Today was break in day for it. Ran about 5 mags through with no problems at all (All factory Glock mags.three 33 rounders & 2 G19X mags). That was with 115 & 124gr ammo.

Then I put the can on and went to 147gr ammo and it ran fine for the 1st few then would not cycle. I had to manually charge it after every round. I figured it just doesn't like the can w/that buffer. Took the can off and tried the 147gr. Still not cycling.

Went back to 115gr (Still no can) and still the same problem. I don't see anything that had abnormal metal to metal contact when I took it apart. The Stern mag adapter didn't come loose.

The lower I am using was used on previous builds and has never given me a problem. Running mil-spec trigger as well.

The Bolt is from Stern. The upper is an Aero EPC9 w/LRBHO that I bought new. Running a Strike flat wire carbine spring and Spikes ST-T3 Heavy Buffer.

I doubt it's the ammo or the mags as the mags have always run perfect & I have run Federal & Blazer brass for a long time like most of us.

What am I missing here so I can fix it?

PHOTOS OF BOLT & MAG ADAPTER: https://imgur.com/a/FADJ6oA

I use a Wilson flat wire carbine and buffers from 7.6-8oz....yours is 5.4oz....otherwise without seeing it I have no idea.

That's the buffer weight Stern recommends and sells on their site. I thought must be buffer too until it no longer ran right when I took the can off.

Did you lube the gun when it started having issues?

You can remove the buffer spring and see if it is binding? Otherwise not sure

I did not. When I got home and took it apart, things were not bone dry or even close, though.

I’d get a 9mm buffer and plain carbine spring to try as that combo works for me in all of my AR9’s.

https://www.spikestactical.com/collections/lower-parts/st-9x-9mm-heavy-buffer/

I'm ordering the KAK K-Spec 7.8oz setup. Hope it solves things

I still find it odd that it ran great for about 150 rounds then started acting up

Describe this not cycling.
Is the round firing, but the bolt doesn't move? It fires and bolt moves, but short strokes? That will help in diagnosing.

It fires, but I have to manually charge it to get the next round in the chamber

Is the round that just fired ejecting? Or you successfully feed the round into the chamber, it fires, but the bolt doesn't move properly, so you have to rack the charging handle to eject the round and then it loads the next round as the charging handle goes back home?


Just a few curiosities - the ST-T3 buffer looks like it's 5.5 ounces and the Stern bolt is 15.5 ounces which puts it under the minimum 22 ounces. Also, what is the length of the ST-T3 buffer? If it is the standard carbine 3.25" buffer, you'll want a .75" spacer assuming you have a standard buffer tube - was your trigger not resetting?

It ejects the fired round so when I manually charge, it also ejects the next on fired round from the magazine.

It's the same weight that Stern sells and recommends on their website so that's why I went with that.

It looks like it was meant for 223.

So you’re saying the trigger is not resetting, correct?
It fires 1 round the hammer follows the bcg down and the trigger is dead the next time you want to pull it, correct?
What fire control group is in it?

I honestly don't recall if the trigger reset or not. Mil-Spec FCG.
Installed the KAK K-Spec buffer & spring today. Going to try again this weekend

As long as you got the pcc version, should fix the problems. 4" long buffer (the one you had prior is 3.25") and weighs 9.5 oz which will put you well above the bare minimum of 22 ounces.

The problems are very likely because the bolt travel and weight were incorrect.

Hopefully, it's just odd that it ran fine for almost 150 rounds then acted up

Happy to report that the KAK K-Spec buffer fixed my problem. Bonus round, it also reduced felt recoil.

Thank you everyone for your help

Great to hear!