Build Your Own 9mm AR-15 Lower
Published by Ninth Stage March 2nd, 2006 in Guns.So you want to put together your own 9mm Parabellum AR-15 lower. It’s about time, everyone is doing it.
It’s a very simple process, we’ll go through the steps here.
Step 1 - 9mm Hammer
Starting with a stripped lower receiver and a parts kit you’ll assemble* it in the normal fashion with the exception of the hammer. Instead of the standard hammer that came with your parts kit you must install a 9mm AR hammer.
If your lower is already assembled just tap out the .156″ dia. hammer pin and replace the standard hammer with a 9mm hammer making sure the legs of the hammer spring rest on the top of the trigger pin. Reusing the hammer spring is fine.
Step 2 - 9mm Buffer
A 9mm buffer is heavier than a 5.56 buffer. A 5.56 buffer weighs just under 4 ounces and a 9mm buffer weighs about 6 ounces.
The AR-15 in 9mm is a blowback operated semi automatic. The action is not locked in battery at firing and the reciprocating mass has to be high enough to keep the action closed until the chamber pressure has dropped to a safe level.
Another part of the puzzle is spring pressure. The buffer spring helps keep the action closed and also stores energy for stripping a round from the magazine and returning the bolt to battery.
All this means that it’s wise to use a 9mm buffer in your 9mm AR. And unless you have a collapsible type stock you’ll also need to install a spacer in the back of the buffer tube. The spacer changes the internal length of your long buffer tube to a short, CAR type, length.
Step 3 - Magwell Conversion Block
This is an obvious one, those skinny mags will just rattle around in that big 5.56mm mag hole, that’s why we need a mag block. This is the Rock River Arms type Mag Block. You can see the feed ramp and ears in the front and the hook shaped extracter ejector towards the rear of the block.
Most mag blocks will just slide in from the bottom of the magwell. Slip it past the mag release until the mag release snaps back into the slot in the block.
On the RRA type mag block there is a small hole with a screw inside. It’s at the front bottom of the block, you can see it from the bottom when it’s in place in the lower. Turn that screw clockwise 1/8″ hex key until it’s firm.
The screw pushes two opposing pins out to the right and left locking the mag block in place. With the mag block locked in place your magazine changes will be a breeze, the mag comes out but the block stays in. (I didn’t know about tightening the screw until I was complaining to Say Uncle about the mag block coming out with the magazine, he told me to “tighten the screw.” I was chagrined that I didn’t figure it out all by myself, but in my defense I’ll note that the mag block did NOT come with instructions.)
That’s it, you now have an AR-15 lower in 9mm, all ready for an upper.
*Illustrated lower receiver assembly instructions are posted at AR15.com.
24 Responses to “Build Your Own 9mm AR-15 Lower”
- 1 Trackback on Mar 3rd, 2006 at 1:14 am
What’s the purpose of using a 9mm hammer? My dealer recommended just using the .223 hammer that came with it.
I believe that, considering the shape of the 9mm hammer, it’d designed to enhance reliability/increase cyclic rate by lowering the reisistance to the bolt’s rearward travel. I’ll check it out later.
The bolt on a Colt pattern 9mm bolt and a 223 bolt are different. You should not use a 223 hammer in the picture you have shown above with a Colt/RRA/ASA 9mm bolt. A DPMS brand hammer is not notched like the 223 hammer you have shown and can be used. The 223 hammer/9mm bolt combo will cause excesive wear and possible egg out your receiver. If you insist on using a 223 hammer, you should have the 9mm bolt ramped by someone like M60Joe.com or a local gunsmith. The ramping will allow the bolt to cock the hammer smoothly.
9mm guy -
I thought I mentioned using the 9mm hammer in the picture above. The picture was meant to point out the difference between the 9mm and standard 5.56mm hammers.
Yet you mention the notch. The notch in the face of the hammer is the same notch you will find, by following the link above, in an RRA 9mm hammer. In fact, the smaller hammer is an RRA hammer.
I can speculate why the notch exists. Perhaps to give the hammer a little extra rearward velocity or to retard the bolt slightly.
If that is not what you meant, please explain.
I bought the 9mm upper without a 9mm hammer, and the notch in the .223 hammer is catching the 9mm bolt and keeping it from closing. I’ve been doing some research and there are several options to keep this from happening.
1. “Ramp” the 9mm bolt so that it doesn’t catch on the .223 hammer notch. Downside: Can still “egg out” the hammer pin hole on the receiver. Permanently altering your 9mm bolt.
2. Replace the .223 hammer with a 9mm hammer. Downside: Difficult to switch between .223 and 9mm uppers.
3. Replace the .223 hammer with a DPMS-brand .223 hammer or a modified M-16 hammer (with the tail cut off for NFA compliance). Downside: Can still “egg out” the hammer pin hole on the receiver. NFA complications for the M-16 hammer.
