AR acting up

So I've been working on this AR build since Christmas, and just recently got it finished. Today I took her out for her first day at the range to find I had on my hands a rather clumsy straight pull bolt rifle rather than the life and liberty gun I had envisioned. Needless to say, I was quite disappointed by this. I was wondering if any of you might have some insight as to what's going wrong. I checked that the gas block wasn't bleeding off gas by wrapping a kleenex around it and firing a few shots. It came off clean as it was when I put it on and undamaged. Also I know that my gun is getting at least some gas because it partially cycles. I was thinking it might be a buffer/buffer spring problem, but my familiarity with troubleshooting the AR platform is almost entirely theoretical, so I might be way off base. As always, any help will be greatly appreciated.



Full disclosure and because it may be relevant information, the gun in question is a 10.5 inch pistol, but I posted in rifle country because A) I plan on making it an sbr and B) I know more AR guys are likely to be trolling through here than in pistols.

 

Thats tough and very aggravating. My guess is...try cleaning (I understand it is new) then lube well and correctly. Can you verify the gas tube is clear? Make sure your buffer and spring are lubed and moving freely. .thats all I've got. Maybe better mechanics will chime in!

Mark

 

Provided that this new build is well lubed and clean, then:

Sounds like a gas system problem. So you must make sure you have the gas block over the gas port, especially if you're using a clamp on style or set screw style gas block. Very common for these to be a little out of spec and not line up. If you take the gas block off the gun, you should see a little ring of soot where the gas port dumps into the gas block. It should tell you if it lines up okay after your test firing.

The gas tube should be blemish free, no gas marks on the tube other than where it goes into the bolt carrier group. A small dimple may happen where you vent gas out of the gas tube. These happen because of corrosion or imperfections in manufacture.

The other thing to check is the gas key not he bolt carrier group. That must be tight and staked so the screws don't vibrate loose. (Thread locker will not hold up to that heat.)

The other thing to check is the bolt and the gas rings. Make sure they fit correctly. Extend the bolt and place the carrier/bolt face down on a table. It should hold up it's own weight, without collapsing.

Good luck and let us know how you make out.

 

Any pictures or video of what happens to the rifle exactly? If we new where in the sequence things were going wrong, maybe we could offer some more insight. It sounds like its failing to eject the fired case, is that correct?

 

Military spec is for the gas block to be spaced off the shoulder by the thickness of the handguard cap. If it's a clamp on then it slides back about .030 which covers the port. The gas block should be drilled slightly bigger but that's no guarantee. It seems a minor issue but that is a common source of short stroking on builds because it's a finesse point and somewhat difficult to measure or check.

Ammo is also an issue, test with full power NATO when possible, not white box/steel case which is a known lower power round.

Also check how the cam is unlocking in the upper - some combinations will have the corners cutting there way out of the recess the first hundred rounds due to a slight out of spec issue. Once broken in the gun then runs like a champ. Lube that area heavily (which is spec - runny wet) and see it it loosens up and tries to cycle.

 

Is the gas block pinned, set screw, or clamp on?

Here's the cheapest, easiest, most fool proof way to set your gas block position over the port for maximum flow:

• Find a buddy - you'll need 3 hands

• Pull the upper from the lower

• Loosen the gas block so it is free floating (hopefully yours isn't a pin-in-place type - still works, but is a pain in the butt to cut new pin grooves.

• Place an empty case with a fired primer in the chamber

• Blow into the muzzle end of the barrel while sliding the gas block fore and aft, and rotating it around the barrel - you'll be able to feel the resistance change as it moves over and off of the port.

• Holding the block in position where the resistance is the least, have your buddy tighten the gas block in place. (For a pin in place block, have them mark two witness marks on the block and barrel with a pen - then redrill the pin grooves in the barrel to hold the block in that position).

• Done, gas flow is maximized.

 

I've never encountered the need for such procedure, every gas block I've installed or looked at, the hole diameter in the block is at least 2X the port diameter in the barrel.

