Failure to eject

[airplanejoe]

Once again I need to pick some brains regarding an anomaly with an AR15 upper I'm putting together. I'm using the lower from a Colt 6920. The upper has a Criterion HBAR 18 inches. Rifle length gas tube.

Took it to the range and at first I was very happy. I attempted to do my version of barrel brake-in. With a sled I fired 10 rounds (fire/clean/fire/…). Everything was normal. Rifle would fire and the bolt would lock back; spent case would eject normally. Accuracy was better than I expected.

I then tried magazines loaded with 5 rounds each. On average one round per mag (sometimes more; sometime less) would fail to eject. It would extract, but then it failed to leave the receiver. The bolt picked up another round from the mag but it would not chamber because of a partial blockage caused by the brass that had failed to extract.

I started researching the problem and the sources I looked seemed split between over gassing and under gassing. A couple of folks even mentioned short-stroking.
I verified that the buffer was an "H" buffer (colt) and changed the buffer spring  (Wolff) to one I think would be stiffer (12.5 inches; 41 coils) (Possible cure for an over gassing condition). I also overhauled the bolt (extractor and ejector). Did not change the rings.

I think that if I replaced the collapsible stock for an A1 or A2 stock with a rifle buffer and a rifle length spring the problem would go away, but because of my height I would like to keep the collapsible stock; but that option remains on the table. I would take the upper from my Colt match/target rifle and see how the combination  behaves.

I'm planning to head back to the range tomorrow to try it.

I really would appreciate suggestions.

aj

[ItsanSKS]

Which ammo are you using?

Especially with a brand new AR-15, I exclusively use M195 or M855 ammo.  (assuming the barrel is rated for 5.56x45)

I've found that new rifles, with their closer tolerances, tend to malfunction exactly as you've described, with .223 spec ammo.  After about 500 rounds of 5.56 ammo, they'll run anything. 

[d2331]

I'm not sure if this will help, I don't remember where I got it from or if it's actually true but I'll throw it out just in case

[Kimber Custom]

Check the gas block to ensure it hasn't moved. My first AR turned into a single shot because of a loose gas block.

[cwcdl79]

I may be waaaay off base, but if you're using a rifle length gas system, shouldn't you 've using a rifle buffer? I had the exact opposite before, as soon as I switched buffers, no more problem.

[airplanejoe]

cwcdl79, Rifle buffers will not fit in a carbine tube (too long). Carbine buffers will fit In  rifle tubes, but you are getting into other problems like possible gas key damage.

aj

[airplanejoe]

I just returned from the range. The good news is that the rifle worked fine. The bad news is that I don't know why.

Here is what I did:
1. The Wolff buffer spring didn't work. I suspect it's too long for the carbine buffer tube.
2. The barrel in question is a .223 rem. Today I fired. 40 rounds; PMC ammo, 55 gr FMJ. No problems whatsoever. I still need to try the other ammo I normally use (Privy Partizan, 69 gr HPBT, match ammo). Also my handloads.
3. Initially I wanted to try a Wolff buffer spring; 12.5 inches with 41 coils. (OEM buffer spring stats are 10.5 inches with 37-39 coils) Didn't work. Even manually the rifle would short stroke. Replaced it with a Colt buffer spring 10.5 inches with 35 coils. That worked.
4. Overhauled the bolt; both ejector and extractor. New springs plus added a small "O" ring to the ejector to make it stiffer; did not change the rings (I didn't have a spare set, otherwise I would have).
5. Brass would eject between 4 and 5 o'clock. met the projection of the diagram d2331 posted.
6. Will update the performance with different kinds of ammo.

I think I now have a reliable rifle.

aj

[navybowhunter]

Magazines?

I've had issues with Magpul mags.  I only use GI mags now.

Just throwing it out for consideration.  I have had this thing called "bolt over round" or something like that....in Rapid fire with 2/8 in competition.  Several NRA competitors have had this issue.

