ran into a weird problem yesterday.

[Lloyd Smale]

Had a colt small pin sp1 lower for many years. About 5 years ago it double tapped on me. Then it started to happen occasionally. Yesterday i had it out and it was doing it every mag. I started parts swapping. I tried three different fire control groups in it. All of them seemed to be acting normal when it was cocked by had and dry fired and the problem only happened when it had an upper on it. Tried a different bolt and carrier out of another gun thinking maybe the firing pin was sticking. No change. Even tried swapping out the saftey selector. The upper i had on it was a new one id never shot before so i figured there might be a problem with that so i swapped another upper on it no change. By now i was getting nervous and didnt want the law coming by so i stripped the lower down to bare and tossed it in the garbage. So now i need another new bare lower. Cant for the life of me figure out whats going on. Ive had that lower for 30 years and only in the last 5 years has it started this. Then i went on psa (who has the only bare lowers i can find) and found what was 45 bucks a year ago is now 95 bucks!! What was happening is it would go bang when i pulled the trigger and again when i released it like a binary trigger would. Scratching my head on this one. Isnt my first rodeo. Ive built at least a dozen ars.

[sigep1764]

Worn trigger group pin holes? Does the pin have any movement in those holes? If it allowed any movement to the rear, the sear could slip.

[Lloyd Smale]

actually had locking pins on it so they couldnt move without the front pins moving too.

[jakharath]

<sarcasm>Its one of those evil black rifles wanting to go fully automatic!</sarcasm>

Soft primers and a sticky firing pin?

[falmike]

I wouldn’t toss the receiver yet.

Drop another hammer/trigger/sear, and maybe a known good BCG, in and see if problem remains. Use factory pins or at least pins other than those already in there as well.

I doubt the receiver is the root cause of your semi automatic rifle’s malfunction that you are trying to eliminate.

Best

[Tatume]

In an Army small arms training at Camp Perry I had an M16 that was doing doubles. The armorer said it was me milking the trigger. I said no, you try it. He did, it doubled, and he gave me a different rifle. I never found out why it happened. I know that's not much help, sorry.

[Lloyd Smale]

I wouldn’t toss the receiver yet.

Drop another hammer/trigger/sear, and maybe a known good BCG, in and see if problem remains. Use factory pins or at least pins other than those already in there as well.

I doubt the receiver is the root cause of your semi automatic rifle’s malfunction that you are trying to eliminate.

Best

i tried 3 different fire control groups (hammer trigger sear and springs) and tried two differnt bolt control groups. Like i said ive tried replacing every part in the lower and even tried other uppers and the only thing left is the lower. Maybe it got bent? But youd think there would be a mark on it or i would remember a drop that hard. the lower was on a gun that had a model 1 upper i bought used years ago and it was on one of the ars i seldom shot. So it might have been that way when i bought. I dont know what to think but everything works when using another lower so i about have no other answer. I know with the liberals taking over it makes me nervous when it does it. Sure isnt worth going to jail over. Like i said it has to be something binding or not working right with the reset because when it happens i noticed it fires on the pull and then again on the release. If you put light pressure on the trigger when you pull it it will take off like bump firing. Sure couldnt take a chance selling it or giving it away because id feel like crap if it happened at a real range to someone else and they got caught or hurt someone. Not for the 30 bucks its worth. Safest thing is the garbage.

[Lloyd Smale]

In an Army small arms training at Camp Perry I had an M16 that was doing doubles. The armorer said it was me milking the trigger. I said no, you try it. He did, it doubled, and he gave me a different rifle. I never found out why it happened. I know that's not much help, sorry.

Heck im willing to listen to any possible thoughts on it. If nothing else i might learn something for next time.

[Winger Ed.]

I've got a late 70's SP1 that hadn't been fired 20 times until I got ahold of it.

It ran fine, until I shot some reduced power reloads, then it would double or triple fire.
I downloaded a bunch of ammo about 25% for short range, target shooting, and wanted to keep some of the stress & strain off it.

I figured the bolt wasn't coming back quite far enough for the hammer to catch the sear,
and it thought it was supposed to 'rock & roll'.

I got a after market buffer spring and chopped about 3" off it.
That solved the problem for me.

[WebMonkey]

^
the above has the most logical explanation.

the receiver itself has so LITTLE to do with the action that it would be a long time before i blamed it.

now, if it was a full auto receiver, then i'd look as the auto sear 'holes' but that's not the case.

the improper engagement of the semi auto sear is the probably cause.

you can put a lighter 'buffer' or weaker spring or both in order to allow the bcg to recoil more freely.
'resetting/engaging' the fire control group more forcefully.

if that 'works', then you could look for a dinged receiver extension (buffer tube) or components related to it.

good luck!

[elmacgyver0]

An SP1 is an original Colt AR-15.
Quite a collectors item.
As far as I know the lowers were never sold separately, what happened to the rest of the rifle?

[BK7saum]

I seriously doubt it is the receiver itself. It is something in the disconnect or like mention above with reduced loads. Don't toss it. I'm sure there is an explanation, and I would want to find out for certain what the reason is.

[DDriller]

If you have a drop in trigger you might try that.

[elmacgyver0]

I don't know how it could affect it but the SP1 had a semi-auto bolt carrier.
Most of the carriers out there are the full-auto variety.
This shouldn't make a difference, but then who knows.

[edp2k]

Hook angle and/or length of hook on the disconnector.

[BK7saum]

When you switched out the fire control groups, did you also use separate disconnectors and springs or did you swap triggers and hammers only.

If the disconnector hook is too short or been chipped/worn, it will release the hammer before the trigger can catch on sear hook. This can happen if the geometry is a bit off, especially if you have a really slow trigger release.

Could you please confirm that you used separate disconnectors and springs each time you swapped fire control groups?

[BK7saum]

When you switched out the fire control groups, did you also use separate disconnectors and springs or did you swap triggers and hammers only.

If the disconnector hook is too short or been chipped/worn, it will release the hammer before the trigger can catch on sear hook. This can happen if the geometry is a bit off, especially if you have a really slow trigger release.

Could you please confirm that you used separate disconnectors and springs each time you swapped fire control groups?

[Lloyd Smale]

i was running a 3oz buffer and a standard weight spring. I had the same thought that i seemed like the bolt wasnt comming back far enough. Problem with cutting a spring is that you then loose your chambering power and you beat up your gun. That and you sure shouldnt have to go lighter then a 3 oz buffer and a standard weight spring to get an ar running. I dont think ive ever even seen a buffer less then 3ozs for sale. I did try different disconnectors and springs each time. I have a number of trigger groups in baggys that i saved when i put better triggers in guns.

[Lloyd Smale]

An SP1 is an original Colt AR-15.
Quite a collectors item.
As far as I know the lowers were never sold separately, what happened to the rest of the rifle?

dont know. Im sure someone didnt want the 20 in fixed carry handle upper and bought the model one. That upper is a stainless 16 inch and is the best shooting ar ive ever had but its on a different lower now.

[Lloyd Smale]

If you have a drop in trigger you might try that.

might try that down the line. I did pull it out of the trash and stuck it away where its safe. Problem is right now drop in triggers are not cheap and replacing a lower is cheaper. But if prices ever settle down and i can get a one piece trigger for under a hundred bucks i might give it a try. At least i could use it on another gun if it doesnt.