I know that you should use a 9mm hammer with a 9mm upper. But can you use a 9mm hammer with a 5.56 upper?

I don't see any reason why it wouldn't.  Try it and see if it works.  Post your results.  You are not the first to ask about that.  It would be good to hear from someone who has first hand data.

I have used the 9mm hammer with the 223 and it worked fine for me.  I only switched to a different hammer when I bought the rLL as I wanted a select fire kit and that required using the M16a2 FCG parts...

but...

I had no problems with the 9mm hammer with 223...

I have been told numerous times that the 9mm hammer works fine with the 223.  But I never have gotten around to getting a 9mm ar so, there, my .0000001 cents worth


Mike

It works fine.  I have a 9mm hammer installed in my lower and I switch the uppers (9mm & 223) without any problems.  Just DON'T use a 223 hammer if you are going to install a 9mm upper.  It will bind and screw everything up.

Quoted: It works fine.  I have a 9mm hammer installed in my lower and I switch the uppers (9mm & 223) without any problems.  Just DON'T use a 223 hammer if you are going to install a 9mm upper.  It will bind and screw everything up.

Well now... don't use an AR-15 223 Hammer with a 9mm upper!  I use a 223 M16 hammer with my 9mm upper and it works fine.  Just remember that you need to remove the spur from the M-16 hammer to keep the alphabet soup guys happy!  The difference between the AR-15 and the M-16 hammer is the extra notch cut into the striking side of the hammer...  M-16 hammers are one smooth surface while AR-15 hammers have a notch.  Ramping your 9mm Bolt will allow you to use a normal AR-15 hammer (or so I understand).

Good Luck!

One more for successful use of a 9mm hammer with a 5.56 upper.

DPMS hammers are rounded so they work with 9mm.  

DPMS hammers have a different radius on the bottom that caused my otherwise bushmaster to double tap. If you want to use a DPMS hammer, I'd advise using all DPMS lower parts, no matter what the caliber.

Quoted: DPMS hammers have a different radius on the bottom that caused my otherwise bushmaster to double tap. If you want to use a DPMS hammer, I'd advise using all DPMS lower parts, no matter what the caliber.

Gots to call horse hockie on that remark.... AR parts is AR parts.  I would venture your doubles were from lack of disconnect engagment.  I am using that style hammer fron the funshow, a long time ago, and I know the rest of the trigger group were not from a "manufacturer", I think they was funshow also.

The DPMS hammer had a distinctly visible different radius on the bottom than either my original bushmaster 5.56 hammer or my new RRA 9mm hammer. The bushmaster and RRA hammer were identical on the bottom end.  I might have it backwards, but the DPMS hammer was more perfectly round, with the others having a flat between 2 smaller radii, or an elliptical shape.

ALL the other 5 DPMS hammers in the order that my dealer buddy ordered directly from DPMS looked EXACTLY the same.  We didn't bother checking out the other lower end parts. Maybe I will one day.

You are correct that the disconnect engagement was causing the double taps, but I think it was caused by the hammer hook being in a "out of spec" position due to the funky radius.  There is very little disconnector engagement with that setup.

I'm not sure what you're getting at by "funshow" parts, but I'd advise against using DPMS hammers without the other corresponding lower end parts, because, at least in my case AR parts are definitely not all the same.  There have been many people who report the same effect, so I'm guessing that is the cause.

Oh, and by the way, my DPMS barrel assembly, which shoots great, will not take a front sight base from any other brand that uses taper pins. I tried to retrofit a chopped FSB for use with a low mount optic on a flat top, and the DPMS taper pins spacing was off to the tune of .030." Of course, you can say that these are drilled for each specific barrel, but none of the others I've looked at had that much variance.  All other rifles I've seen have been interchangable. I had about 15 USGI FSB's to use for comparison, by the way.  AND my DPMS FSB isn't on straight. THAT particular fact took a long time to diagnos and confirm, because it's just a hair, but enough to require my windage to be set WAY right.

Don't get me wrong, my DPMS upper shoots great, but their specs aren't the same as mil-spec, or anybody else's spec.  I know it's wordy, but I think the evidence supports my point.

Should a 9mm hammer work with a ramped carrier?  I have a preban Colt 9mm upper which shared time on my (formerly) preban lower w/ 5.56 AR hammer.  

I just built a RRA lower w/ 9mm hammer, and the disconnector wouldn't engage when hand cycling for funx check .  It worked fine when I put a 5.56 upper on it.  I swapped the 9mm hammer for a regular RRA 5.56 hammer and it worked fine too.

Is this normal?

Can't help you with that ehay. I don't  fully understand the benefits of ramping, and what it's really supposed to do other than relieve "overcocking." (Anyone, feel free to correct me if I'm wrong here.)  I'm not sure if that is an issue with the 9mm hammer or not.  

Seems to me it's possible that the ramping might, I say might, be preventing your bolt from cocking the 9mm hammer fully. ???

M60Joe is your man for that as he performs the machining.

Quoted: Can't help you with that ehay. I don't  fully understand the benefits of ramping, and what it's really supposed to do other than relieve "overcocking." (snip) ...it's possible that the ramping might, I say might, be preventing your bolt from cocking the 9mm hammer fully. ???

I think that's exactly what is happening.  If it prevents the 5.56 hammer from overcocking, it must be preventing the 9mm hammer from cocking enough.  It's enough for the hammer to engage the sear, but not the disco.

The 5.56 hammer is back in now, and I'll test it tomorrow at the range.  If it works, I'll probably just leave it alone.  Sorry for diverting the original thread...

-E

Don't use an old Colt SP1 style hammer with a 9mm upper, that will bind up and be a serious pain to fix.  Ask me how I know...

how do you know?

Quoted: how do you know?

I  too was crazy enough to try it, then I put the "round top" style  hammer in, of course it was laying on the table at the time.