AR-15 Gas key screws sheared

[Gardner]

We went to the range today and one of the rifles we took was an AR-15. I assembled this particular AR in 2012.

We had shot several rifles, and I bring the AR to the table, load a mag, insert mag and pull back on the charging handle to chamber the first round. The CH came to the rear an inch or so and stopped. I saw the ejection port door didn’t open so my first thought was the EP door was jammed. No amount of tugging/jerking could move the BCG more than an inch or so.

I finally decided to separate upper and lower. When I did something fell out of the upper. At first I didn’t recognize it but then it dawned on me…...that’s the gas key!!

As you will see in the photos, both screws sheared flush with the BC.

That’s certainly a first for me. But not really, this same BC sheared the front screw when I first bought it. It took forever for me to figure out where the gas leak was.

What’s odd is that before every range trip (the afternoon before) I remove the bolt and check for lube, whether it’s a bolt gun or semi, run a dry patch down the bore and do an overall look see for safety.

The gas key was attached to the BC when I assembled it and put in its case.

To be clear I did NOT fire a single round today before I found this failure.

Luckily I had a spare BCG in the tool box.

Now I have to find a smith to remove the broken screw stubs, I don’ have to tools for the job.

 

[Mauser fan]

It isn't a difficult task to remove the broken screws from the bolt carrier. There are a couple of ways to skin that cat. Remove the firing pin from the bolt carrier and soak the carrier in some oil is the first step. Then read this link from start to finish. Scope Base Screw Stripped While it isn't the exact same issue it does cover screw extraction in great detail.

Jeremy

 

[Blue68f100]

It's been a while since I've bought a BCG. But it may me around the same cost as removing the sheared screws it they are really stuck.

Soak it in Kroil for a couple of days and use a pick and see if they are loose. You may get lucky and they will spin out with just a pick.

 

[Gardner]

I have spares, but I hate to trash a BCG that cost me $175 way back then.

I think I should leave this to the professionals who have the tools and experience

I jusr wanted to share

 

[a5werkes]

I jusr wanted to share

Thanks for sharing.

 

[Kp321]

If the BCG has a history of shearing gas key bolts, something is out of spec. I would just scrap the old carrier, broken screws and all.

 

[derek45]

If the BCG has a history of shearing gas key bolts, something is out of spec. I would just scrap the old carrier, broken screws and all.

this

 

[Gardner]

Good point

 

[Armorer 101]

All it is is sheared carrier key bolts, changed a bunch out. Just drill center and use a left hand bit or easy out to take the carrier part out. The stakes in the bolt head should be drilled to remove and the heads should fall out.Then replace the sheared bolts with hardened carrier key bolts and stake them, no thread locker.

If you do not have a mill, suggest taking it to a machine shop.

 

[powermad]

Does the lower look like it has been getting smacked with the gas key?
Happening twice is not normal.

With it separated take the carrier, put it in the tube and compress it firmly.
The gap between the lower and gas key should be around two Quarters wide.
If not put one of the Quarters at the end of the tube then make sure the bolt hold open functions.

 

[DoubleMag]

My guess, unless you want to tinker, contact YM on a warranty claim?

 

[a5werkes]

Does the lower look like it has been getting smacked with the gas key?

I bet it does.

I have a gauge to specifically check that gas key - lower receiver ring gap tolerances. A fancy two quarter test.
There are a number of parts where tolerance stacking of good parts will result in carrier key impacting the lower ring as well.
Here are a few that can be checked. I fabricated a aluminum rod rule to quickly check buffers & spring lengths.

Gas Key-Lower Receiver Gap is a result of stacking of these measurements when the carrier is
fully to rear on the lower in its stroke length

1. Carrier OAL > 6.66”
Overall length determines how much the buffer is displaced rearward from its retainer pin when the assembly is in battery. The front edge of the carrier is the impact point with the barrel extension upon going into battery.
It is undesirable for the face of buffer to lose contact with the rear of the carrier.

If carrier is too short it may result in buffer face impacting the buffer retainer pin i.e. buffer face is separated from rear surface of the carrier as the carrier goes into full battery; alternatively, the buffer retainer pin mortise may have been located (drilled) to far rearward, with the same impacting result

2. Receiver extension length; carbine = 7.125 ” v. “medium” = 7.9375” v rifle = 9.625 ”

OAL mil.spec 2.5” differential carbine v. rifle

3. Buffer OAL; carbine = 3.250 ” v. rifle = 5.92” 2.67” differential carbine v. rifle

Gas Key-Lower Receiver Ring Min Gap Go @ 0.1350” No-Go@ 0.125”

U.S. quarter thickness ~0.0650” x 2 = 0.130”

When minimum and maximum allowed part tolerances are combined….

Minimum= 0.125” v. Maximum=0.250” gap between carrier key-lower receiver


Check for carrier-gas key travel gap 0.135” or better; the No-Go (<0.125”) it is possible in some cases depending on ammunition type / suppressor mounted that higher back pressure could cause the buffer tip to collapse and cause malfunctions.

1. Carrier is too short, <6.66” OAL

2. Buffer is too short; carbine = < 3.250” ; rifle= < 5.920“ OAL

 

[FROGO207]

When doing this kind of work investing in standards makes a lot of sense. Not usually worth the cost for a one off build though. Hope the OP figures out the problem quickly.

Having just recently gotten into the AR building mode all this is fascinating to me.
Kinda like big boys tinker toy sets.