Need some help...

[JackOfAllTrades]

I am new to the AR platform.

I built up an AR over the winter and now that I have been able to get out to the range, there are some problems.

The symptoms I have (in the order I noticed them)

a) The bolt does not lock back on an empty magazine.
b) Sometimes the bolt does not pick up the next round (bolt closed on an empty chamber)
c) Steel cases only extract about 3/4 of the way (neck still in chamber, full stop)
d) sometimes I get the bolt ramming into the side of a round that is starting up out of the magazine putting a nice big dent in the side of the case.

a) and b) are clearly the bolt not moving far enough back.
c) might be not enough force on the bolt to extract the less slippery steel cases
d) I am not sure about, but it might be related to a) and b)

When I first test fired it (right before the big blizzard), it fired and ejected, but was effectively a single shot rifle... it wouldn't pick up the next round or lock back on empty. I lubed the crap out of it, and now it generally feeds multiple rounds but won't lock back. It also occasionally munches the side of a case it is feeding with it wedged between the feed lips, feed ramp, and bottom of the bolt carrier.

All of this leads me to believe that I am under gassed.

This is all on:
20 inch barrel
rifle length gas system
rifle buffer
rifle buffer spring
rifle buffer tube
M-16 style bolt carrier

Ideas?

 

[richc]

Any chance it is the ammo? Or the mag? Have you tried this ammo and mag in a different AR?

 

[Knicks]

Tulammo?

 

[griffinwerks]

All symptoms could be indicative of insufficient gas, Or excessive inertia, Or a bad chamber. Do you have a "heavy" buffer? Did you measure gas port size when building?

 

[xtry51]

It's not the ammo. Any AR should cycle any commercial ammo including steel.

Here are the things that can cause what you are seeing:

1) You didn't oil your gun and its drier than the Sahara. There is no such thing a an AR that is too wet. It's like saying someone it too rich.

2) you built the upper (or paid a bad gunsmith to) with a cheap barrel that has an undersized gas port

3) you built the upper and the gas block port is not properly aligned with the barrel port

4) you have a rifle length buffer spring in a carbine buffer tube.

5) You have the wrong weight buffer for your rifle setup. It is too heavy.

Basically either your buffer setup is wrong, or you're not getting enough gas to the BCG.

Other very weird and unlikely things:

You have an out of spec upper or BCG that is binding.

You have a really shitty charging handle that is bent and restricting BCG travel. I have personally seen this happen TWICE to other people's rifles and it's the reason I will only run BCM charging handles.

Your chamber headspace is incorrect, meaning your bolt is binding while locking/unlocking. THIS IS VERY DANGEROUS. Did you check the headspace (if you built the rifle)?

 

[JackOfAllTrades]

Federal brass cased 55grain FMJ for ammo. We will ignore the steel cased stuff for now.

Magpul PMAG 10 for the magazine.

I haven't tried the ammo or magazine in another AR yet.

I wasn't aware that there was a "heavy" buffer for rifle length systems. I thought it was just "rifle" unless it was a carbine, then you had multiple options.

I did not measure the gas port, but I can.

I do have a low profile set screw secured gas block, but I was careful in installing it and I don't think there is a misalignment. I adjusted for maximum flow before securing it.

I will dig in and measure the gas port tomorrow.

 

[SJan]

Who made the parts?

Who assembled the parts?

Crap parts properly assembled may not run.

Hi end parts poorly or improperly assembled absolutely will not run.

 

[JackOfAllTrades]

It's not the ammo. Any AR should cycle any commercial ammo including steel.

Here are the things that can cause what you are seeing:

1) You didn't oil your gun and its drier than the Sahara. There is no such thing a an AR that is too wet. It's like saying someone it too rich.

I did that already. I too fell for the "you can run them dry" line and it was a single shot rifle. Lubed it sloppy and it worked much better.

