9mm AR

[Marine24]

With my son enlisting in the Marine Corps, I'm putting together a couple of ARs starting with RGuns engraved USMC lower receivers. Rather than build both in .223/5.56, I'm planning on building one in 9mm.

I've done some research and seems pretty straight forward. I'm leaning towards the Spikes enhanced magwell conversion block because of the magazine selection and still deciding on the upper.

What isn't clear to me is what else is needed. Based on what comes in some of the kits, they also recommend the following:

Ramp bolt/bolt carrier assembly to use AR trigger
Heavyweight buffer to reduce cycle time

Anything else I'm missing?

[DeusExMachina]

9mm ARs typically use a specific hammer. If you want to be able to use any lower, you need to get a "ramped" BCG. The carrier is the ramped part.

Since 9mm is blow back and not gas impingement, you need a heavier buffer otherwise it'll be overgassed. I have a 9mm buffer, as I switched to a an extra heavy tungsten one on my suppressed SBR. Let me know if you want to buy it off me.

They are pretty simple. The upper consists of a barrel, receiver and BCG. No gas system to speak of. Lower has a 9mm hammer (not necessary with some BCGs, as mentioned above) and a magwell block. The Spikes is probably a good one, but I haven't used it. I use CProducts mags without issue.

[Marine24]

Appreciate the info. Saw a similar thread in the Build Your Own Forum that mentioned the Oly option. I'll keep you in mind on the 9mm buffer. Brownell's has a Yankee Hill kit that includes it, but haven't settled on buying the kit or pieces individually.

[SAnd]

CMMG makes a 9mm "Enhanced 9mm Bolt/Carrier" that lets you use a standard AR15 hammer. The carrier/bolt are one piece. It's ramped also. It has a weight built in so it is about 4.5 onces heavier than the standard AR bolt. I still run a heavier buffer but I don't know if you have to with the CMMG bolt.

I run a bottom loading mag adapter. Some times if I remove it and reinstall it I have to fiddle with it to get it in just the right spot to run. I am told the top loading adapters don't have this problem.

You might check on the empty brass deflecter and ejection port door. I have always had one so I don't know if they are really needed. They aren't expensive, $12 for the deflector and $9 for the door. I cut a standard door shorter for my upper.

Have fun.

[DeusExMachina]

I use a standard upper with full door. I've never seen any reason for the deflector, mine kicks out a few feet at a 45* angle just fine.

[Marine24]

I've seen a couple with the deflector, similar to what the old Rem 597 semi 22s used to have. Odd looking. Expect if you tuned the extractor properly, it would be a non-issue (assuming that is possible)

[ThunderSquirrel]

The 9mm "deflector" isn't for brass, its for gas on the blowback system. If you run it unsuppressed, and shoot right handed, its not really needed.

[DeusExMachina]

I run a standard upper with a F/A gas vent, since the F/A doesn't work with the 9mm BCG anyway. Get a nice puff of gas forward when running suppressed.

[Marine24]

Guess my left handed son would appreciate that.

[Marine24]

Any suggestions on magazines? I'll be running a CMMG 9mm upper with a dedicated 9mm lower. Colt/Metalform mags plus modified Uzi's will work and expect the same is true for Pro-Mag and C Products. I'd prefer the bolt hold open after the final round in the magazine, but not sure if the modified Uzi magazine will do that or not.