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tcoz
15 November 2014, 15:50
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
15 November 2014, 15:58
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
15 November 2014, 16:04
Gas system... buffer weight... ammo... could be numerous issues.

alamo5000
15 November 2014, 16:35
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
15 November 2014, 16:54
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
15 November 2014, 17:13
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
15 November 2014, 18:03
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
15 November 2014, 18:07
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
15 November 2014, 18:14
Sounds like a head space issue.

alamo5000
15 November 2014, 18:14
But I'm still not ruling out gas system either.

Ride4frnt
15 November 2014, 19:13
Or the bolt isn't fully closing into the lugs of the barrel extension. Had that problem a few times.

UWone77
15 November 2014, 19:18
Sounds like a head space issue.

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

tcoz
15 November 2014, 19:24
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
15 November 2014, 19:33
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
15 November 2014, 19:42
Guy puts one AR together and now he's troubleshooting others. [adore]


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

UWone77
15 November 2014, 19:44
Don't forget the part about the 20+ year gunsmith teaching me things along the way...I ask a lot of questions.

I'm still amused :o

Thompson
15 November 2014, 19:57
...I ask a lot of questions.
This is true haha

Ordnance
15 November 2014, 20:17
I would run a different brand of ammo first before jumping into swapping out parts. Also, who manufactured the barrel?

alamo5000
15 November 2014, 20:19
I'm still amused :o

Should I do my Good fellas Joe Pesci routine? LoL ;)

alamo5000
15 November 2014, 20:20
This is true haha

Yup. Its a blessing and a vice both at the same time.

tcoz
18 November 2014, 18:33
I would run a different brand of ammo first before jumping into swapping out parts. Also, who manufactured the barrel?

Barrel is MAS Defense 16" carbine gas.

UWone77
18 November 2014, 18:44
I assume you attempted to try different magazines?

tcoz
18 November 2014, 19:01
20 rd Lancers and 30 rd Pmags

UWone77
18 November 2014, 19:05
20 rd Lancers and 30 rd Pmags

And you were able to try out another BCG as well?

tcoz
18 November 2014, 19:28
At the present time I don't have access to another brand of BCG, but I was considering buying either a BCM or Fail Zero to dedicate to this upper rather than swapping my current PSA back and forth.

Because the problem only occurred 3 or 4 times out of 30 rounds fired, I was wondering if it's possible that the bolt didn't lock up those times. I've never heard of that occurring, but it seems possible since the bolt has fired at least a thousand rounds through my 5.56 upper. Maybe it needs to "wear in" a bit before it runs smoothly in the 300 BLK upper?

tcoz
23 November 2014, 14:28
I cant get back to the range for another week but I plan on being more prepared to diagnose the issue and document exactly what happens. I wound up buying a PSA Premium BCG to dedicate to the 300 BLK upper since I have a feeling that my problem might be due to my 5.56 bolt not always locking to the 300 barrel. Whether or not that's the cause I don't want sit at the range continually lubing and swapping bolts. I also picked up an H buffer since I'm currently running with a carbine buffer. I'll take a few different brands of mags and a few different types of ammo. Now all I have to do is figure out a protocol for testing everything without making it a confusing mess.

Former11B
23 November 2014, 14:54
Fiocchi makes quality ammo. It's always possible to get a bad batch but Fiocchi is not a crap brand.

tcoz
23 November 2014, 16:26
I know that Fiocchi isn't a crap brand otherwise I wouldn't have used it in a brand new upper. I don't know what the allowable load variation is for commercial ammo but if in fact the problem was caused by the bolt not locking up, is it possible that the few rounds with the lightest loads didn't have enough "steam" and that combined with the light carbine buffer prevented complete lock up? Then I'm not sure how or if that would relate to the bolt not locking back on either of the two mags I went through.

Set me straight if you think I'm barking up the wrong tree with this hypothesis. Although not new to the AR platform, I am brand new to 300 BLK and it seems like there are just so many more variables and potential causes of malfunctions.

Former11B
23 November 2014, 16:39
You mentioned it wasn't the best stuff available...I was just indicating it's certainly not the worst. No harm no foul


I have a feeling that my problem might be due to my 5.56 bolt not always locking to the 300 barrel. Whether or not that's the cause I don't want sit at the range continually lubing and swapping bolts

300BO is modified 5.56/.223 brass. It doesn't make a difference that its a 5.56 bolt because that's what is required. The first thing I'd do is swap a whole different BCG into it (simple matter of swapping them out...takes just a few seconds and can be prelubed) and see if the problem persisted ...do you have a buddy that can loan you one? If you were a hair closer I have a Colt F/A BCG you'd be welcome to (I'm near Columbia).



If it keeps happening, I'd check the headspace.

tcoz
23 November 2014, 17:24
Thanks for the offer, but I did buy a new BCG so the first thing i'll do is try it out without making any other changes and using the same mags and ammo as before. The point I was making about the 5.56 bolt that I swapped into the new upper is that it has about a thousand rounds on the clock and has conformed to the 5.56 barrel. If the tolerances are just slightly different in the new upper, maybe that could explain the few times that the bolt didn't lock up. If that's the case, it probably would just need to be run, but I decided that I really didn't want to be constantly swapping the bolt back and forth anyway so I bought one. I just wish I didn't live 45 minutes from the range so I could get this sorted out.

tcoz
6 December 2014, 07:10
Well, my big test day was a flop. It's a long story, but let's just say I never made it to the range. I'll get there eventually but in the meantime...

Earlier, I mentioned that three of the remaining rounds of Fiocchi ammo that I have wouldn't drop into the Sheridan gauge. In each instance, they stuck out at least the thickness of the case head. They couldn't be pushed down into the gauge either. If I attemp to chamber a round like this, wouldn't it keep the bolt lugs from locking? In that case, what would happen when I pulled the trigger? Would I get a "light click" as opposed to the normal sound when the hammer drops? I definitely remember the click on my rounds that didn't fire being noticeably lighter than normal.

tcoz
20 December 2014, 18:16
Update---Well, I finally made it back out to the range to see if the changes I made took care of the two issues. I put 70 rounds through it without a single malfunction and the bolt locked back every time. I used both Lancer mags and Gen 2 Pmags along with 147gr and 150gr FMJ ammo. The changes I made were removal of the Tactical Link battery assist device and the addition of a new PSA BCG. I also removed, smoothed and lubricated the bolt catch, hand cycled the new BCG at least 200 times and lubricated it very well. I took an H buffer with me just in case although I didn't think I'd need it and I didn't.

Thanks to everyone for your input. I got a lot of good ideas from your posts.