Ravenwood - Thanks for the added info from someone who’s tried it.
Why does it “egg out†the hammer pin hole when you use a .223 hammer - even one that works like the DPMS hammer?
4. Buy a stripped 9MM lower receiver from RRA or CMMG and build a new 9MM lower. It will probably wind up a whole lot cheaper than trying to make it work and less the aggravation. $119.00 for lower and $60.00 for internals.
Olympic Arms does make a 9MM/223 combination that uses the same .223 hammer. But they use a totally different setup for the 9mm with different a magazine.
Why does it “egg out†the hammer pin hole when you use a .223 hammer - even one that works like the DPMS hammer?
Because of the way the bolt travels on the hammer. It creates more stress on the pins. Should the stress damage the pins (bend them slightly) they will start to egg out the holes in the receiver.
The only two ways around this, are to either use the 9mm hammer, or to have the bolt ramped. (ramping the bolt changes the way in which the hammer rides it.)
Thanks for the info Sully.
hey guys. can anyone answer me what the notch in front of a ar15 hammer is for? not the rear notch. (on a m16 hammer) just a plain ar15 hammer. i have one with no notch. i have seen my buddys ar with a notch. what is it for? anyone.
also. will a plain ar15 hammer with no notch in front. work with a 9mm upper reciever. rra 16″ upper.or do i have to ramp the bolt.
kiki - The consensus is that a plain faced AR15 hammer should not be used with a 9mm upper. Other than that I just bought the stuff I needed from RRA. The hammer is not that expensive and not that dificult to remove and replace.
Re the notch, I’ve seen all kinds but I don’t know. Maybe someone else does.
thanks,marc. i have everything but the upper,and the hammer. who do you know has the cheapest rra uppers on the web. i have a complete reciever, vw hytech mag block and 8 uzi mags . waiting to play. p.s.
anyone who uses uzi mags, finds out the hard way that. the mag follower is at very sharp angle. and sometimes it wont strip the last 2 or 3 rounds. very easy solution to fix. take apart uzi mag. take follower and heat the sides. straighten mag follower lips, perfectly stright. up and down. re insert follower and boom your problem is fixed. i had 2 that i needed to file down some areas to reinsert, but it was forth it. thanks again
Kiki, my suggestion and a request for more info are here on the front page.
Hi Guys,
I own a Colt M16 (original machine gun, not conversion). It has the machinegun hammer. Is this hammer O.K. to use with a 9mm machine gun bolt? Will it damage my receiver?
Thanks
Mark
IIRC You might need to lengthen the opening in the bolt where the hammer goes. I’d check out Colts select fire 9mil setup before I tried it what with the current value of your registered (right?) receiver.
Excellent step-by-step — thanks! I’ve been considering setting up a 9mm AR as a plinker.
Max, you’re welcome. Mine is a blast as a plinker. I like shooting the pistol caliber plate racks at our range with it. For a hoot we set bowling pins up at 100 yards and plink away - quite successfully too. Buy extra ammo though ‘cuz everyone will want to try it!
Will the 9mm hammer work for .223? Or is it not heavy enough to set off the primer in the .223 case?
If it would work, then not much problem with swithcing uppers.
Thanks.
YES it will but the best choice to go with from my experience in the army is use th .223 hammer. the basis is that if you want a 9mm assault rifle then use a 9mm hammer. But if you want to reach out and touch somebody 700 meters away then go with the m4 carbine,m16a2,or the semi auto ar15, dont go swapping out parts for other weapons. unless you are prepared for the headaches that will follow. HOOAH!
Glad I found this page, I’ve been having serious problems with my ASA 9mm upper. Now that I see the buffer, spring and hammer need changing out, I’ll go that direction before I toss it up for sale.
The largest problem I’ve been having is rounds slamming in the mag block on their way to the feed ramp, thus damaging the nose of the bullet.
Sometimes the round will go straight up and jam part way into the chamber, I have thought about hollowing out part of the feed ramp like what is done on the pistols but it also occured to me that the barrel could use that same only less of it.
I’ve broke 1 hammer pin and the new one I installed constantly walks out, I do a lot of bumpfire mag dumps and most of these problems are happening when I’m firing normal.
Guys, what can be done to implement a bolt lock back feature into the lower so that it locks back on an empty mag? I know that the factory guns have it, but what about a conversion? Where and who makes the replacement latch if it’s possible to put one into a .223 lower?
Greg,
They make a bolt catch for 9mm, but they are prone to breaking. I would recommend using the Hahn Precision bottom installing mag block or the RRA new style mag block with C-Products mags. They utilize the standard AR-15 bolt catch very well. C-Products are making all of the new Colt mags anyway, and if you buy from them they are a lot cheaper. I have 5 of the 32 round mags and they all work flawlessly bolt catch and all. I also use the Hahn bottom installing mag block.