Hammer Pin and Receiver Hole Questions

I decided to get another Colt LE6920 last week just because I could. It had a really nice finish and a "CR" prefix serial number--and knowing that people have gone Ape Shit on the net about not having an LE serial number prefix, I decided to make it a shooter that I don't care if it gets the pretty smacked off of it.

After two short 100-round-each range trips, I noticed the hammer pin had walked out of the left side.

This is not a slam on Colt. I painstakingly lined up the pin to the receiver hole and pushed it back in. The next day I made another 100-round range trip to see if it would walk out again. It did not. I even did a rapid fire 30-round mag dump to see if I could force it to walk out. It stayed. I'm guessing the factory worker who assembled the lower had simply not pushed the pin in all the way and it took two short range trips for it to walk out.

What I'm about to post next is mostly out of curiosity, and that's why I'm posting it in the tech section. I'm assuming the rifle is GTG at this point since the hammer pin didn't walk out again even with a rapid fire mag dump, but I'm now curious about some of the technical aspects of this.

1. Would the gun have functioned normally with the hammer pin out of the left-hand receiver hole? In other words, how many rounds did I fire with the hammer pin halfway out? I want to know this just as a kudos to the weapon's design, or to realize that maybe the hammer pin must have finished walking out on the last round or I would have had a malfunction. It's just my curiosity is killing me on this.

2. If it would have fired in that condition, is there any danger that the hammer pin in its half-installed state might have done some damage to the hammer pin hole in the right side of the receiver? Should I anticipate that I might experience a hammer pin hole ovalling out in a few thousand rounds? Or is the anodizing sufficiently deep enough and the nature of the weapon's design such that I probably can just rock on knowing that I "fixed" what a Colt employee should have done to begin with?

Just eyeballing things under a magnifying glass, it appears that the only obvious "damage" is that there is a minimal loss of finish around the inside of the hammer pin hole on the right side of the receiver. Under a bright light it looks as round as the trigger pin hole.

Again, I'm assuming all is well and thank the Lord I know enough about this type of thing to simply push the hammer pin back in all the way and soldier on as if nothing happened.

But I'd love to know more about some of the technical aspects of this.

An AR fire control group is extremely forgiving in my experience. The first sign something's wrong is usually a dead trigger or an unexpected burst. Your pin had probably just come out if you didn't notice anything funky. I seriously doubt you'll need to worry about future hammer pin hole issues.

There are only three reasons why a hammer pin could come out.

1) Bad pin. Maybe the J-spring groove is too narrow/shallow, has a chamfer/nick/collapse on the groove that allows loss of tension while rotating.

2) Bad J-spring. Broken/Loss of tension/Worn where it interacts with hammer.

3) Installation. Wasn't seated. Should be a tactile click when pin is seated.

Good J-spring. Has a nice snap with minimal end play when a hammer pin is installed.

http://i.imgur.com/q1J4eRx.jpg


A lower like one in the following picture can cause what can be perceived as pin walking. The FCG side walls are around 0.010" different. One side may just be a little wider from forging or maybe the pocket is off center. Plenty of room to function correctly even if pocket is off 0.010". With a FCG installed, the pins will appear to have slightly walked towards/out the thinner side when compared to the thicker side.

http://i.imgur.com/xwfe8Jl.jpg


Now for some "Does that even work?" Yes, yes it does. I screwed up an 80% lower. Right side pin holes are 0.030" closer to mag well than left side. It did require a set of KNS pins. The geometry is correct (both right holes equally off) so it functions fine. What is interesting is the obvious cant of the FCG.

Lower has around 9k on it. Notice the uneven finish wear between the red arrows. This is caused by the carrier not making full contact while cocking the hammer due to the tilt. The blue arrow shows where the hammer contacted the lower until it mashed enough material out of the way.

http://i.imgur.com/wgDEsEN.jpg

So all in all, I think it boils down to a design that allows for stuff to be not quite right.

You seem to know a lot about this.

Is my assessment correct that the pin probably wasn't properly seated to begin with?

If it were a defective component, wouldn't the pin have walked again at the next range session, especially when I did the 30-round rapid fire mag dump?

Quote:

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Originally Posted by Leaveammoforme

A lower like one in the following picture can cause what can be perceived as pin walking. The FCG side walls are around 0.010" different. One side may just be a little wider from forging or maybe the pocket is off center. Plenty of room to function correctly even if pocket is off 0.010". With a FCG installed, the pins will appear to have slightly walked towards/out the thinner side when compared to the thicker side.

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It would be interesting to see if the trigger slot was displaced the same amount.

I've been told that the thicker wall on one side is due to the forging not being properly placed in the CNC machining fixture.

Any machinists lurking?

Quote:

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Originally Posted by Doc Glockster

You seem to know a lot about this.

Is my assessment correct that the pin probably wasn't properly seated to begin with?

If it were a defective component, wouldn't the pin have walked again at the next range session, especially when I did the 30-round rapid fire mag dump?

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I wouldn't say I know a lot but I have made general observations. As an L1 Master my inquisitive mind investigates all things mechanical.

I'd assume the pin was never fully seated. Hold trigger released while moving hammer through its range of motion and pressing on the pin. I'd say you're good if it doesn't release from the J-spring with minimal pressure.

Quote:

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Originally Posted by 26 Inf

It would be interesting to see if the trigger slot was displaced the same amount.

I've been told that the thicker wall on one side is due to the forging not being properly placed in the CNC machining fixture.

Any machinists lurking?

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Looking at blueprints, the trigger slot *should* be centered with takedown slots, RE, magwell... the variance in forgings determining the trigger slot wall thickness. which looks like its supposed to be .095" +/- .003

How the manufacturers actually do it..???

Attachment 52094

Quote:

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Originally Posted by Leaveammoforme

I'd assume the pin was never fully seated. Hold trigger released while moving hammer through its range of motion and pressing on the pin. I'd say you're good if it doesn't release from the J-spring with minimal pressure.

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When I re-seated the pin, I heard a faint but audible click (evidently the J-spring engaging the detent in the pin). That was before the range session where all went well. I tried your test as well and the pin didn't move.