I'm having trouble with my 9mm SBR, the hammer is following the bolt home, and you know what it does when that happens. It's not a problem with the lower, because it's the same lower I built my first 9mm SBR on several years ago and this lower has never given me any problem. I finished my 5" barreled upper yesterday, and I'm using a Ti-Rant suppressor. I'm using a 5" Adco barrel, ramped 9mm bolt, 9mm buffer, and the Ti-Rant I mentioned. Ammo was 147 grain Blazer aluminum cased ammo. From what I can tell, there's not enough back pressure to completely cycle the bolt enough to re-cock the hammer. Do I switch to a lighter buffer?

Have you function checked the trigger group just to be sure there isn't something wonky with your disconnector?  Blowback pistol calibers can be hard on fire control parts.

If it's not cycling far enough to cock the hammer, I don't know that the forward bolt speed will be high enough for the hammer to have enough speed to slam-fire when it follows.

Do you have functional last round hold-open on this lower?  Does it lock back on the last shot?

If you're having this problem, you might want to just put the "malfunctioning" upper in the safe, as a back up, you know, just in case you need to use that upper.

Yeah, I checked the trigger group just to be sure, and when time allows I'm going to put the 10.5" upper back on just to verify. And I can assure you, it fired again when the bolt went back into battery. It scared the hell out of me, and thankfully I had only loaded eight rounds, and the only reason it stopped firing was because it malfunctioned. I checked the face of the bolt to make sure there wasn't a piece of casing or primer stuck in the firing pin hole, acting like a firing pin, and it was clear.

Back when I originally built this lower I put in a 9mm hammer, you know, one of those bobbed off the back narrow hammers. At the time my bolt was un-ramped. Well I decided I wanted a 10.5" 5.56mm upper to use on this lower too, so I swapped out the 9mm hammer with a neutered M16 hammer, and went ahead and had my bolt ramped by Adco at the same time I ordered the 5" barrel from them. The 10.5" 5.56mm upper has been on that lower for the last two years or more, and I haven't tried the 10.5" 9mm upper with the different hammer and ramped bolt since I made the switch. I certainly never had any problems like that with that lower and the 5.56 upper.

Also, I don't have the last round hold-open feature.

Did you try some hotter ammo ?  That aluminum case is probably a reduced load.

No, since I was using a suppressor I wanted to stick with subsonic loads.

There are lots of other subsonic loads.

Ya I'd start cheap and easy first. Try different subsonic ammo to rule that out as a possibility. Then start working on things like lighter buffer, or lighter spring.

I'm shooting hand loads, using hogdons or hornadys data, the rounds are quiet. 125's and loading some 147's. They are out of a G19 and 10.5" 9 ar upper, using a YHM Cobra. Shooting 125 28 spl out of a 20 lever they're super quiet.
Have a few lbs of TB i will eventually get to. For now it's titegroup.

Don't know if will help you but I'm using an ADCO 5 in barrel, J&P bolt, 8,5 oz buffer, wold xtra power spring on a DDLES Glock lower. I'm shooting reloads of 147 gr at 960 fps with an Octane 45 suppressor and it functions great. The trigger & hammer are ALG-ACT.

Are you using a mil spec style trigger group? Many aftermarket triggers do not work well with 9mm bolts.

The easiest way to check the the bolt interface with the hammer is to empty the gun. Dry fire and hold down the trigger. Pull back on the charging handle (still holding the trigger back).  If the hammer follows the bolt home then the hammer isn't being pushed low enough to be reset by the bolt.

It sounds to me like the hammer might be the problem.  With ramped bolts, I know the notched hammers don't usually work right.  If I understand correctly, the hammer you're using isn't notched but it still could be the cause of the problem.  

Make sure the gun is empty.  Pull the trigger and hold it, then manually cycle the gun.  Did the disconnector grab the hammer?  If not, then I don't think you have an issue with the ammo or any of the action parts.  I think it would indicate a problem with either the disconnector or the geometry of the hammer relative to the ramped bolt.

Is the 'neutered' M16 hammer what you have in it now?  What's done to it other than removing the hook?  Do you have a standard AR15 hammer without the notch that you can put in?  That's an easy thing to try without live fire.

ETA:  FWIW, I have an Adco ramped bolt in my 9mm upper and it works fine with standard AR15 hammers as well as an M16 hammer in an actual M16 lower.  I've also used my 9mm upper with Geissele G2S triggers and I don't remember having any problems.

Just to make sure it's clear, you need a hammer that looks like this:



Not one that looks like this:

Im also running an ADCO ramped bolt with a standard hammer. Runs great. It had an after market trigger/hammer before and it was known to bump fire on its own. I would tell it to stop and it just wouldnt listen. Didnt think that excuse would work if some LEO happened to be at the range.

The hammer in the top photo is what I have. I initially said neutered M16 hammer, but it's a DPMS no-notch hammer. In addition to the Blazer ammo, I fired some WWB 147 grain ammo and some Tula 115 FMJ ammo through it just to confirm my suspicions. Both subsonic loads caused problems, the high velocity stuff did not. I put the 10.5" upper back on and all three loads functioned perfectly. Subsonic loads just don't produce enough back pressure to fully cock the hammer when fired in the 5" barreled upper.

It's not worth risking getting my ass thrown in prison so I just put the 10.5" upper back on. The lower functions like it's supposed to with that upper, and when I test it manually. I took my suppressor off and there's going to be a 5" Adco 9mm barrel for sale in the EE soon. It wasn't as quiet as I thought it would be anyway.

Ok, it does sound like it's short stroking with the subs then.  You could just use a lighter buffer.

That said, these aren't as quiet suppressed as a locking bolt or delayed blowback action, that's for sure.  I have an MP5 clone and when using the same can and subsonic ammo, there is a huge reduction in port noise (felt recoil too).