There are a couple different style of 9mm AR15s out there. Used to be as simple as having two variations that were not interchangeable, the Colt pattern and the Olympic pattern. Now there are other variations, but the majority have somewhat interchangeable parts with the Colt pattern.
In the Colt pattern, you are suppose to use a spacer in the buffer tube. I finally bought a proper spacer, but previously I had been using a stack of quarters.
Proper spacer on the left, $1.25 on the right.(My old “spacer”) Looks like the spacer is the same height as 4 quarters, and it has a section to slip into the spring.
My first 9mm AR was a Colt upper on a standard lower using a VM Hytech (sp?) Uzi mag block. It let me used unmodifed Uzi mags, but I couldn’t use the normal AR15 magazine release.
I sold that Colt upper and picked up a 10.5 inch RRA upper. That RRA upper had a very heavy barrel and ended up being much heavier than the Colt 16″ barreled upper. I was not too thrilled about that. Ended up getting rid of that some time ago.
An old picture of a messy desk and the RRA 9mm Upper.
A little while back I saw a guy selling one of the new Colt 9mm uppers that had a flat top for a good price. I bought it and also purchased a Hahn Precision top loading mag block.
The top loading mag block allows me to use the standard bolt catch, and mag release with Colt pattern mags. Installing the top loading mag block requires removing the bolt catch, so it isn’t very quick to install or remove.
So my 9mm AR is a Colt 6991 upper on an old SBR lower. When using a 9mm upper you need to make sure to be using a compatible hammer in your fire control group. I use a no-notch hammer. Notched semi auto hammers will cause the action to lock up and make disassembly the firearm a pain in the ass.
One other change I made was to add a gas tube stub to the front sight base. On a normal AR15, the gas tube prevents the front cap for the hand guards from rotating. On a 9mm or .22 AR, there is no gas tube. Some years back I got from Spikes Tactical some sort predrilled rod sections that you install like a gas tube to prevent your hand guard from rotating. Not necessary, but a nice little little upgrade.
As shown in the picture, I have a KAC RAS quad on it, and I am running an Aimpoint PRO as the optic. The Aimpoint PRO is an excellent lower cost option for a hard use optic. If you shop around you can find them in the $350 range.
I don’t shoot the 9mm AR much, but it is a whole lot of fun. It is excellent for when you are wanting to shoot steel targets at much closer ranges.
In the end, if I were to start from scratch, I would probably either just buy a factory Colt 9mm carbine or perhaps something like the SIG MPX.