If you are worried about the hand-guard cap not being there, just put a shim of about 0.030" to make up for it. But I think most clamp on gas blocks are made to not be used with the retainer as there is no need for a low profile gas block if you want the retainer, usually these are used with a free float tube that extends past the gas block. The exception may be the gas blocks with built in rail sections, but these aren't low profile and I've only used one of these on a 9mm SBR so it wasn't really acting as a gas block, so I paid no attention to the alignment.

Bolt carrier or cam pin binding in the upper is a possibility, but there should be obvious signs of wear after some shooting and it shouldn't take long for the steel parts to carve out a clearance.

^^^ this

 

I checked the gas block for soot and didn't see any. From what I've seen I think it may be that the gas hole in the barrel is too small. I was using IMI Systems m193 ammo in it which I believe is full power, however I haven't used this before and I may be wrong. I will try again with ammo which I know works in the other AR, and if the problem persists, I will take a video of it and link to it here. Yesterday with around 20 rounds through it, every time but one the carrier would cam back and the bolt would unlock from the barrel, but the assembly stuck there. the one time this didn't happen the carrier traveled probably close to 2/3 the distance required to fully extract the cartridge case. there was also one light primer strike, but this only happened once and I am less worried about this.

 

It would be helpful to post a full list of the parts you used to build this gun. Did your barrel come from a reputable source that is known for making high quality SBRs that run well? Or Jimmy's Discount Barrel Bucket? SBRs have very short dwell times, so they can be finicky and sometimes require some trial and error to find the right combo of gas port size and buffer weight. It sounds like your port might be undersized, which is a better problem to have than being oversized. You can always make a hole bigger, can't make it smaller.

 

If your worried about the gas hole, and can get the block off, it looks like the size is supposes to be about .07, but you may want to research a bit. Go to a fastenal or similar, and get a number 3 drill bit, or 7/32, and see if it fits in the hole.

 

" checked the gas block for soot and didn't see any. From what I've seen I think it may be that the gas hole in the barrel is too small. I was using IMI Systems m193 ammo in it which I believe is full power, however I haven't used this before and I may be wrong. I will try again with ammo which I know works in the other AR, and if the problem persists, I will take a video of it and link to it here. Yesterday with around 20 rounds through it, every time but one the carrier would cam back and the bolt would unlock from the barrel, but the assembly stuck there. the one time this didn't happen the carrier traveled probably close to 2/3 the distance required to fully extract the cartridge case. there was also one light primer strike, but this only happened once and I am less worried about this"

The last thing I would do would be to change the diameter of the gas hole in the barrel. My guess is there is another issue. If you have not cleaned and lubed, lubed. lubed the rifle I would start there.

 

are you following this? similar issue, and getting good answers.
http://www.thehighroad.org/index.php?threads/ar-malfunction-please-help-diagnose.812101/

 

The rifle in that thread cycles and locks back on an empty mag. Entirely different issue.

Sammael, for an AR to function, the gas must flow through the gas port, into the gas block, down the gas tube to the gas key and then into the carrier where it must pressurize the expansion chamber to get the carrier moving. If the flow is blocked anywhere, or there is excessive leakage, the rifle will not function.

The most common cause of blockage is a gas block misaligned with the gas port. This can be due to a mislocated port or a misaligned gas block. Some gas blocks ate designed to be used with a hand guard cap. If the gas block is installed without the cap in place, locating the block against the barrel shoulder will restrict gas flow. If the gas block is designed to be used without the cap and a cap is used, the result is the same.

The most common cause of leakage is a loose gas key. Sometimes, you can try to wiggle the key, but it's still possible that the key is loose enough to leak but feel any movement when wiggled by hand. Instead, try tightening the gas key screws. If they turn, they were loose and the key is leaking. If this is the problem, it's best to remove the key, clean the making surface on the carrier, install a new key & screws and stake the carrier.

