Click but no bang

[tcoz]

I took my new 300 BLK upper to the range for the first time today. It's 16" carbine length gas. I was shooting commercial Fiocchi 150gr FMJBT ammo which isn't the best stuff available. The lower I was using has fired at least 1000 rounds of 5.56 without a single hiccup including 50 rounds today. ALG ACT trigger. I only had time to put 30 rounds down range. 15 rounds through each of two 20 rd Lancer mags. Out of these 30 rounds, I had three clicks with no bang. After ejecting the rounds and loading them back into the mags, they fired without a problem. Also, the bolt didn't lock back after the last round fired on either of the two mags. Any thoughts? I'm leaning towards the ammo being the problem, but I didn't have time to troubleshoot. BTW, the ammo is loaded pretty short at a COAL of 2.06"

[alamo5000]

It could be any number of problems. Specifically what kind of upper is it? What kind of barrel? Did you build or buy it?

Did you have your upper head space checked?

It could be anything from hard primers on down to many other things... including but not limited to cheap ammo.

[Danep21]

Gas system... buffer weight... ammo... could be numerous issues.

[alamo5000]

If your bolt didn't lock back I would ask more about that upper and where you got it or who built it. The gas holes might not be aligned properly with your gas block and barrel. It sounds to me like the upper is part of the problem...

[Thompson]

Also, the bolt didn't lock back after the last round fired on either of the two mags.

It could also be that the magazine springs are worn. Are those relatively new Lancer mags, or have you had them for a while?

Also - you said that after no bang, you re-loaded the rounds. Did you do an inspection of the rounds at all? Any light primer strikes?

[tappedandtagged]

A few questions:

1) What did the primers on the three would be duds look like after the first attempted firing? We're they dimpled as much as a fired round usually is? Or were they light primer strikes? The former indicates ammo issues, the latter indicates a weapon issue.

2) What buffer do you have installed? Carbine, H, H2, H3?

3) Does your lower have an extra power buffer spring?

4) Is the upper a build or a buy?

Now some speculation:

For the failure to lock back; Like Alamo5000 stated, the gas block being out of alignment could account for the failure to lock back. So could a heavy buffer/spring. I'm assuming that you are conducting regular cleaning/maintenance on it and keeping it lubed. If not, that could be your issue; but a dry upper that's new should be able to easily handle 30 rounds.

For the failure to fire: Headspacing could be it. Find someone with a 300 BLK no-go gauge. If it goes, it's the headspacing. If it doesn't, try to swap out BCGs with another and shoot some more of the same ammo. Fiocchi isn't the best, but it isn't Tula junk either. It should go bang a lot more than 90% of the time. The short COAL wouldn't have an effect on the failure to fire.

[tcoz]

The primers looked just like they always do after ejecting a chambered unfired round. Light dimple from the firing pin hitting it when the bolt slams forward.

Carbine buffer with standard spring. Ejection pattern is 2-3 o'clock indicating appropriate gassing.

This is a build by someone quite experienced building 5.56 rifles but not so much with 300 BLK.

When building an upper, what part of the process affects the headspace or is it more a function of the barrel and whether it's in spec or not?

[tcoz]

It could also be that the magazine springs are worn. Are those relatively new Lancer mags, or have you had them for a while?

Also - you said that after no bang, you re-loaded the rounds. Did you do an inspection of the rounds at all? Any light primer strikes?

They're brand new mags. Today was the first time I've used them. I did look at them and the primer strikes looked just like they do after ejecting a chambered unfired round. Very light strike from firing pin during chambering.

[alamo5000]

Sounds like a head space issue.

[alamo5000]

But I'm still not ruling out gas system either.

[Ride4frnt]

Or the bolt isn't fully closing into the lugs of the barrel extension. Had that problem a few times.

[UWone77]

Sounds like a head space issue.

Guy puts one AR together and now he's troubleshooting others.

[tcoz]

Or the bolt isn't fully closing into the lugs of the barrel extension. Had that problem a few times.

I thought about that. How would you correct it or would it correct itself as you continue to use it? I'm a reloader and although I haven't started reloading 300 BLK yet, I have everything ready to start. I use a slotted Sheridan case gauge and I just dropped my remaining 20 Fiocchi rounds into it. What I found were two of those rounds that wouldn't fully drop into the gauge and were very tight. The head stuck out about 1/16". Could it be that these out of spec rounds are the problem?

[Ride4frnt]

I thought about that. How would you correct it or would it correct itself as you continue to use it? I'm a reloader and although I haven't started reloading 300 BLK yet, I have everything ready to start. I use a slotted Sheridan case gauge and I just dropped my remaining 20 Fiocchi rounds into it. What I found were two of those rounds that wouldn't fully drop into the gauge and were very tight. The head stuck out about 1/16". Could it be that these out of spec rounds are the problem?

Could be a tight chamber, out of spec ammo. Once the hammer drops you've gotta charge the weapon again, but if it happens again make sure the bolt is closed. If not, tap the forward assist to close it. I would say lube it up pretty good, and it should break in with time. I don't think you're having an issue that isn't easily resolved.

[alamo5000]

Guy puts one AR together and now he's troubleshooting others.

Don't forget the part about the 20+ year gunsmith teaching me things along the way...I ask a lot of questions.