How importatnt is the 9mm hammer? I see lots of people talking about it. I know my RRA 2 stage hammer  hangs up on the bolt carrier. I talked to Warren from Ameetec and he said they use regular hammers. I cant see how a regular hammer will hit the pin and push it in far enough to hit the primer. When I hold the carrier in my hand with a regular hammer it doesnt go inside the recess enough to make the pin pop out the end. Maybe the hard smack compresses the spring?
I dont know shit about 9mm AR's but I am slowly poecing one together to go with a Gemtech trinity can I have on the way. Here is my set up so far.

Ammetec upper and 7" barrel
YHM free float
Ameetec bolt carrier group
Hahn top loading mag block with BHO
SBR'd lower
9mm buffer and spring

I just need a few more parts to make it happen . Any advice on the trigger issue or an explanation of the difference would be great.

if im not mistaken the notched hammers would hang up on the bolt/carrier

I hear that term notched a lot. Is it the step on the bacside of the hammer?

If the Ameetec 9mm bolt is ramped, you can use a regular semi, notched hammer.  See the 9mm FAQ tacked at the top of the forum for details on the bolt and hammer.

For semi use, a 9mm hammer from RRA or an M16 hammer (no notch on back) is usually required.

I have a question along the same lines... I've read 9mm FAQ, as well as many other posts, but the whole "ramped bolt" and "notched hammer" thing is still a little unclear to me.

I want to build a 9mm SBR; if I go with ramped bolt, can I use a regular hammer? Or vise-versa - if I get a 9mm hammer (RRA for example), do I not need to worry about ramping my bolt? To be a little more specific, if I go with RRA 9mm bolt, will I also need to get their 9mm hammer? Is RRA 9mm bolt already ramped? Also, will RRA 9mm hammer work with their NM 2-stage trigger? Basically, what I want is a dedicated 9mm lower (and upper, obviously) with a 2-stage trigger.

Quoted: I have a question along the same lines... I've read 9mm FAQ, as well as many other posts, but the whole "ramped bolt" and "notched hammer" thing is still a little unclear to me. I want to build a 9mm SBR; if I go with ramped bolt, can I use a regular hammer? Or vise-versa - if I get a 9mm hammer (RRA for example), do I not need to worry about ramping my bolt? To be a little more specific, if I go with RRA 9mm bolt, will I also need to get their 9mm hammer? Is RRA 9mm bolt already ramped? Also, will RRA 9mm hammer work with their NM 2-stage trigger? Basically, what I want is a dedicated 9mm lower (and upper, obviously) with a 2-stage trigger.

If you use an RRA 9mm hammer, you do not need a ramped bolt.  Most 9mm bolts are not ramped from the factory.  You could use an M16 hammer and would not need to aquire a ramped bolt or  9mm hammer.  I don't know about the 2 stage trigger.

While I don’t have RRA NM trigger but from the looks of the NM hammer’s disconnector and trigger notches, I would say the RRA 9mm hammer will not work with the NM trigger assembly.  Perhaps, someone who has both the 9mm hammer and NM trigger assembly can confirm that.  As fq1234 said, the RRA bolt does not come ramped.  With 9mm hammer, ramping is not required since the hammer does not get reset until the bolt travels rearward for approximately 1.5”.  If you go with a neutered M16 hammer, ramping is recommended since the hammer will reset with less than .5” of bolt travel (according to m60joe).  This increased throw-back speed will/may put undue stress on both the hammer pin and holes causing the pin to break prematurely and the pin holes to egg out.

So if RRA's 9mm hammer is not compatible with their NM 2-stage trigger (hopefully someone could confirm this, or otherwise), can I go with a "regular" 2-stage trigger setup - whether it be RRA, LMT, or even CMC - and just have the RRA 9mm bolt carrier ramped? Sounds like it should work...

Since the NM trigger hammer is designed for semi auto 5.56 bolt carriers which are ramped and ramped 9mm bolts have similar profile, I would think it will work with NM hammers and for sure with notched 5.56 semi hammers and neutered M16 hammers.

i have a colt bolt but it will not work with rra's 2 stage,  it will lock up.  if rra's is a true clone of colt it will not work either.  the hammer noth locks up on the little notch in the bolt.  ive done it!  though once i ramped the bolt to take out the little notch it works fine.

I recommend using the standard fire control unit.  As long as the trigger pull is smooth with a clean break and a fair amount of overtravel you can shoot extremely well and reliability will never be a problem.

The standard round hammer should just be the default hammer for all AR-15 rifles.....toss the notched ones, forget ramping your bolt.......it's that simple.

Match triggers are problems waiting to happen.  They belong on match rifles.......not assault rifles.

Quoted: I recommend using the standard fire control unit.  As long as the trigger pull is smooth with a clean break and a fair amount of overtravel you can shoot extremely well and reliability will never be a problem.

+1 on that.

If you are going to have a dedicated 9mm setup, I would opt for a 9mm hammer.  It's much cheaper than ramping the bolt.  At $15/9mm hammer, why take the chance of egging out the pin holes with a neutered M16 hammer?  Sure the added throw-back force on the M16 hammer may not harm the hammer pin or holes on a semi only FCG; but again, why take the chance in order to save $15 bucks on something that cost hundreds?

Here' some more input.
The ramped bolt aleviates the "trigger slap" effect from the bolt hitting sqaure onto the hammer. It is smoother shooting.
With a ramped bolt you can use a standard AR15 223 hammer.
I asked RR if the NM trigger set-up would work with a ramped 9mm bolt - they answered they didn't know - I have not tried.
The NM trigger set-upwill not work, from I was told, with the std. 9mm bolt.
One issue you have not addressed it the bolt catch release - the 9mm uses and extended one, but one of Hahn's newer block can use a std bolt catch release - check with Hahn.
My 2 cents..

I have heard about the trigger slap with neutered M16 hammers with unramped bolts but not with 9mm hammers. My RRA unramped bolt with 9mm hammer runs just fine and even works with 5.56 upper.