Worn or damaged gas rings will also cause leakage, although rarely to the point the rifle won't cycle at all. To test for worn gas rings, the manual tells you to remove the BCG, extend the bolt and hold the BCG vertically with the bolt end up. If the bolt slides back under its own weight, the rings are to be replaced.

Beyond that, check each part for blockage and the gas tube for holes

 

I had that same issue on my first build. The problem then was a hammer or trigger spring arm? was slightly out of align in the lower, I had a bolt action rifle until I moved the arm of the spring . That was awhile ago, so I don't remember the exact details. I will look and see if I can find the info

 

Since yours is a new build, it is unlikely you are having the problem I had. It was my first build and much to my surprise it functioned perfectly!

After about 1000 rounds, one day at the range I had similar problems. Once home, cleaning began and I noticed the castle nut on the buffer tube had worked a little loose.
Tightened it, problem solved!

When you say your build, did you use a complete barreled upper, or go from all separate barrel and so forth?

Besides that, wondering how well does the gas tube fit into the key? Is it possible the tube was crushed timing the barrel nut?

Could you state a little about the gun, barrel length, gas tube length (pistol, carbine, mid, rifle length, etc? Also which brand and length of barrel and make of gas block, and finally, what buffer, spring and buffer tube are used in the build?

Whoops, I see 10.5 barrel...how about the rest?
Russellc

 

This sounds like it's under gassed. One other gas related issue not mentioned above is the gas key. Make sure it's torqued down.

 

http://www.midwayusa.com/product/19...-1-in-7-twist-105-chrome-moly-melonite-coated

That link is to the barrel I used (10.5 inch melonite AR-Stoner). The bcg was ordered from Palmetto State on sale (not sure precisely which model, but I'm sure the problem isn't with the bcg). Buffer tube is a Phase 5 Hex-2, and I used the buffer and spring that came with it. Gas block and gas tube are from some chinaland outlet. Has block is low profile with two hex screws holding it to the barrel.

 

So we have a bunch of budget oriented parts making up the critical function components of the upper?

Not to be a downer here, but I would suggest you critically check every part of the operating system. If all of these parts were from top tier manufacturers I'm guessing the weapon would function just fine.

 

Update: So, having checked everything else, to no avail, I decided to try opening up the gas port. I discovered that the port was 0.069, so I broke out my set of machining drill bits, left over from my brief stint trying to learn metal fabrication in college, selected size 0.073 and set to work. I guess the bits were older than I had thought and become brittle with disuse. So long story short, I have an unknown length of 0.073 drill bit broken off inside a 0.069 hole, and a broken heart. So new problem. Should I try blasting it out by shooting a round through (the barrel is unobstructed), or do I just need a new barrel?

 

"Should I try blasting it out by shooting a round through (the barrel is unobstructed), or do I just need a new barrel?"

Don't do that...your eyes and fingers will thank you. You now have a single-shot AR; a new barrel is required and will be inexpensive insurance.

Let us know if the new barrel solves the problem...

 

Buy a new barrel preferably one that is of higher initial quality.

 

A good machine shop or gunsmith can probably remove it.
I've never seen an Ezout that small. If they do make them that small I would try that.
McMaster Carr is a good place to look for hard to find items.

 

This is way too easy to do, I doubt there was anything wrong with your drill bit. I use a pin vice on the drill bit and then do it by hand as if it was a reamer, one drill size step at a time using a generous amount of cutting oil.

If all else failed I might fire a shot through it, but *ONLY* if you can verify that none of the drill bit protrudes into the barrel -- this is harder than you might expect as you could have serious issues if it protruded into the groove but not enough to be seen above the lands.

So try this first, when it has happened to me, since the drill bits are hard and somewhat brittle, I used a 1/16" pin punch as if I was driving the broken bit into the barrel -- it fractured the bit into pieces that would fall out or could be pushed through, although you will booger up the end of the punch.

If you have to buy a new barrel, don't despair, just consider it tuition for the school of hard knocks