Failure to feed and erratic ejection. With Pics

[rabiddawg]

I decided to start a new thread since I figured out the cause of the accuracy problem with this gun. Lets just say that level cross hairs are a good thing.

Here are pics of what my gun is doing:
[IMG][/IMG]
[IMG][/IMG]


Here is what i know about the gun:

Alex Arms upper and lower 16" barrel. AA built gun.
10.75" buffer spring
2.95 oz buffer or 83.7 grams. Weighed it on a Sartorius paint mixing scale
Hornady 123 sst. Bought from Selway Armory
Bolt has Never failed to lock back after the last round

What i have done to this point:

Tried different mags to no avail.
Radius the ejector and trimmed 1 coil off the spring.
Smoothed the extractor corners to eliminate the "claws" on my brass
Checked the gas key. It is tight and shows no signs of leakage
Looked at gas tube for uneven wear. Found none. Just a nice bevel around the sealing edge
Cleaned it, lubed it. Sprayed brake cleaner down the gas tube to verify the port was clear.

Now, what it is doing. My research and brain picking has brought me to the conclusion that my gun is displaying what is referred to as "bolt over base" where the fired case is ejected but the next round is not fed properly and gets jammed by the lugs on the bolt leaving two dents near the shoulder. I can manually duplicate it by not pulling the bolt completely back and letting it slowly ease forward. Basically the bolt head is too far ahead of the next round to properly chamber it.

What else can I do to fix this?

Thanks!

[LRRPF52]

Bolt over base is usually not enough gas.

Is this a MLGS? Pull the gas block and look at the gas port.

Trimming the ejector spring only serves to reduce the throwing capacity of the ejector.

Your malf is happening because the bolt carrier face is catching the head of the cases, after the bolt misses it.

As the bolt carrier drives the cartridge up the feed ramps, the bolt smashes into the side of the case wall, causing the damage you see.

[rabiddawg]

From my measurement (gas block to receiver =9.5") i say it is a midlength system.

This gun ran fine for 150ish rounds tgen started this late last season. The erratic part is it throws one case out at 1oclock and the next at 3. Did this for the four rounds i watched my son fire last sunday. It didnt fail to feed while i watched; it only does that when we shoot deer with it.

Does the erratic ejection tell us anything?

[bwaites]

The other thing that can cause it is an out of tolerance mag/latch interaction where the mag is sitting a little low.

[Sniper338]

Ive never seen an AR that throws brass in the same spot everytime... its always scattered a bit.. you can buy a brass catcher pouch to put in it if thats a big deal...

[rabiddawg]

Sniper, its not a problem for me. I mention it incase it is a symptom of the problem.

[SG4247]

In my mind, this is just short stroking.

Carrier comes back just far enough to catch the round on the face of the carrier. If it would traverse back an additional 3/4", the bolt face would catch the round like it should and feed it up into the chamber. This is what LR is saying.

So, it's right on the edge of having enough gas pressure to fully stroke the carrier and the random ejection pattern supports this theory.

Possible causes of under gassed AR's are:

Leaking old dirty carbon sooted up rings on the bolt.
Loose or leaking gas key.
Wear or damage to the gas key tube inlet port. (I had one out of round once)
Carbon build up or damage in the sealing ring area of the carrier bore.
Wear or damage to the gas tube nipple.
Leaking, plugged up or mis aligned gas block.
Too small or plugged up gas port in the barrel. Often this is a burr or copper build up from bullet jackets, that block the hole. This is easily cleared by removing the gas block and clearing the hole with a proper size drill bit by hand.

Hope this helps.

[rabiddawg]

The other thing that can cause it is an out of tolerance mag/latch interaction where the mag is sitting a little low.

Thanks, i will check that out. May just get a new one to be sure.

Btw, that barrel i bought from you has been at PF since mid november!

[rabiddawg]

SG4247, thanks for all the suggestions. I guess i may as well order me some tools. Removing the gb will be a good time to add a free float hg.

I will report back asap.

[bwaites]

Thanks, i will check that out. May just get a new one to be sure.

Btw, that barrel i bought from you has been at PF since mid november!

for?

[hbp]

Are you resting the rifle on the mag. while firing? This could be causing enough of misalignment or carrier bind or bump to prevent the next round from feeding.

[cst]

Couple things I would do..get a brand new extractor....I had the same problem from a bad batch of extractors where there was insufficient grab..the brass prematurely released and screwed up the pick up of the next round.
I would also check different mags..if the feed lip angle is off at the front..it could also cause the round to pop up our of the mag too early.

Remove the ejector and make sure it's not binding..polish and lube it.


If it come down to being a gas issue..asuming it's got the correct sized hole drilled then u Need to check that gas block installed, buffer etc... hope it works out

[rabiddawg]

for?

Dude im sorry. Mark is building me a rifle. I decided my first attempt at building one shouldnt be with a tube that nice.

[rabiddawg]

Are you resting the rifle on the mag. while firing? This could be causing enough of misalignment or carrier bind or bump to prevent the next round from feeding.

No, been using the 5&10 rd mags and they dont touch the bench when shooting at the camp. Iirc the only times it has done this is when we have shot it out of a box stand with the hg resting on the window frame.

[bwaites]

Dude im sorry. Mark is building me a rifle. I decided my first attempt at building one shouldnt be with a tube that nice.

Scared me there!