AR15 malfunction: adjusting the rifle to the load.

Thanks - a lot of Colt 11.5's are .0720 - which is REALLY big.
The Mk18 Gas port (10.3) is alledgedly .070, but one of our Engineers pulled a real one and it was a lot smaller.. which I guess means don't beleive all you read on the net.

I will see went I get back to Fl if I can get those numbers, I know our numbers for 14.5 and 11.5 and they are a lot smaller...
 
The DPMS 11.5" barrel that I have has a port size of .085". The data we have shows this to be in the middle of the range of port sizes used by most manufacturers. So while this may be large compared to the numbers Kevin is seeing, it is not the largest port size out there for this barrel length.
 
Just acquired Glen Zedicker's "The Competitive AR15". There is a section on gas port diameters. Interesting reading.
There is a wide observed range of gas port diameters. According to this source, spec. for the M4 is .062. He reports most custom builders use .085 - .100, Kreiger at .096, Martin at .110. These are in service rifle length barrels.
He reports rifle weight is also a factor - a heavy match rifle needs more gas.
In the few barrels I have drilled, I started at .062, and increased in stages until reliable function was obtained.
Barrel length is a factor. So is barrel diameter at the port.
 
.092/.093 is M16 standard, having played with lighter 20" barrels and heavier - it does not matter.

0.062 is the number I have heard quoted for the Mk18, but I don't want to guarantee it until I pin gauge one of ours first. That is a pretty big hole when using M855 or Mk262, Mk317, or 70gr Brown-Tip...
Especially when the gun is suppressed 90% of the time at least.
 
KevinB said:
.092/.093 is M16 standard....
Click to expand...

That's interesting, that's where I wound up by trial and error, opening the port in stages until reliable function was achieved.
I do believe that the AR gas system is self compensating to a degree.
 
I installed a Spike T-2 buffer. Advertised as 4.3oz, it's between the H and H2.
Today I shot 90 NATO rounds and had 3 failure to eject. As previously, empty case dive in the front of ejection port, cycling too fast. The problem is less frequent than with the standard carbine buffer. I'll install the black donut around the extra-power extractor spring to help.
We'll see next time!

The best part is that I got some MOA groups with EOTech and magnifier.
 
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Gas port sizing

I had an issue with a 7.5" AR barrel and when I went looking for the gas port sizes I found this

For 5.56mm
Colt's gas port sizes are:
* 10.5" - .093
* 11.5" - .081
* 14.5" - .063
* 16" - .063

I just drilled the gas port out the the next largest size in my standard Canadain Tire drill index which turned out to be 5/64" and it fixed the cycling problem right away.

The gun runs great now
 
unfortunatly i canot help here...
I just wanted to thank everybody that contributed to this thread. It is one of the most informative thread on this forum (my own post excluded:redface:) , and might prove very usefull to me in the near future as I am currently building my first ar and whish to go with a 11.5" Questar barrrel.

so tagged for interest, keep posting guys, I won't... but I'll keep reading.
 
Been some excellent info here and has inspired me to play with my new Norc M4.

I have found the functioning of the rifle excellent with zero issues but was very surprised that cases were screaming hot after ejection. Hot enough to melt into my plastic tarp I was using to catch the cases.

They also show quite a bit of smoking on the case.

Recoil was also alot harder then I was expecting.

the gas port in the barrel is around 1/16" (no pin guages) so not out of the ordinary. however, the YHM gas block has a very large entrance port. A 1/8" drill will not fit but the next smaller drill bit has tons of room.

Since this is a split gas block, shimming the gas port was no problem using steel shim stock. I cut and put in a piece of 5 thou shim stock to cover part of the gas port (1/2 diameter). The shim just reaches the gas port hole in the barrel.

gasshim1.jpg


gasshim2.jpg


Recoil was immediately tamed, cases aren't very sooty at all and temp has dropped way down (don't burn your hands). I can only assume that the orig set up was overgassing the rifle and pulling the cases out while there was still alot of burning powder in the barrel.

Functioning continues to be 100% over 100rds.

with this success, I have now made another shim with a hole that will be smaller then the hole in the barrel. If I have lined up the holes, it should soften the functioning somemore - maybe too much????

I also expect the exhaust gases to open the hole in the shim a bit.

the goal is to let the action function properly but eliminate the buffer slamming into the back of the stock tube and reducing recoil somemore.

depending on what happens next time out, I may add a heavier buffer or add lead to the orig, softer buffer tip and heavier spring.

I am reloading for this rig and have an accurate load which is at moderate pressures.

Tuning the gas system to this load should yield a nice soft shooting rifle.

Jerry
 
A few hundred rounds will eat the steel shim away. I'm not a metallurgist, but a high heat resitant alloy will be your best bet.

Most gas blocks have a .125 hole in them, generous, but the pressure and flow is pretty much controlled by the port in the barrel.

However most MG's and the FN SCAR have large holes in the barrel, and smaller ports in the gas block, with the gas block being made of a higher temp alloy than the barrel.


