help with an AR-15 that goes full auto

[sako_75]

I have an early Colt AR-15 SP1 that wants to go full auto on me. It will shoot two to three rounds just fine then it wants to shoot in 2-4 round bursts. I have given the rifle a through cleaning and it still behaves the same way. Any help as to what to look for would be appreciated. I am more of a bolt action guy, so needless to say I don't have this problem with my other rifles

 

[dfariswheel]

The "usual" is a problem in the trigger assembly.
The most common cause is a disconnecter that's worn/defective.

Open the rifle up, hold the trigger back, and push the hammer back until it's caught by the disconnecter.

SLOWLY release the trigger until the hammer is released by the disconnecter and is caught by the sear.
If it's a disconnecter problem, the hammer will not be caught by the sear and will drop all the way.
(Note: catch the hammer with a finger so it doesn't drop all the way and impact the lower).

If the hammer is caught by the sear, it's possibly a worn/defective trigger or hammer sear.
In these cases, what's happening is the impact of the bolt closing is "jarring off" the hammer/trigger sear and allowing the hammer to fall.
This is in effect a "Slam fire".

In any case STOP SHOOTING THE RIFLE, until you get it repaired. This condition of firing multiple rounds is VERY DANGEROUS, to both you and the rifle.

 

[eviioiive]

check hammer, disconnector and disconnector spring.

 

[sako_75]

When holding the trigger rearward and cocking the hammer the disconnecter catches the hammer. Upon releasing the trigger the sear does catch the hammer. The trigger and hammer sear both appear sharp and unworn. This rifle has had less than 1500 rounds through it in its lifetime. Is there anything else to look for or is it time to take it to a smith?

 

[geekWithA.45]

Check that the firing pin is undamaged, and that the firing pin channel is free and clear of obstruction.

In general though, I'd say it either needs a 'smith, or a trip back to the factory. A relatively new rifle has no business doing that.

 

[sako_75]

Its far from being new. Its a SP1 from the late 60's just never fired very much.

 

[ArmedBear]

4-step fix:

1. Put the selector back on semiauto.
2. Cover in preservative grease and enclose in watertight container.
3. Bury in remote location.
4. Note GPS coordinates.

 

[Chipperman]

A gun that goes full-auto unpredictably is no good to anyone, even if the balloon goes up.

Get it fixed post-haste. Do not take it out anywhere until fixed. ATF would enjoy making you sweat over an unregistered machine gun. If you plan on taking it to a gunsmith, break it down before transport. (That's being super-paranoid, but better safe than sorry).

 

[Punkermonkey]

I had the same thing happen with an old Colt .45acp. It wound up that the firing pin kept getting lodged through the guide hole and was strike-firing when the action closed. Full auto from a hand gun is quite a surprise!

 

[ArmedBear]

Full auto from a hand gun is quite a surprise!

If you have a Kimber, it shouldn't be a surprise.

Seriously, Glock actually makes a select-fire pistol. I've always wondered, legal issues aside, what the market for it could possibly be! (I guess it could solve the problem of Glocks naturally pointing a bit below where you'd like them to.)

 

[mc223]

I think armedbear was onto something till the Kimber thing.

 

[PvtPyle]

Simple fix, replace all the internals and you are done. They are worn and unsafe to fire like this.

 

[Nhsport]

As others have said, either trigger/sear (most likely) or some sort of fireing pin hanging up and doing a slam fire deal. If you can borrow another bolt/carrier from someone and run it in your gun that will indicate or rule out the fireing pin. If this is the case it is likely brass shaveings in the bolt or a burr on the fireing pin.
If it is an early colt I believe any lower parts will fit,standard trigger group is only $45 or so , even if you bite the bullet and get the Colt stuff it shouldn't be more than $70.
Trigger group just drops in,no adjustments or fitting,thats one of the nice things about the AR .

 

[jungle]

If all the parts are sharp and it only has 1500 rounds through it I'd guess the disconnector spring is broken or faulty. Cheap fix to r&r and degunk the channel the disconnector rides in. If this doesn't work then replace the disconnector then hammer, then the trigger.