Need help troubleshooting AR

[Chape1rm]

I have a 450 BM upper I purchased on here and I'm having failure to feed issues when pinned to a stag arms lower with an A2 stock.

When I pin the upper to my DD carbine lower it cycles just fine.

To me and my limited knowledge on the AR platform I'm thinking the buffer spring is to long/stiff for the 450 but I wanted to get some opinions on where to start. If the same lower cycles .556 wouldn't there be more gas from the 450? Is it cycling too fast?

 

[wyogoat]

I think you’re on the right track with it being the buffer spring. It would seem to me it’s too stiff but someone on here may know more and have a better diagnosis.
Have you pulled that buffer spring in the problematic lower and made sure it’s clean and not being held up somewhere?

 

[Chape1rm]

The buffer and spring look good. Not seeing any hang ups.

 

[ABK]

Swap the buffer and spring from the DD lower and see if you still have the problem. The try different combinations of buffer and spring to see which is the problem.

 

[StupidLightweight]

There's a lot that goes into this. First, let's discover where the empty cases are landing from each lower. That'll help diagnose the issue.

 

[Northpark]

Just real quick check for an adjustable gas block too. Could be just enough to cycle one but not the other.

 

[Chape1rm]

The brass is from the DD lower is landing almost perpendicular to the receiver and closer together.

The brass out of the problematic lower is landing further back 18 inches to 3 ft or so and the group is further spread out.

It is a mid length gas system and not an adjustable block.

 

[Bearwhisky]

Most likely buffer weight or buffer spring issue.


Sent from my iPhone using Tapatalk

 

[StupidLightweight]

Yeah, the issue is obviously lower related. Like the OP's guess, I think the A2 buffer is causing more resistance, maybe not traveling far enough rearward to have enough energy to snag and chamber the next round. If you'd like the lower to work with the .450 upper, I'd suggest a variable buffer from KAK. This will allow you tailor the mass to work perfectly.

AR-15 RIFLE BUFFER BODY ONLY

Enhance your firearm's performance with our high-quality AR-15 rifle buffer body. Built for durability and reliable recoil management.

kakindustry.com

 

[Chape1rm]

Yeah, the issue is obviously lower related. Like the OP's guess, I think the A2 buffer is causing more resistance, maybe not traveling far enough rearward to have enough energy to snag and chamber the next round. If you'd like the lower to work with the .450 upper, I'd suggest a variable buffer from KAK. This will allow you tailor the mass to work perfectly.

AR-15 RIFLE BUFFER BODY ONLY

Enhance your firearm's performance with our high-quality AR-15 rifle buffer body. Built for durability and reliable recoil management.

kakindustry.com

So I would need to get a few weights and add/ take away for fine tuning with this buffer?

 

[Musky]

What BCG do you have in the 450 upper? I agree it's either buffer/spring or also could be a BCG issue.

 

[Chape1rm]

What BCG do you have in the 450 upper? I agree it's either buffer/spring or also could be a BCG issue.

There's no markings to identify who's BCG it is but the specs I got when I bought it were that it is a M16 BCG 8620 black nitride steel with a staked gas key and 9310 black nitride steel MPI tested bolt.

Could you shed some light on how the bcg would effect cycling in one lower but run fine in a different one? Not doubting it just trying to learn more of what I'm working with here.

 

[Musky]

If it's an M-16 BCG then it's heavier than the standard Ar15 version and thus slows cycling down some. Not that there is anything wrong with it, generally speaking an M16 or full auto carrier is smoother to shoot as the recoiling rate is generally slowed down some. I generally prefer the M16 version carrier to the lighter ar15.

It'll be much cheaper to swap out different buffer or spring combinations than picking up a different carrier. I was merely curious.

On your stag lower with the A2 stock does your buffer look like the photo in the link above? When I first got into it, I didn't realize you actually need an A2 buffer/spring as the standard carbine buffer/spring do not generally work in the full size A2 stock.

Below is the difference in carriers if you're curious.

 

[StupidLightweight]

So I would need to get a few weights and add/ take away for fine tuning with this buffer?

Yes, sir. That's exactly how you'd tune the buffer. They sell weights made of aluminum, steel and tungsten. It's a very clever way to fine-tune an AR.

 

[Chape1rm]

Here's what I'm working with. Unfortunately after 20 rds the other lower started having the same malfuntion. I also noticed the bolt wasn't holding open on an empty mag. The A2 buffer/spring is on the bottom.

 

[Musky]

You could pop the pin on the A2 buffer (the longer one) and take a weight out to lighten the load.

I was going to suggest possibly a loose mag catch lever, however if both lowers are having this issue that almost rules that out. But that issue you could also see failure to feed and no bolt hold open.

Easy enough to drop the pin on the buffer and take the weight out to see if it will decrease pressure and allow the bcg to cycle further backwards.

Lastly, there is a very real possibility that the gas block is causing this issue. Especially since you bought it privately. The gas block may not be in the correct position giving less gas to the bcg than is required to cycle. This would absolutely be something to check.

 

[StupidLightweight]

Yeah, it sounds like your upper is "under gassed," meaning there's not enough gas pressure to operate the action. Potential causes could be:

1. Ammo (are you using trusted loads?)
2. Gas block leakage (black soot will be visible)
3. Undersized gas port (this will require removing gas block)
4. Buffers are too heavy (solution is to make them lighter)

Good luck. Keep us posted!

 

[Chape1rm]

I appreciate all the input guys. The gas block was out of alignment. The rifle is back in order and I harvested two whitetail does for the freezer.

 

[StupidLightweight]

Great to hear you succeeded on the four "Fs" of fixing your firearm and filling the freezer.

 

[justinspicher]

I appreciate all the input guys. The gas block was out of alignment. The rifle is back in order and I harvested two whitetail does for the freezer.

This is awesome news, I was just thinking about sending you a pm asking if you got it figured out.