I want to build a 9mm with a 10.5" barrel, and a Sig brace...That  Uses Colt style magazines.
(I already have the Sig brace, and KAK industries buffer tube)

As far as the upper receiver... Do I have to buy a 9mm upper? Or can I just get a 9mm bolt carrier assembly, and install the 9mm barrel, shell deflector, and dust cover on a 5.56 upper that I already have?

The 9mm magazine block... Do they actually WORK? Or will I be building a jam-o-matic if i use one of those with a 5.56 lower?

Does anything else need to be done to the lower BESIDES the 9mm hammer, 9mm buffer, and magazine block?
Does it use the same bolt catch?
Will the bolt hold open on the last round?

Is the buffer SPRING the same?

Anything else I need to consider, before I start ordering parts?

Thanks in advance.

Thanks for the input!!!

I forgot to ask:

Are there any issues with different types of ammo? (hollow points? +P? etc)

And...Most of the barrels I have found ar 1:10 twist, but a few have different twists... Is 1:10 the optimal?

Tell us how it works out and how well it feeds HP ammo.
I'd be looking for one that will feed the Remington HP as it seems to be under 15 a box now.
I'd also be interested in knowing if it feeds that really fast

forever4 pretty much nailed it with his answers, but I'll throw in my $.02.

On the mags, the old plastic Pro-Mags were often a problem, especially when you packed them full and they expanded on you.

As for the plastic ProMag well adapter, as I said, mixed results. But then you never know if the problem is the actual part (possible) or if the installer didn't get it adjusted just right (also very possible) or if the combination of parts just didn't want to play well together (also very possible).

When I built my first 9mm AR I used all Rock River parts. I had some issues with RR's magwell adapter when I used it in a Rock River lower. I did get it to work with a little fine tuning. I learned a lot from that frustrating experience. But later I installed that same adapter in a Cav Arms "plastic" (actually they are nylon) lower and that thing just ran like a champ. I never knew if it was just the lowers being a bit different or if I had just learned from the first bad experience on how to adjust the well adapter.

9mm AR's are fun but sometimes they can be a head ache when they are first built. Its understandable as you are often using a mixed bag of parts that came from a lot of different sources. Don't ever believe all these companies are producing "identical" parts to some mystical "standard" (a.k.a. MilSpec). While I am sure they all looked at the same drawings when they actually build the parts there are natural variations. Sometimes you get a "stack" problem where these variations add up against you and you get headaches sorting things out. Other times the gun gods smile on you and you end up with this sweet running firearm. Toss a coin.....roll the dice..... But, if you pay attention to detail and read up on what previous builders have learned you can avoid  some of the problems they had.

Best of luck to all the 9mm AR builders out there...

You could get a Quarter Circle 10 Colt mag lower and forget about adapters.

Get a lula loader ASAP, loading those shits by hand is hell on your thumbs



 

KNS pins keep your hammer pin hole from egging out. The blow back 9 m.m. hits the hammer violently, and can break hammer pins and wear out the pin hole in the lower.
Yes, even if you have a ramped bolt and a heavy buffer.

Also put 7 quarters in the receiver extension before you insert the buffer spring and buffer. This eliminates the difference in length between the 5.56 round and the 9 m.m..
The bolt would otherwise go the extra distance and on its return SLAM the BHO. Eventually breaking the BHO in half at the pin hole.

And the other guys mentioned AMMO. YES, buy all you can. The 9 is usable in an indoor range, and is as cheap as a centerfire can be.
Also 9's rock in a SHORT barrel with a can.

Yes...I looked at those, But right now I am looking to build more of a classic Colt 9mm carbine looking thing.

If I do eventually get a QC10, I might go with the Glock magazine lower.

Thank you for these tips.

Ottosear, since I'm pretty much an AR illiterate, and having just completed a 9mm build, can you please explain a bit more about the bolt "slam" and the BHO breaking at the pin hole? I have used the search for BHO and looked at an exploded diagram, and have no clue to what the BHO actually is. Also on the quarter thing, could I just turn a nylon rod to the thickness required?                                                                                                                                                                                                                  
Thanks for your time. cec

Bolt hold open...He is saying the bolt will travel forward with more velocity, and break the bolt release.

Thanks, madmedic!                                                                                                                                                                                                                                                

Ok, trying to get my pea brain wrapped around this. If you add a spacer behind the buffer assembly, wouldn't that cause the bolt to return with even more force?                  
And, I assume, this would only be an issue with the last round?

No, the spacer(spikes makes one), or quarters(I use $1.50), will keep the 9mm carrier & buffer from traveling too far back. The extra travel allows for the bolt to gain too much momentum. This extra force will eventually snap off the bolt catch. When I built my 9mm AR, I was told to add quarters until the bolt would only go past the bolt catch by .25 of an inch. I ended up doing 3/8 of an inch, but it works great. Still on my first bolt catch.

You guys are GREAT! Thank you so much. cec

Another question. How do you measure the bolt overtravel?

It doesn't have to be exact. I pulled the charging handle all the way back until it stopped. Then looked through the ejection port to see the distance between the bolt face and bolt catch. I eyeballed it at first. Then I took a small metal machinist ruler that I had, bent it so it would fit in the ejection port.

Think I've got it!  Locked bolt to rear, charging handle out. Measured left-over distance from locked bolt to end of travel of bolt. Eight quarters = 3/8" of travel.                        
Now understand the bolt-to bolt catch "slam" referred to. Will test fire tomorrow, and if everything goes correctly, will fabricate spacer. Again, many thanks!                          
                                                                                                                                                                                                                                                                               
Tip on loading mags: Position round on mag with slight pressure, tap on hard surface, and the round will pop in on it's own, all the way to the 32nd one.

Yes, but you don't lock the bolt to the back. You pull the charging handle back until it stops & then hold it there. This means the rubber bumper on the buffer is against the back of you buffer tube(max travel). Then you look at the distance from the bolt to the bolt catch. If you put quarters in your buffer before you put you buffer spring in, this reduces the distance your bolt can travel backwards.

I just set my gun on the table with the buffer tube in my gut. Pull back charge handle and hold it. Look in the port and eyeball a quarter of an inch.

Like I said above, I use 6 quarters in my Phase5 pistol tube. Depending on what tube you have, the quarter stack will differ some.

Although the Promag blocks feel very cheap and cheesy, I too have had them work perfectly. If you already have a lower the Promag block is a cheap way to get started.  You can see how you like the 9mm. If you love it and save some money, then buy a nice quarter circle lower.

I built a CMMG lower up with tons of issues then i bought a CMMG mk9 pistol. This pistols runs and shoots with no problems. i am really lovin this pistol.

OP, the 9m.m. is my favorite for these reasons...

Cheap to shoot, Even 147 subs wont break your budget.
Can use it even if it's raining at an indoor range .
It FEELS good. A .22 LR is fun, but it doesn't have the real big gun kick.
Suppressed, it is pretty dang effective ! The expanding 9's of today are not the FMJ's of yesterday.  
And it opens a new door for registered lower receivers.