2) you built the upper (or paid a bad gunsmith to) with a cheap barrel that has an undersized gas port

I built the upper, and it is a cheap barrel. I should have checked the port size. Will do that.

3) you built the upper and the gas block port is not properly aligned with the barrel port

I did build the upper, but made sure I got maximum flow through the block before tightening it down.

4) you have a rifle length buffer spring in a carbine buffer tube.

It isn't that. I got matching rifle length buffer tube, spring, and buffer.

5) You have the wrong weight buffer for your rifle setup. It is too heavy.

Possible I guess, but I thought rifle length gas systems need rifle buffer assemblies.

Basically either your buffer setup is wrong, or you're not getting enough gas to the BCG.

Unless someone tells me that I have something wrong with a rifle length gas system, rifle length buffer tube, rifle buffer spring, and rifle buffer, it sounds like it is gas.

Other very weird and unlikely things:

You have an out of spec upper or BCG that is binding.

You have a really shitty charging handle that is bent and restricting BCG travel. I have personally seen this happen TWICE to other people's rifles and it's the reason I will only run BCM charging handles.

Your chamber headspace is incorrect, meaning your bolt is binding while locking/unlocking. THIS IS VERY DANGEROUS. Did you check the headspace (if you built the rifle)?

I don't feel any binding in the BCG or charging handle. I can gently close the bolt on an empty chamber and the buffer spring is sufficient to fully close and lock it. While assembling, I was able to tilt the upper at a 45 degree angle and it would slide home and lock without assistance.

I do admit that I have not checked the headspace. Again, I heard differing arguments about you do or don't have to do this on an AR.

The buffer spring is not sufficient to close the bolt on a round when eased in, but gentle pressure on the forward assist will close and lock the bolt. Letting the bolt slide home from anything more than about half way back will close the bolt and lock it every time.

I did inspect the brass after the first few shots. No signs of overpressure that I can see. No popped primers, No claw marks in the extractor groove or any other evidence of the bolt trying to open while under pressure.

I should really find a set of gauges... but I don't think that is the problem here.

I am going to measure the gas port to see what it is. I suspect that might be the problem.

 

[xtry51]

Ok, it's gas port or the buffer weight then. Don't be afraid to run a lighter buffer even if you bought a set. Some rifles just like light buffers. I think Supermoto was running a rifle with a CRAZY light buffer that I wouldn't even have considered possible at first.

The bolt won't normally close fully on a round if released slowly, so that's not unusual.

The headspace shouldn't be an issue if your parts are all in spec. Personally I've never seen a combination out of spec, but I know people here have.

 

[mac1911]

Where are you located. How does the mechanical check out just manually manipulating the controls.
1. Find someone with a functioning AR willing to switch around the uppers.
Should show it's face qickly if it's a upper or lower issue.

What stock are you using?
Is you gas key tight, straight. Are you sure you the gas tube is clear. You adjusted the gas block? Are you sure you adjusted it correctly?
Any chance it's over gassed and the buffer is rebounding faster than the mag can bring up a cartridge.?

 

[1903Collector]

Unless I missed it, what kind of stock is on the lower?

It's that or your low on gas, make sure there's no shit in the gas tube, Gas key not loose, gas block alignment good and clamped down.

 

[mac1911]

Go back and adjust your gas block correctly. The entire point of adjustable gas block is to use as little gas as possible to cycle the action with the load you intend to shoot.
Soaking a AR in lube will get you no where. A properly functioning AR should function with just the slightest film of oil on the correct places for many many rounds.

I adjusted my gas like this.
Adjusted to minimum then back off 2 turns. Fire one round loaded from magazine.
Bolt lock back? If it does I turn screw in till bolt won't hold open. Then 1/4 turns until it bolt stays locked back again. I don't switch ammo with this rifle.
Also depending on quality of the BCG it self it can be very tight inside the bolt bore and gas rings.
Friends Ptac bolt was very hard to remove the bolt from carrier.
Start over recheck everything...