WildPhil - I would bet your issue is extractor/ejector related. Your ejector is just not getting the round out in time. On a recent 5.56mm program we did for a government entity I had the 'luxury' of sitting thru hundreds of highspeed videos of weapons being fired. A lazy extractor - either due to spring pressure or extractor design was the culrpit of most of the 'border line' extractions, regardless of bolt carrier velocity.
Sometimes brass shavings find there way into the ejector, so a good way is to exercise it, by taking a few drops of clp on it, and rocking a casing back and forth from the 'locked' position to unlocked and letting the clp work out any crap.
Once that is done, if your still having issues, and you have the black insert and the O ring, then change extractors, as yours may have a chip or incorrect geometry to flip the casing out.
I've seen guns run at over 1200 rpm (piston guns) that do not 'overspeed' ejection or the magazine spring, so its not a carrier velocity issue, its a parts issue with your gun.
 
From all this, it seems that the adjustable gas block would actually be the best 'fix' for this. Then you could just adjust to a setting that you deemed optimum, and if the need ever arose for a little more/little less boost, you adjust it as necessary.
A buffer is cheaper than an adjustable gas block (unless you're handy with a drill and tap and know exactly how and where to put the hole), however, you get way more adjustment with the gas block.
 
Thank you Kevin.
The heavier buffer got my FTE from 30 to 3%. Now I have to try with the black donut added. If it don't solve the 3%, I'll follow your instructions.

In all your testing, you didn't use stronger ejector spring?
 
Kevin, yes, I see this being a short term fix but at least it can narrow down where I need to put my attention. the new shim that I have has a hole in the middle of the part so maybe it will last ....maybe not????

I think an adjustable gas block is a wonderful idea and was expecting to see it more prevalent in the aftermarket but there seems to only be the JP stuff.

something like what is on the XCR - notched/indexed adjustment nut - seems like such a good idea. Dial it down till the ammo doesn't feed properly then back out a notch and have fun.

I see that the orig gas blocks do not leave any material above the hole to tap but that is easily solved in the aftermarket blocks as JP has done.

The hole in the barrel should limit the gas flow but the port is obviously too big in my case and the even larger hole in the gas block helps the problem along. I am using my reloads so can play around to suit. My reloads are a moderate light load and the rifle is still overgassing. I have a load that uses sig more powder. Runs just fine as well but you sure know when you pull the trigger.

With the millions in use over the decades, I am sure all this has been designed with good reason but I am surprised at how overgassed my rifle is.

acrashb, I am looking into a heavier buffer and a heavier recoil spring. I want this rifle to shoot like a 10/22 when I am done. A muzzle brake would be the last evil addition and only if everything else doesn't do what I want.

It's already pretty noisy.

My suspicion is that reducing the gas flow from the barrel is all that I will need to get this rifle to slow down and kick less. Accuracy has gone up too but could be more about it bucking less making for better follow through????

I am pleased with the overal function of this Norc M4. It simply has been a plug and play toy and keeps on running - dirty and with min lube. Will update on how this new shim works or doesn't.

Jerry
 
We have dual extractor spring in our 5.56mm guns, so our guns are different, however we did a lot of work with others guns too to ensure we where better.

The spring tension is not really the issue with the insert added. It's a rocket wire spring, so it has a very finite life and without the O ring really takes a beating.



On adjustable gas ports -- yes they seem like the perfect solution. However the adjustment is easier said that done. Pistons guns with a gas plug are much easier to work this on, as it vents the excess gas after rearward movement of the piston has started. Pistons in this manner are much more efficient, and easier to setup for adjustable gas system -- look at the C1A1 - two position gas plug, and an adjustable Gas Regulator (from 0-12 IIRC, its been a long time). The Gas Plug was for Grenade Launcher or Not, and I cannot count the number of times people reversed it in the dark -- nice idea, poor implementation, as you then had to turn it around, and make sure it did not launch into space. The Gas Regulator allowed for venting after the piston started to move so you could dial it so just enough gas was used.

Direct Impingment systems operate differently, as gas travels down the tube, and presures the bolt against first the bolt face and then pressure forces the bolt carrier back and the cam pin unlocks the bolt from the carrier has travelling reward and pushing the cam pin into the cam in the bolt carrier.
Its not just as simple as smashing an op rod back into the carrier.

I have yet to see a well designed adjustable gas port for a DI gun, we went down this road several years ago on the early Battle Rifles (02/03 time frame) as that gas system was not optimized to run both suppressed and unsuppressed without adjustments.
The issue is how to adjust it easily and identifiably, and secure it so it is where you want it. Also moving parts in the block do not do well with sealing the system, so some tend to leak like a sieve.
I have my own design for an adjustable DI gas system, but not sharing it right now ;)

So what are the other options:
I know some entities have ports sized to a minimum, and one of the best open source examples, is Grant Timblerlake from G&R Tactical has a 10.5" gun that has a very small port, it is very clean even suppressed, although it does not lock back when unsuppressed. The idea is to both have a clean running gun, with minimal recoil.

Thought with lower power ammo the gun may not function at all.
 
KevinB said:
So what are the other options:
I know some entities have ports sized to a minimum, and one of the best open source examples, is Grant Timblerlake from G&R Tactical has a 10.5" gun that has a very small port, it is very clean even suppressed, although it does not lock back when unsuppressed. The idea is to both have a clean running gun, with minimal recoil.

Thought with lower power ammo the gun may not function at all.
Click to expand...


I know you dont like them but this is where piston ARs shine and really show their worth.
 
Not really -- more recoil that a compareable DI with the same carrier velocity.

We have piston M16FOW 5.56mm guns for government sale - but I don't like the them, why make that platform in a piston, if you want a piston use a different platform.
 
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