Hesitant AR15

[aviator41]

Shooting a very basic DPMS upper with reloaded ammo. Ammo is 62gr with a 23.8 grain charge of BL-C(2). This ammo has performed very well is several other AR's this weekend, so I think I have a touble child upper.

At various round count (1st, 5th, 27th) on both Magpul Gen 2's and Isreali steel magazines the bolt ejects the spent brass normally but the carrier sort of pauses in the fully retracted position (i.e. it's got the buffer all the way compressed) a slight tilt of the rifle will cause the bolt to slam home and the gun is fireable. You can then go two or three rounds and it happens again.

It's really annoying. This is my favorite rifle, it goes with me everywhere on the property.

Any idea what could be causing this? round count is just of 600. I've never fired commercial ammo through it, don't really have a want to do that, but could if needed. The rifle is always cleaned after firing, and I keep the BCG lightly oiled. I haven't done any work the the ramps or any other part - perhaps something needs polished? Wear on the BCG looks exactly like the wear are my more expensive platforms. this upper has been doing this regardless of what lower it's on.

 

[uncle money bags]

Did you try swapping the bcg and/or charging handle out with another known good sample?
Also, is there any unusual wear in the upper behind the ejection port?

Does this gun have a forward assist?

 

[Fyrtwuck]

Is this a full size rifle or a carbine? If it's a full size, do you have the full size buffer and spring in it? They are different lengths. Or, a possible gas problem. The bolt and carrier should be pushed fully forward after each shot. Is there a forward assist? Are there any marks where the assist hook could be hanging up on the side of the bolt carrier.

 

[Shootin 4 Fun]

Sounds like the bolt is hanging up on either the buffer detent or the forward assist. Is the tube threaded in right up to the post on the buffer detent?

 

[uncle money bags]

He stated the problem happened on other lowers as well.

 

[aviator41]

Okay, let me try and answer everyone's questions, and thank you all for the help:

Forward Assist: yes, it has a forward assist. there are no marks on the Bolt Carrier where it would be dragging on it, including the front of the carrier. I inspected the Forward assist pin and it's still virgin. (I've never needed to use it)

I don't see any odd wear markins in the upper where the bolt carrier would be dragging, but I will inspect closer this evening during tear-down. Is there a particular spot i need to be focusing on?

I have switched out the BCG and Charging handle with several others, the problem persists. I have not had a chance to put THIS bcg in another rifle and fire it. if it's causing problems, I don't want to risk damaging my other rifles.

This is a carbine sized rifle with a carbine size buffer tube, buffer and buffer spring. I did, oddly enough, notice that the buffer is not solid sounding when shaken. It has a rattle to it, imagine a couple of bolts inside a prescription bottle. kinda sounds like that. is it possible that the buffer has gone Tango Uniform?

Closer inspection does show some gas escaping from the gas block out of a bleed hole. but the bolt seems to be cycling the carrier to correct distance.

It buffer tube is threaded all the way up properly. enough to hold the detent in, not so far as to cause binding.

I was talking with my son about this last night and he mentioned that he had one round jam in the rifle pretty good. it took both hands pulling on the charge handle to get the bolt to rotate and eject an unspent round. I chalked it up to a round that didn't like being neck sized. it happens. The bolt does rotate smoothly and that was the only bolt-forward issue he reported.

Is it possible that the bolt locking lugs are not fully engaging, therefore it requires less gas pressure to cycle? since it takes less pressure, more pressure is used to slam the bolt carrier into the buffer tube, causing it to jam at its rear-most position?

 

[NikatKimber]

If, as you said, it cycles back far enough, then there are two possible issues: something is either hanging, or not generating enough return force.

Has it done this since the beginning? Or did this start recently?

Changing lowers eliminates the low return force, as the only thing generating return force is the buffer spring; the spring is in the lower, if you changed complete lowers, then the spring isn't the issue.

If changing BCGs doesn't fix the problem, there is an issue with the upper. I would look for burs or debris in the bolt carrier channel.

I guess low lubrication *could* cause this (higher friction), but I would think to generate enough force to completely stop the bolt it would have to be nearly or completely dry.

 

[uncle money bags]

You have eliminated all possibilities except the upper receiver itself, as far as i can tell.

There is something catching the bcg at just the right time between its recoil and trip back into battery.
My bet is an incorrectly milled receiver.

Is any part of the bcg in contact with the magazine when stuck?
What direction do you need to tilt the rifle to make the bcg return to battery?

 

[jakerz]

I would say take a good look at the buffer/spring, but you already said you have tried it on other lowers. It could be a gas issue. If it's not fully pushing the BCG back to the rear, then the spring will not have enough compression to push the BCG back forward to pick up another round and re-seat.

 

[henschman]

Look at the upper receiver from the rear. Is it bored noticeably off center? I have seen defective ones that have this issue before, and it can cause the kind of thing you're describing.