 

[JackOfAllTrades]

Ok, I took my gas block off and had a look.

Based on the soot, the gas block was centered on the port in the barrel. It looks like there has also been some blow-by around the tube at the block. Not much, but I don't know how much is too much. All I saw was a trace of soot about 3/16 from the gas block. Since there are no actual seals on the tube at the gas block, I think this might be normal.

I used the shanks of numbered drill bits to gauge the size of the hole in the barrel.

The exterior hole is between 0.096 (no-go) and 0.093 (fits well).

I say exterior hole because the 0.093 inch hole only goes as deep as the barrel wall. I have to go the next size down to 0.089 to cross the bore.

So the port is larger the 0.089 but smaller the 0.093.

My guess is that they needed another couple of thousandths of an inch more depth in their drilling cycle. I bet the tip of the bit crossed into the bore, but not the full width. That gives you the right size on the outside and something smaller on the inside.

Everything I have read says a 20 inch barrel should be 0.093 for a port size. I guess I am under by a bit.

I am thinking of carefully completing the hole at 0.093 with a wood dowel in the bore to protect the other side.

After that, I put it back together and test.

It doesn't seem like 0.004 could make that much of a difference, but I guess at several thousand PSI, it makes enough.

 

[7.62x39]

I did that already. I too fell for the "you can run them dry" line and it was a single shot rifle. Lubed it sloppy and it worked much better.

I do admit that I have not checked the headspace. Again, I heard differing arguments about you do or don't have to do this on an AR.

You keep listening to the wrong crowd. Running an AR dry is so ridiculous I wont even go there. As far as headspace goes, Yes an AR will HS correctly most of the time, but not always, I've run into two that didn't.

Most of the time, is just not good enough. The HS should be checked on every single build or anytime a barrel or bolt is swapped out.

 

[mac1911]

I have all 3 gauges if your local to hanover MA we could check it during the week at my work?

 

[99105]

I had similar issues, with 2 causes
1 - lousy mag I picked up at a show. New magpul and it was happy
2 - gas tube issue. - see below
Apologies ahead of time for inaccurate component descriptions

The barrel nut was leaning against the gas tube, so when the bolt carrier group moved forward or back, the carrier key (part that the back end of the gas tube fits into) would bump and drag against the tube each way.

A way to check for this is:
- clear the weapon
- remove the lower half (2 pins) and set aside
- take out the BCG and charging handle
- take out the cotter pin and cam to remove the bolt (front cog type piece)
- I think you have to put the cam and pin back in as I recall
- without the bolt, slide the rest of the BCG back into the upper receiver
- make sure it goes all the way forward.
- now get ready to catch and tip the muzzle up. If there isn't a drag, the BCG should slide right out, hopefully not to the floor.
- if it catches on the gas tube, you are on your way to finding your problem

Good luck

 

[xtry51]

Go back and adjust your gas block correctly. The entire point of adjustable gas block is to use as little gas as possible to cycle the action with the load you intend to shoot.
Soaking a AR in lube will get you no where. A properly functioning AR should function with just the slightest film of oil on the correct places for many many rounds.

I adjusted my gas like this.
Adjusted to minimum then back off 2 turns. Fire one round loaded from magazine.
Bolt lock back? If it does I turn screw in till bolt won't hold open. Then 1/4 turns until it bolt stays locked back again. I don't switch ammo with this rifle.
Also depending on quality of the BCG it self it can be very tight inside the bolt bore and gas rings.
Friends Ptac bolt was very hard to remove the bolt from carrier.
Start over recheck everything...

That's great IF you only plan on ever running that one ammo type. For most people this isn't the case.

 

[mac1911]

That's great IF you only plan on ever running that one ammo type. For most people this isn't the case.

Right so what's the point of adjustable gas block if your just going to run it wide open.
Question: is it possible for the adj. block running wide open to over gas?
I also need to correct myself. Earlier post I asked about the M16 bolt. I mentioned it being lighter when they are heavier.
Possible the bolt gas seals are bad?

