since not even the first round was fired.

Hope that clear things up!



I think you are right that this is the next thing I have to try. I feel silly for not thinking of it right away and trying it right then and there.

As you said, the other two uppers run fine. If the trip surface of the carrier was just several thousands of an inch that would trip the hammer at the wrong time in auto. Semi releases the hammer off the sear surface of the trigger. Auto releases the hammer off the trigger surface for the first round. All rounds thereafter are released by the auto sear. The firing sequence ends in auto when the trigger is released.

So from what you are describing, the auto sear is not releasing the hammer at the proper time. Since the other two uppers run, it would seem to me that the auto sear is not releasing the hammer at the right time to continue the automatic firing sequence. So I would compare the trip surface of the BLK OUT carrier to the trip surface of the other two carriers that do run in auto. Please let us know how you make out.

Scott

Ok, that sounds like the auto sear is being tripped very late in the bolt cycle, to the point of not being tripped at all when manually cycling the rifle.

One easy test to confirm this theory is to charge the rifle with the selector set on "AUTO". When the rifle fails to fire the first round and has a "dead trigger", flip the selector to "SEMI". If the rifle now fires, you know that the auto sear is being tripped too late (or in that case, not at all).

If the lower works on auto with other uppers, then the position of the auto sear within the lower, relative to the hammer, should be correct. The only other part which can affect auto sear timing is the bolt carrier, via its trip surface.

It looks like you've isolated the problem to that particular bolt carrier.

I think the sear is releasing too early causing the hammer to follow the firing pin, giving light primer strikes. As stated above, try a different BC.