Colt AR failure

[gillman7]

OK, bought a brand new 6920 and took it to the range today.

Shooting M855 to sight in, first three shots, good pattern, needs some adjustment. 4th shot, no bang. Drop the mag, pull the bolt, case is in the barrel. Pulled it out, no primer strike, no scratches, nothing. Replaced the mag, let the bolt go, fired twice, third shot, same thing.

Thought it might be the mag, changed it out, same thing. Tyromeo and Adluginb were there, so I changed out the lower, and everything was fine. In looking at the Colt lower, found that it has a heavy buffer. Ty thought that it might be too heavy and causing it to short stroke. It would cycle, just not lock up tight.

I am thinking this could be the problem, but am curious if anyone else has had this issue?

 

[mmchambers06]

So it didn't cycle far enough for the hammer to catch on the disconnector? Sounds like shortstroking. You should have tried using one of their buffers in your lower, that would tell ya for sure.

Was it the M855 that I traded ya?

 

[gillman7]

Yup, it is consistent ammo, I like it.

You are correct, it seems to be shortstroking. It functioned fine with the other lower, so I am thinking for some reason it doesn't like the heavy buffer.

Oh well, if so, it is a cheap fix. Of course, if I have an extra buffer laying around, it would be wasteful not to build a gun around it. Since it is a Heavy one, I guess I will have to think about an SBR?

 

[mmchambers06]

Of course, if I have an extra buffer laying around, it would be wasteful not to build a gun around it.

You, sir, are correct.

I have a couple extra accu-wedges that need homes as well!!

 

[NikatKimber]

Was the hammer not falling? I think you'd be able to tell that.

 

[gillman7]

Was the hammer not falling? I think you'd be able to tell that.

Hammer would fall, no primer strike. Bolt did not lock up enough to engage the round. Even the forward assist did not ride the bolt forward. I really think it was just shortstroking.

The bad thing was that I tried Pmags, GI mags, M855, and XM193, and it all did the same thing.

 

[NikatKimber]

I'm not an AR expert, but if it was short stroking wouldn't the hammer fail to engage the sear and not remain cocked? Or is there a secondary notch?

1) too strong of spring) Bolt goes back, not completely. Spring has stored all the energy of the recoiling bolt, and now slams it forward. The spring *should* return the bolt completely to lock up, even if it is short stroking. The issue would be if the bolt's travel was enough shorter than required the hammer would not be completely compressed into position to catch the sear.

2) Too soft spring) Bolt travels completely to the rear, and is stopped before the spring can absorb all the energy, therefore the spring returns the bolt with less energy than necessary to lock up completely. Hammer is completely cocked, but the hammer falling only completes lockup as opposed to striking the primer.

Now again, I'm not an AR expert AT ALL, just thinking the process through. If I'm wrong, someone correct me. And if possible, explain why, just since I like to know what's going on.

 

[RustyZX9]

Yup, it is consistent ammo, I like it.

You are correct, it seems to be shortstroking. It functioned fine with the other lower, so I am thinking for some reason it doesn't like the heavy buffer.

Oh well, if so, it is a cheap fix. Of course, if I have an extra buffer laying around, it would be wasteful not to build a gun around it. Since it is a Heavy one, I guess I will have to think about an SBR?

Sounds like you need to build a 9mm AR... lol...

 

[gillman7]

#2 is what is happening, I am just wondering if the heavy buffer is absorbing too much of the recoil or slowing the spring down on the return to keep from locking it up.

The hammer is cocking, but does not contact the round in the chamber. When it happens, I can look at the bolt and see that it is not far enough forward, but the forward assist does not push it into battery either.

Sounds like you need to build a 9mm AR... lol...

I like the way you think!!!!!!

 

[NikatKimber]

Ah... LOL! My bad, i was thinking "heavy spring" when you said "heavy buffer." Yeah, I get you now.