PMAGS suck! LOL....one of my coaches suggests using them only to smoke the front sight.

FWIW

[airplanejoe]

99% certain mags were not guilty. When trying things out I take 7 mags with me and use them all. All steel.

aj

[Nashville Stage]

Hrm... My hunch would be that the added friction of a round in the magazine pressing up against the underside of the bolt carrier is what's slowing down the bolt enough to cause ejection failures. That would seem to fit the pattern of "works fine as a single shot, but not with a mag in place" (other than the mag itself causing issues, but you seem to have eliminated that as a possibility).

I would guess that the ratio of gas supply to the resistance of the bolt/buffer/spring, etc. isn't high enough. I doubt that it's an over-gassing situation, as the extra resistance of rounds in the magazine would make the situation better, not worse.

As has been mentioned, you could check the gas port to make sure it's not blocked or misaligned. If it's an adjustable port, you could also try giving it more gas to see if that fixes the problem.

The other side of the equation is to reduce the amount of energy needed to move the bolt & buffer (as you've already started tweaking). You might try swapping out with a non-heavy buffer and/or a lighter spring.

Something else that's niggling at me... having a rifle-length gas tube with an 18" barrel might not be providing the bolt with a good enough gas pulse. If the amount of time between when the bullet passes the gas port and when it leaves the barrel is too short, the gas pulse might be too short. This, combined with a port opening that perhaps isn't working 100% and a tight-fitting new bolt carrier might be tipping the scales too far. Of course, changing the length of the barrel or gas system might be more work than you want to get into, so you could experiment with the other variables first.

I'm curious to hear more after you've tested it again.

[ItsanSKS]

Do you have any way of measuring your gas port diameter?  It would be interesting to see what it mic'd out at.

For reference, see this list (which doesn't include an 18" bbl with rifle length gas):
http://www.tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html

[Vinnie]

My guess is that adding the small "O" ring around the extractor spring is what fixed your problem.  Sounds like the extractor was coming off the spent casing a tad too early, but now with the o-ring in there you have a tighter grab on the casing, not slipping off early.

Just for grins, take out the o-ring and see if your problem returns.... I bet it would.



Vinnie

[airplanejoe]

I'll begin putting together another upper the week after next. This time with a 20" barrel. I'll measure the gas port diameter, but I'm thinking, the gas block port diameter will be roughly the same. Thinking aloud, if you have two parts where you align holes to create a path, wouldn't the smaller hole rule the results?
aj

[ItsanSKS]

if you have two parts where you align holes to create a path, wouldn't the smaller hole rule the results?

Yes.  This is why I linked the port diameters- upon review, you'll notice that the 16" barrel 5.56 with carbine gas system has a port diameter (as drilled in the barrel) of 0.065-0.070";  the 20" barrel 5.56 with rifle gas system has a port diameter of .0935-0.0980".

With the malfunctions that you are experiencing, it could potentially be a too-small gas port drilled into the barrel.  Based solely upon the numbers, I would expect that your gas port should be somewhere in the mid-to-high 0.08" range; that said, notice that the gas ports for the short barreled rifles (11 and 14.5") are in the mid-to-high 0.08" range.  There must be some kind of rhyme or reason to serve as a guide for what size gas port is optimal for your rifle, but an 18" barrel is just too uncommon to have this kind of data readily available.  So, we experiment, and document the results.   

[Nashville Stage]

...notice that the gas ports for the short barreled rifles (11 and 14.5") are in the mid-to-high 0.08" range.

That's interesting. Any idea why the gas port diameter doesn't continue to get progressively smaller as it moves closer to the chamber?

[airplanejoe]

There must be some kind of rhyme or reason to serve as a guide for what size gas port is optimal for your rifle, but an 18" barrel is just too uncommon to have this kind of data readily available.  So, we experiment, and document the results.   

