Today I was able to test out my AR9 with the fostech ECHO trigger and my new NFA 9mm bolt at the range. I could do full mag dumps in ECHO mode unsupressed but when I put the YHM wraith XL suppressor on the SBR I would have double feeds and one even looked like a triple feed . What could be causing this issue when firing in the fast ECHO mode suppressed. It did not do it in semi mode suppressed when I fired as fast as I could. So its something going on at high speed with gassss.

The spec of the 9mm SBR are:

QC10 lower
MVB 9mm PDW stock (MVB buffer 5.3 oz)(MVB "special" buffer springs)
NFA 9mm bolt
matrix arm side charging upper
cmmg barrel 8"? (I don't remember where I got this one)
Fostech ECHO trigger
YHM wraith flash hider
YHM wraith XL 45 suppressor

Do you guys think it might need a heavier buffer for suppressed? MVB has a 8.5 oz to be release soon so I want to get that to solve the over gassed issues this SBR already has.

Here is some video of two of the mag dumps. No hammer follow or bolt bounce (from what I could tell) So far the ECHO is awesome in the AR9 I just need to fix my suppressed problem to make it strictly dependable. Nevertheless without the suppressor it was fun as hell.

ECHO AR9 mag dump 1

ECHO AR9 mag dump 2

Colt mags have a tendency to vomit rounds out if you hit them hard enough to take pressure off the feedlips. My guess is that with the suppressor on there, the bolt is slamming into the back of your gun repeatedly, rattling your magazine, causing the feeding clusterfucks you're describing.

The solution is a heavier buffer, like you think.

Thanks!

8.5 oz  PCH buffer from MVB is ordered. I hope I'll have it before the end of the week

I hope it works out for you. Please report back if you have the time. I am curious if I will have the same issues once I get my can.
I have a very similar setup with the mvb stock on a colt pattern lower and the echo trigger installed. (Octane 45 pending 6/3). Weather here has kept me from the range since I installed my echo trigger, but I am hoping it runs as smooth as your videos make it look...

Those cases pouring out is sweet looking!

Will do. The buffer should be delivered tomorrow. I will not abe able to get out to the range for a week or so for testing. Your going to love the ECHO on your AR9. What barrel length is your setup?

I'm also interested in how it goes, I'm wanting one of these types of triggers for my 4.5" Glock mag AR. It does see some unsuppressed use, but 95% of the time it's wearing the can.

Barrel is a 5.5" QC10.

I know that barrel well. My other QC10 SBR lower has that same barrel. It's a suppressor under the hand guard. It's sitting on my bench in pieces. I need to put that upper back together.

I got the MVB tungsten buffer 8.5 oz installed last night. Its a heavy ass buffer so I hope that fixes the issue with the can. I will test it next week. I want this setup to be dependable in suppressed echo mode and unsupressed Echo mode.

update will come next week for my PDW setup. The MVB stock limitations have been the issue so far but I'll know if it fixed when I get out to the range with the 8.5 oz buffer and the can attached. However without the can the ECHO rocks.

Edit: You probably will not encounter the same problem as me since you are using Glock mags, so you may be GTG

I was able to test the 8.5 oz tungsten buffer yesterday in ECHO mode and had a kaboom twice with and without the suppressor attached. It did not hurt me or the lower but dam it was scary. The tungsten PCH MVB buffer is solid tungsten and I think I had bolt bounce to have these two out of battery explosions. I'm contacting MVB industries tomorrow.

I placed the 5.3 oz back in ,which is not solid, and everything worked fine without the suppressor.

Whoa! Glad you didn't get hurt.
Sounds like a bolt bounce, yeah. I wonder if it would be worth slowly modifying/adding weight to the 5.3 buffer and testing it out each time? How loose are your magazines in the magwell? Do you think the added back pressure of the suppressor is bucking the magazine around too hard and causing the double feeds?

I hit the range a couple days ago (finally) and my setup ran fine in echo mode while un-suppressed. I can't explain how fun this trigger is!
I pick up my Octane 45 tomorrow . I will go out again afterwards - snow be damned! Report back soon.

The problem was that the buffer is Maxed? out with the 5.3 oz MVB buffer. It's all tungsten weights inside. The buffer is a proprietary buffer with the MVB stock. I can't use anything else. I talked with MVB industries and they are going to make an 8.5 oz with weights so that should do the trick. I still think the suppressor is knocking the hell out of the buffer tube and that is knocking extra rounds up. It does not do it during semi mode fast fire. Let me know how yours does with the octane. I love my octane 45 too

I found with the can that I needed to run a standard 3.5 buffer.  With the heavy I was getting kaboom and bolt bounce. Sometimes a dead trigger. Switched to a 3.5 buffer and it's smooth now.

I run the Bianary trigger however.

That's odd. What buffer? Solid mass makes sense for BB but you would think the heavier the buffer the more dead blow effect. Does 3.5 oz kill on recoil with a can?  I ran my BFSIII in my other ar9 that day too but unsuppressed. Did not get to run it for testing as much as I wanted so I've waited to say anything about it's performance. The BFS III would run but you had to really time it or you'd get hammer follow. I want to go lighter on the buffer and run it suppressed but time and the ECHO kabooms stopped that for the day

Spikes buffers the ST-T2 is what I use.  They are powdered tungsten.  I bet it solves your problem.

I had one kaboom with the heavy buffer I attributed it to bolt bounce. I was getting hammer follow issues as well until I switched to the 3.5 buffer.  It has moving weights in it not sure what actual brand it is. My thought was the carrier was able to be faster with the lighter buffer.  

It's worked for me.

First, an update on my Octane 45 suppressed set-up: Success. I went through two mags in a slower-paced-firing between ECHO Mode and semi. 100% function. Then I did three full-stick-mag-dumps in ECHO Mode and it ran flawlessly! I let it cool off for a bit, then took the can off and went through one mag at a slower pace in ECHO Mode. Ran fine.

My upper build:
X-Products side charging upper standard 5.56 length ejection port
QC10 5.5" 9mm barrel
QC10 colt 9mm bolt (15.3oz vs your NFA's 14.3oz)
Octane 45 HD

Same MVB stock as you. A few more theories on your build. My bolt is one ounce heavier - maybe that makes all the difference? Also, maybe the smaller port door on your upper isn't allowing gasses to escape fast enough causing harder blow-back? I have no education in fluid mechanics though, so maybe the port door thought is a garbage theory . And lastly, see if it happens with your Octane versus your YHM.

I also think it might be the barrel length too 5.5" vs 8" . My NFA mod bolt weights about 16.7 oz so now I dont know I have another bolt the weights less, so on the next outing I will try it. I have my other SBR upper which is basically the same setup as you (Gibbz SC upper and 5.5" QC10) It runs my Octane 45. that will be a something I will try too. Thanks for the update. When its run in ECHO its awesome!

my choice in stocks kills that idea for me.  Would the tungsten still act as a dead blow hammer?