I have a Stoner 9mm upper from Midway and am running a 5.4 oz 9mm buffer. The gun runs 100% on brass cased ammo, but experiences numerous ejection and issues with steel cased ammo. If I tap\rack\bang I can keep shooting, but I'm getting malfunctions around 20% of the time. What I think is happening is I'm not getting enough dwell time and due to that I'm getting weak ejection with the spent case staying in the action, causing the malfunction. 

I've seen this exact same malfunction with 22lr, and it's always cured by faster ammo. I also had a simiar issue with a NIB 9mm Glock that cured itself within the first 500 rounds by breaking the gun in with hotter ammo. I tried this with the 9mm AR upper and it's still experiencing the malfunctions. My question is has anyone had this issue, and can it be cured by going to a lighter buffer?

Sorry, can't help much. I built a Colt 9mm on a Colt stripped upper and lower. I used all Colt parts from Brownells and their buffer shows 5.5 ounces. The first time I shot it, it had a few malfunctions that I quickly discovered were due to my laziness of not properly oiling the innards. I basically started it dry. - After dropping in some CLP, it ran 100% and still does..suppressed or not, whatever I feed it, it pews out with no issue.

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Originally posted by SgtGold:
What I think is happening is I'm not getting enough dwell time...

Likely too much. Inelastic steel cases are sticking to the chamber walls, slowing ejection, bolt travel and everything down. At least that's my theory.

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Originally posted by cas:

quote:

Originally posted by SgtGold:
What I think is happening is I'm not getting enough dwell time...

Likely too much. Inelastic steel cases are sticking to the chamber walls, slowing ejection, bolt travel and everything down. At least that's my theory.

By dwell time I was referring to the amount of time the bolt spends in the rearward position. So, in a way your pointing out that the steel cases are slowing down the bolt may be correct. The question still remains as to if a lighter buffer will restore enough of the bolt speed to get the rifle to run on steel cased ammo?

Could possibly be the complete opposite of what your thinking... the "sc" stuff may actually be hotter. I would check the "sc" casings to see if there are any extractor swipe marks. 5.4 oz buffer is considered light for a 9mm setup. Most are running at least a 7.5 oz (some even running 10 oz) extended buffers with a standard carbine (with some even going up to a 308) spring to slow things down.

quote:

Originally posted by SgtGold:

By dwell time I was referring to the amount of time...

I thought you were referring to the amount of time the case stays in the chamber once the hammer drops. The reason they have heavy bolts, buffers and springs,to add as much as possible to that on blowback guns.

These are really in the end incredibly simple blowback guns. Me personally I would do the normal tests...one round does it eject and lock back. Where did it go. Two rounds etc. Then adjust the buffer. I can't say if your ammo is hot or not, so its not clear to me which way to go. I don't shoot steel case.