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Killed my M4 today.


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I finally got the chance to get to the range for the first time with my M4, after my Form 1 was approved, and I was able to install the 14" barrel.  The only other changes I've made were the A&S trigger guard, and a set of wolf springs.

On the second shot, the gun locked up, and the trigger was dead.  I had to pull the trigger group to get the action open.

My trigger broke at the disconnector pin hole, the disconnector pin is mia, but the springs and plunger were still in the bottom of the trigger group.  I'm pretty sure the trigger and disconnector are FFT parts, from right after they started selling them years ago.  The hammer is a geissele.  It's run fine for years, up to today.

It looks like I'll have to go with a Briley, since FFT is OOS, and it sucks I cant buy just a U.S. made trigger, I have to buy all the trigger group parts together.

 

LNGwKRr.jpg

Edited by nimslow
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@nimslow If I was you, I would reach out to FFT and see about getting a replacement.  There is an apparent lifetime warranty. At least get a new trigger from them, and they you could sell it to someone who might need it. Out of curiosity, how many rounds do you figure the you have shot with the FFT parts?

Edited by MNtadpole
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1 hour ago, StrangerDanger said:

Wow. Never seen that before. What's the face of the hammer look like? Any damage? Pistons look ok and freely move? Any signs of damage to the face of the bolt carrier where the pistons hit?

Do you still have your OEM trigger and disconnector to drop in?

The wear on the hammer face looks normal, the pistons look fine, and move freely.  The bolt carrier face has the same marks it's always had from the pistons.

I'm scratching my head on this one.  First look at the range, and because the disconnector was loose inside the action, I'd thought it failed at the disco pin hole.  Once I got it out, this is what it looked like.  The little circles above the trigger are whats left of the disconnector pin.  The pin sheared, and each end was left in the holes in the trigger.

QWK3zot.jpg

It seems the hammer hook hit the top of the trigger, and that caused the failure.  I'm wondering if it's the A&S trigger guard.  In the front of the trigger pocket, the A&S has two steps, and there are some drag or impact marks there. Using my MK1 MOD 0 eyeball, the FFT trigger looks slightly longer than the OEM one.  So perhaps the nose of the trigger was binding enough to let the hammer hook make contact and fracture it.  At least thats my best guess right now.  Everything felt fine during function checks, and it looked to be working normally after I put it all together.

MxpyWxx.jpg

OEM on the left, A&S on the right.

quC6sKJ.jpg

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Those gouges in the bottom of the A&S frame appear to align with where the trigger broke in half. So my guess is that is where those came from. Sheering that disconnector pin is insane. It's impossible to reassemle the trigger pack if that pin is slightly out of the trigger. The trigger won't seat into the trigger frame if it is poking out one side.

The amount of energy required to sheer that pin and to break that trigger like that would have to be immense. Even a weak trigger should not have allowed the disconnector to be sheered off like that.

I'd double check your new barrel to eye ball those gas ports to make sure they look similar to your 18.5" barrel. I'd feel out the action by racking the bolt to see if you have the proper amount of tension from the main spring. Look over the bolt carrier real well to make sure everything is moving properly. A failure of the trigger is one thing, but a secondary sheering event at the disconnector pin seems like it was being hit way too hard.

The rounds fired, did they feel odd at all?

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1 hour ago, StrangerDanger said:

Those gouges in the bottom of the A&S frame appear to align with where the trigger broke in half. So my guess is that is where those came from. Sheering that disconnector pin is insane. It's impossible to reassemle the trigger pack if that pin is slightly out of the trigger. The trigger won't seat into the trigger frame if it is poking out one side.

The amount of energy required to sheer that pin and to break that trigger like that would have to be immense. Even a weak trigger should not have allowed the disconnector to be sheered off like that.

I'd double check your new barrel to eye ball those gas ports to make sure they look similar to your 18.5" barrel. I'd feel out the action by racking the bolt to see if you have the proper amount of tension from the main spring. Look over the bolt carrier real well to make sure everything is moving properly. A failure of the trigger is one thing, but a secondary sheering event at the disconnector pin seems like it was being hit way too hard.

The rounds fired, did they feel odd at all?

The two rounds of Federal flite control buckshot that fired felt the same as always.  Everything else looks and feels ok, as far as I can tell.  Since I had to order another disconnector pin, I went ahead and got a new recoil spring.

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That shouldn't be anything too extreme. Good call on the recoil spring.

Right where it broke on the trigger is where the hammer pivots down between the disconnector and the trigger sear. The only thing I can think of is the trigger was weak for whatever reason and broke in half when the hammer was cocked by the bolt. No idea why it would sheer that disconnector pin at the same time though.

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I got my new disconnector pin, and recoil spring yesterday.  I could only measure the force to retract the bolt with a trigger pull gauge, but the old spring was about two pounds less force than the new one.

I put everything back together with the OEM trigger.  After some experimentation with both trigger housings, I put everything back in the factory trigger housing.  I'm leaning towards the trigger pocket in the A&S trigger housing not being long enough.  The nose of the trigger contacts the front of the pocket in the A&S housing, before the back contacts the travel limit pin.  It also looks like the front of the trigger has more room to travel down in the factory housing.  When I get some time, I may open up the front of the A&S and see if that makes a difference.

 

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1 hour ago, Milspec said:

How old is that A&S housing?  I'm just wondering if it's from recent production.

I got it direct from A&S in February or March this year, don't remember exactly when.  In  the picture above, you can see the marks where the front of the trigger contacted the front of the pocket in the trigger group.  It is "stepped" with two ledges, both are deformed where the trigger hit them.  There are no such marks on the factory trigger housing,  that has had a couple of thousand rounds through it, with the exact trigger that broke.

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It's interesting that the trigger made contact with both "steps".  One would think that it would have been aligned with one or the other. 

I wonder if it was rocking due to the fact that the disconnector pin sheared?  Maybe it was rocking for a while after the shear until it finally broke?

It almost implies there was "extra" play going on with they trigger.

Total speculation on my part of course.

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1 hour ago, StrangerDanger said:

Maybe make contact with A&S and see if they’d be willing to swap it out?

You have probably seen way more of the A&S units than I have.  Are the "Steps" in the front normal?  Or should it be even?

 

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11 minutes ago, Milspec said:

It's interesting that the trigger made contact with both "steps".  One would think that it would have been aligned with one or the other. 

I wonder if it was rocking due to the fact that the disconnector pin sheared?  Maybe it was rocking for a while after the shear until it finally broke?

It almost implies there was "extra" play going on with they trigger.

Total speculation on my part of course.

Since I only got two rounds out of it,  It had to have occurred on the first, or second shot.  I'm still thinking the front of the trigger didn't travel as far down as it should have, letting the hook on the hammer strike the trigger,  breaking it and the disconnector pin.

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