 

[xtry51]

Right so what's the point of adjustable gas block if your just going to run it wide open.
Question: is it possible for the adj. block running wide open to over gas?
I also need to correct myself. Earlier post I asked about the M16 bolt. I mentioned it being lighter when they are heavier.
Possible the bolt gas seals are bad?

Running a can that you don't keep on.

 

[mac1911]

Running a can that you don't keep on.

would that not be the same as changing ammo around ? You need to have less gas with a suppressed AR correct ? Another thing I been thinking about. The port on the barrel is only going allow X amount of gas through...so would there not be a level of adjustment on the gas block that would max out anyway or could you actually loose pressure having the gas block to far open ?

 

[griffinwerks]

.004 is an 8.5% reduction in area for the gas port, not insignificant. Barrels are drilled that way intentionally, slightly under(.089 or so) then to size, this delivers a greater accuracy to the finished hole as well as reducing the flash/burr pushed into the barrel. It seems they failed the second step. Finish the drilling and let us know.

 

[mac1911]

.004 is an 8.5% reduction in area for the gas port, not insignificant. Barrels are drilled that way intentionally, slightly under(.089 or so) then to size, this delivers a greater accuracy to the finished hole as well as reducing the flash/burr pushed into the barrel. It seems they failed the second step. Finish the drilling and let us know.

I have a question: considering the barrel port to be correct size: If you install adjustable gas block that has a "chamber" large enough before the gas enters the gas tube could this create a pressure drop or even a pressure "pulse" of some sort and cause issues ?
Just a thought.

 

[JackOfAllTrades]

Ok, some updates here...

I rechecked the gas port. I tried it at 0.093. No good. I opened it up to 0.096 (the upper limit of what Colt allegedly allows for a barrel 0.75 at the gas port). No good. Gas block alignment is fine. There are no obstructions or leaks in the gas tube or key. The bolt seals are staggered. Everything lubed and moving smoothly.

At this point, my brass cases are ejecting into a pile about 18-24 inches away from the rifle and it never picks up a second round or locks back. The brass is going out at about the 3:00 position after bouncing the case mouth off the deflector.

I figure my choices are to open up the gas port some more, clip some turns off the buffer spring, or take some mass out of the buffer.

Two of those choices are non-reversible... buffer it is.

I took the buffer apart and replaced two of the steel weights with plastic spacers. This brought the buffer down from 5.175 oz to 4.0 oz.

I just tried this out and my Federal .223 feeds perfectly and locks back on empty.

The steel cased MFS .223 Russian stuff ejects now, but won't feed a second round. I care less about this.

All my cases are now ending up about 6-8 feet away at about the 4:00 position and no longer have a ding on the case mouth from the deflector lump.

Unless I see evidence of the buffer bottoming out or something else that would worry me I guess I will keep running this way. I don't know if the steel case stuff will ever work, but now I can actually break in the rifle.

 

[xtry51]

Break it in with a few hundred rounds then try the steel again. If it won't cycle it still, personally I'd lighten the buffer further until it does. There is brass case ammo out there that is similar in load to the steel stuff, so it's doubtful your failure to feed will be limited to the steel if you start branching into other brass ammo.

I would absolutely NOT go bigger on the gas port hole. At .096 you're really limited by the gas tube now not the gas port size. That and opening it up too large can create duration issues where you're getting gas spike for too long which can lead to action problems. I've seen a gun not operate reliably because it had an out of spec oversized gas port.

 

[beaker]

If you narrowed it down to the buffer you can also try a light force buffer spring. Wolff sells them as well as others and they are nit expensive.

Sent from my Galaxy S4 using Tapatalk Pro - typos are from the GD auto correct unless they are funny substitutions those I'll take credit for.

 

[oilspill]

Count the coils on your spring, rifle spring should be 43 and about 12" long(forget the exact measurements something like between 11 1/2 and 13 1/4).