Agree ItsanSKS. I have no solid reason for the results I'm getting. The only thing I can say is that, as it is now, works. I'll keep experimenting in hope that I can eliminate what we used to call in the service "PFM" (pure f#####g magic).

aj

[airplanejoe]

I now think that the gas port on the barrel is probably on the large side. Why different length barrels have different sizes (look at the chart that ItsanSKS posted)? I don't have a clue. The other factor in the equation is the gas block. Is the gas port on all the gas blocks the same? Again, I don't have a clue, but I doubt it. So what can be done about it? There are adjustable gas blocks on the market; with one you can adjust the volume/pressure of the gas to suit your specific set-up.

The other path you can take is what I'm in the process of doing. Play with bolt carrier mass, buffer weight and buffer springs. Tune your particular weapon.

D2331 posted a graphic that portrayed under/correct/over gassing as a result or where ejected rounds were falling. I've seen that graphic before. I don't know whether its accurate information or not. But I guess it's better than nothing.

Some of the choices out there are:

From www.tactical springs.com you can get the following springs (color coded): 1. White (std power), 2. Blue (enhanced power), 3. Red (extra power).

From www.spiketactical.com you can get the following buffer weights: 1. ST-T1 (overall weight 3.0 oz; reciprocal weight 1.9 oz), ST-T2 (overall weight 3.7 oz; reciprocal weight 2.8 oz), ST-T3(overall weight 4.6 oz; reciprocal weight 3.5 oz).

From www.jprifles.com you can get the bolt carrier.   

There are other vendors out there (let your fingers do the walking).

In my particular case I began with a wolff buffer spring and a light buffer. That didn't work (physical dimensions).

Swapped to a colt carbine stock spring and an "H" buffer (colt).

At the same time rebuilt the bolt and added an "O" ring to the extractor. Vinnie posted a picture of what it looks like.

Those two things solved the puzzle. Working rifle. Which one? No clue, maybe both, maybe one of the two. I now know that I should have done the fixes one at a time. Next time I'll do that, in the meanwhile I'll follow the advise "don't fix it if it's not broken". By the way, ejected rounds fall at about the 4 o'clock position (where the chart suggests they should fall).

The recommendation I've seen regarding parts is that you should use the heaviest buffer/spring combination that will handle the lowest power ammo you are using (disclaimer).

[ItsanSKS]

That's interesting. Any idea why the gas port diameter doesn't continue to get progressively smaller as it moves closer to the chamber?

I'll hazard a guess, but that's all it is. 

The port diameter is based on the amount/pressure of the gas behind the bullet- in a 20" rifle, the gas has expanded to its maximum extent, and the pressure as measured at the port, is on the downhill side of its cycle- so you need a larger port size to allow as much of the gas into the gas system as possible. 

As you move to the 16" barreled rifles, you get higher port pressure and volume, but a shorter dwell time (the time when the bullet has passed the port, but has not exited the barrel).  This requires a smaller port diameter to increase the pressures seen by the bolt carrier. 

When you move to the 11 & 14.5" barrels, you have an even shorter dwell time, but you are also in a position where the gas is still expanding- hence the larger port diameters, effectively allowing the gas system to make better use of the rapidly expanding gasses prior to their dissipation.   

Again, these are only my guesses, based on my limited understanding of the gas system on these rifles, and how pressure curves work. 

In airplanjoe's case, (an 18" barrel with rifle length gas system) the dwell time is shorter than any of the above; (I think- I'm not sure what the dwell time on an 11" barrel would be?) I would therefore expect that the port size should also be on the small size, to increase the pressure exerted on the bolt carrier.  If I had to guess, his port has probably been drilled to the 'standard' (for a 20" rifle) of .09+" size.  If true, I would expect a short duration, relatively weak push on the bolt carrier- hence his ability to make the gun run 'correctly' with a lighter weight bolt buffer &/or weaker buffer spring.  I'd also expect that running 'hotter' rounds would allow the rifle to function 'correctly' too.