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How Do I Build a 9mm Upper?


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I decided a little while ago that I want to turn a spare lower into a 9mm SBR. I am going to have a trust drafted, transfer the lower into that, and then do the paperwork to turn it into an SBR. That part I have pretty well covered.

What I am having a little trouble with is the 9mm upper. I have been reading as much as I can about it, and I have yet to find a simple set of build instructions. I've read about all the different mag blocks, ramped BCGs, 9mm buffers/springs, &c... But I'd like to see if I've actually figured out the basics.

A 9mm upper is blowback, so there's no gas system. You can get upper receivers with no gas port. So, is it really the case that all I need for the upper are (1) receiver, (2) 9mm barrel, (3) handguard w/ barrel nut, and (4) 9mm bolt/carrier (ramped if I want a standard AR15 hammer)?

Am I missing anything? Is it that easy? Buy it all, torque the barrel to 75lb.ft. and go? I've seen gas-clamp rails used on some builds. There's no gas ports to cover, so is that just to put the front sight on the barrel rather than the handguard?

Lastly, it seems like a lot of the 9mm sources have dried up. Spike's doesn't have a lot of dedicated 9mm parts on their site anymore, but there seem to be a lot of variations of the Rock River Arms stuff floating around. Any other suggestions for parts sources if I want to build it myself?


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I did it the easy way. I bought my 9mm upper from JP They have a complete upper and all the other things you will need. Earlier 9mm were not reliable this one is. It is a blast to shoot and

works every time. It shoots minor loads,silver tips and my open 9 loads.(170pf in the pistol and 191pf in the carbine.)

What a hoot to shoot :cheers:

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  • 2 weeks later...

Questions first? Have you decided on barrel length? Are you going to run it suppressed? What length handguard are you going to run?

As for the build, if you have put together AR's before its really no big stretch, Actually it's probably a little easier. The fact is you dont need a dedicated 9mm upper at all. A normal 5.56 upper will run without any issues and they are far cheaper. Don't worry about the gas tube hole as its not an issue. If it makes you sleep better fill it with red RTV and forget it. You can easily add a shell deflector to a 5.56 upper check out M&A Parts. As for 9mm bolts Brownells carries a CMMG bolt that functions very well or you can get a RockRiver direct from them. The CMMG is already rammped sufficiently. I have run both and prefer the CMMG. DO NOT BE AFRAID TO POLISH. it may seem unnecessary with all the spring weight involved but it really does help. I polish heavily the bolt travel surfaces, the feed ramps, the chamber mouth and the breech face. This is not where the polishing ends. I have found that the feed lips on almost all 9mm ar mags require at the very least polishing and sometimes recontouring. cproducts mags are the most readily available but in my experience normally need work. Your gun may like metalform mags, none of mine do. If you find yours runs them let me know and I'll give you a deal on a couple. ;) be ready to experiment with buffers as well. I recommend starting with a heavy buffer and no buffer extension, make adjustments from there as necessary. (A lot of this will vary if you are running suppressed and what type of suppressor you are using.)

Barrels, I recommend TROS barrels highly. (as I am yet unworthy of posting links you will have to google them) Excellent quality at a great price. Just torque as normal, figure out what handguard you need and clamp it on. Keep in mind ID if you want to recess the supressor under the the guard.

Mag Blocks, Do your homework, know what mags run best in them and if there are reported problems. There are a million of them out there, pick one and be ready to adjust it until the mags are feeding right then locktite.

Important notes: If you are familiar with the ar platform and its bolt carrier group assembly, keep in mind the firing pin on a 9mm bolt is under spring load and the spring is not captured. This means when you remove the retaining pin keep your finger over the rear of the firing pin or you will be searching for the spring in the carpet or worse yet you won't realize that there ever was a spring in the first place. This will result in mysterious failures to fire if you reassemble without the spring... ask me how I know...

Long story short, I say go for it. be ready to tweak and trouble shoot a little, but if you build it yourself you will definitely enjoy it more. Personally next to my .22 SBR my 9mm is by far my favorite to shoot even over my match guns. Shot a couple local multigun matches last year with the 9mm and I'll say running a shoot house stage with a 5" suppressed 9mm is a blast. Don't hesitate to contact me if you have questions or need help. and Have Fun.

This is the last one I built along with the .22 Both suppressed SBR's


Edited by sb3
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I got the 9mm bug about 4 years ago. At that point, I had built all of my AR's so I looked into building first. It was about $20 cheaper to build one than it was to order a RRA ($820 vs $840), so I went with a RRA. Started out with the 16". When my tax stamp came back, I bought a RRA 10.5" pistol upper. The only difference I have had with the SBR is a change in accuracy. My 16" shot about a 2" group at 100yds. The shorter barrel is more like 4-5". It is still a blast to shoot and I'd really like a can for it someday.

It takes some tweaking to get them running sometimes, especially with the magazine adapter. There is a little movement up and down that I had to adjust to get it running. I tried all the different mags and the best are the old Colt/Metalform mags. I had issues with the CProducts (Colt had gone to CProducts at one point - not sure what they are using now). The ProMag ones worked, but I had 2 crack - They were replaced by ProMag BTW.


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