I didn't want to sidetrack this thread any further, so I've started this one to keep track of my own issues.

I'm having some issues with my new 9mm upper doubling when suppressed. Runs okay unsuppressed, but as soon as I put the can on it, I start getting doubles pretty regularly.

Barrel: 4.5" Faxon
Can: Trident 9
Bolt: Odin hybrid
Lower: CMMG dedicated (Colt mags)
FCG: stock DPMS (stock trigger, rounded hammer)
Stock: RRA Operator (commercial carbine recv. extension)
Buffer spring: standard
Buffer: custom solid steel, 7.4 oz, 3.95" OAL

I've tried two different loads - both subsonics. First, factory Winchester "Super Suppressed" 147gr, and then my own hotter 147gr loads (last time I chrono'ed them out of a pistol, they averaged 1047 fps). Same behavior with both.

I also tried swapping back to a normal 9mm buffer - one of the regular-length ~5oz ones. Same thing.

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Based on discussion in the other thread, things to be tried this week:

- Going to turn down the buffer to 3.75" OAL. Statements from FM-Products regarding their experience is that 3.8"+ is too long.
- New disconnector spring. Found a spare lying around. Hard to tell for sure, but does seem a bit stiffer than the one that was in there. (Maybe I had a bad spring?)

Might be able to get out on Thursday to try these changes, otherwise won't be able to get out to the range until Saturday or Sunday.

was going to say try a new disconnector and spring. seems like it's related to excess bolt speed.

I don't know much about BB 9mm setups but a quick look shows buffer weights in the 8-12oz range with the 12oz being recommended for suppressed use. maybe bore out the center and put a few tungsten weights inside.

Maybe try a SringCo Red buffer spring.  That is what I run on both of my AR9 builds along with 9mm buffers.  My can is still in jail, so no experience running suppressed.

My first thought is disconnector spring. I had an S3G on a 5.56 where the spring went bad. Liked to double and triple.

I would guess it's not the disco, since it does it 0% of the time unsupressed. You're probably at the ragged edge of reset with that long buffer and with the added back-pressure of the suppressor, you're picking up bolt speed which is why it's now following. Either turn yours down or try an A5 tube or another buffer to see if it solves your problem.

Also check your pins aren’t slipping. Hammer pin slip will cause doubling. I use anti walk on blowback

No luck with the other disco spring and turned down buffer. (Buffer is now also down a quarter ounce: 7.15oz).

One observation I failed to mention before: running with an empty mag in place, it will lock open 100%. So it certainly appears that I have plenty of bolt travel - it isn't short-stroking. It's following after the point where the disconnector should have caught the hammer.

Yes, it would certainly be related to bolt speed, given that it is only happening suppressed. Combined with the fact that it will lock open on an empty mag, it seems to me that the problem is related to some kind of bolt bounce - something is getting the hammer off of the disconnector. While bringing bolt speed down should undoubtedly improve the situation, I am also going to look at a different FCG - it could be that something is on the very edge of spec.

I modeled my buffer after the Spike's ST9X, which is similarly solid steel, and rated at 7.7 oz. I know there are even heavier ones out there, but I figured this would have been sufficient.

Increasing the buffer weight myself right now isn't really an option - I don't exactly have tungsten just lying around.

No problems there - pins are not walking in the slightest.

I will have to look into picking up some other springs to try. The thing I'm wondering here, though, is that increasing the spring strength is a bit of a double-edged sword. Yes, it will resist the bolt moving rearward, but it also means that you can gain more forward velocity when returning to battery.

Can you put your hands on a hydraulic buffer to try out?

Update:

Got a new FCG - thanks @FM-Products - and looks like all is well. Was able to run to the range for a quickie this morning - only ran 10 rounds through it, but all 10 ran 100%.

I had ordered some other buffer springs to try as well, but they didn't arrive until this afternoon, so I haven't even had a chance to do any testing with those yet.

@FM-Products

So I've got to ask, how many bad bolts did you send Brownells?
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Mistakes happen. Brownells is sending a replacement.  As long as its machined correctly,  I'll be up and running.

ETA, you can see the issue, right?

That makes a lot of sense. Does your design side charge?