I have a full auto AR-15, it fired fine till i got a new upper for it. I then switched back to the original upper and it still wouldnt fire auto anymore. It fires semi just fine then i switch it to auto it fires the first round then love taps the primer of the second round. Is it the hammer spring or the sear release? Any advise would help.

to the Armory
Sounds like the Hammer is not hitting it hard enough, so I would check the spring and replace it. Next time you try it after the first shot slap the forward assist and see if it chambers the round completely.

It fully chambers and locks the round, I dont understand how it can fire semi fine but fail to ignite the primer on the second round on auto. Will check it out next time i go out.

That is odd, also look at your selector and make sure there is nothing catching the sear. Look for wear marks on the selector and sear.

All the pieces are fine a little dirty, could it be the timing?

I would not think the timing could mess it up like that, order you a new Hammer spring or take the upper off the lower and check the resistance you have with the hammer going forward and locking into the firing position.
hate to ask, but you are still running the M16 BCG right?
Also check your Buffer Spring to see if it seems weak and what buffer are you running?

I would guess it may be Bolt Bounce, if you are not using a M-16 BCG, this can happen. Do you have a heavy buffer? Will it do this with all magazines?

The "tap" on the primer is from the free floating pin, it will do this regardless of the hammer.

What is bolt bounce? I'm not sure what BCG it is, or the buffer and i've tried every mag i got. It will rarely spit out a 2 round burst, then fail to fire the next round. So i'm thinking hammer spring.

BCG is Bolt Carrier Group
Buffer is the gold looking thing that is in your stock which the BCG hits and rides on.
Buffer Spring is the long spring inside your buffer tube which is what your stock sits on.

I know what those are, I'm in the service. I'm just curious what bolt bounce is, I've never heard that before.

The AR buffer acts like a hammer...same principle of the forward assist, but mechanical. The weights in the buffer remain towards the rear of the buffer, until they hit the Bolt Carrier Group(BCG) slamming it forward to ensure the BCG is seated.

The bolt carrier moves foreword and the bolt will lock, but it will want to bounce back because of the energies involved. As this is happening the sliding weights in the buffer move forward counteracting the energy stopping the bolt from bouncing.

If for any reason the bolt gets there too late, it creates bolt bounce and will cause malfunctions in FA ARs.

When you have bolt bounce it may be caused by a lighter BCG, lighter spring, lighter buffer, heavier loads, too much pressure from gas or a combination of all. A great way to counteract this is with a Heavier BCG, Heavier Spring and Buffer and maybe a "Pig-tail" style gas tube.

Hope this helps.

and yes the short answer, it is all timing. If all parts of you FA do not play well together, they will not work properly.

But it used to work awesome, then after switching uppers it stopped working. Everything is back to original now. Thank you all for the advice. I will give you guys a range report ASAP.

Can you post a pic of the new bolt carrier?



TS

My first inclination is that you've got an AR-15 bolt carrier and a M16 bolt carrier and have somehow mixed them up. This is the most likely solution for "used to work on FA but doesn't anymore, but still works fine on semi"

Verify you are using a M16 bolt carrier with an in-spec and undamaged trip surface.


After that, perform a function check. Make sure that everything is operating as it should.

M16 function check goes like this:

remove magazine, clear weapon
set the selector to safe, cycle the charging handle
pull trigger nothing should happen
click to semi, pull trigger. hammer should fall - continue holding trigger
cycle the charging handle again, hammer should not fall.
slowly release the trigger until you feel the disconnector pop
pull trigger again, hammer should fall. release trigger.
set selector to auto, cycle the charging handle again
pull trigger, hammer should fall. continue holding the trigger and cycle the charging handle several times. hammer should fall each time.

If you function check your rifle does the sear trip appropriately on auto?

if yes, go shoot the gun.
if no, get back with me.

if everything works as it should, go shoot the gun.

if you are unable to fire f/a and the rifle passed a function check, it is likely experiencing other problems.

come back here and tell us what happened and we'll go from there.

I think someone already said this, but I just read in the latest NRA mag, page 70, where Colt improved full auto on the M4 carbine by adding a heavier buffer. Semi auto would work fine, but full auto would cause the bolt to bounce causing light hammer strikes.

This post has been edited by Energizer: September 25th 2009 1:39 AM

It is a full auto BCG and it has passed functions check, i have taken it out and it still doesn't fire on auto,

show us a primer ding pic, and a bcg pic

I'm interested in seeing these as well.

My most likely suspect here is either (still) the wrong BCG or a gas issue, too much = bolt bounce, too little = it will fire sometimes but not reset properly.

Double check the gas key and make sure it is tight and staked

load one round in the mag, fire it

does the bolt lock back?

if it does, you're likely not short stroking, and you may be experiencing bolt bounce.

Are you using a rifle stock and buffer assembly or are you using a carbine buffer assembly?

if carbine, acquire a heavier buffer, H2, H3, or 9mm. See if that will fix it up.