9mm AR Build Questions

[toastie]

I have been kicking around the idea of an AR in 9mm and I have a stripped lower already. Is it as easy as pinning on a complete upper in 9mm and getting a conversion block for the mag well? Any pros / cons to glock or colt style mags? Should I just get a new AR9 lower? Any pros /cons to building as a pistol or rifle length barrel?

TIA

[thor0812]

Personally, I prefer dedicated pcc lowers and glock mags. The mags are plentiful, and so are the parts. I have a carbine length I built using an fm9 upper and stripped fm lower. 16" barreled 9mm is stupid quiet and a blast to shoot

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[blaqkpanda]

The thing you're going to have to look at with choosing uppers and lowers aside from mag compatibility is how the last round bolt hold open (LRBHO) operates. The QC10 has it in the lower and was unreliable for me. I switched to an Aero receiver set that has the LRBHO as part of the upper and it's been 100% reliable. Another thing to keep in mind unlike other ARs is that your barrel length is measured from a different datum point in the upper receiver than a normal AR so you need a shorter handguard with your barrel length choice. I made this mistake myself.

[andre3k]

I have Glock and Colt style 9mm lowers and I prefer the Colt. Glock mags swell a bit when you load them. Some tight tolerance Glock lowers will have sticky mags that are hard to drop. I've never seen this issue with the colt/uzi mags. And the Colt sticks look cooler, IMHO.

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[imyomama]

i built one using the hahn drop in block , you basically load it as you would a magazine , as the mag catch clicks , you use an allen wrench to secure it ... it uses colt mags and has ran flawlessly for me for a decade . it uses colt mags and has last round bolt open feature as well . i run it with a tros barrel , 5" with built in 3 lug machined into it, not a screw on.
pull the upper, few turns of the allen and it's back to rifle calibers in just a minute.

the cons. you have to make sure that you check the allen screw once in a while as i've had to re-tighten it a few times .. it never hurt functionality and you feel it long before it becomes an issue.
Also if you run drills and drop the mags , they spew rounds some times.

if I was to go dedicated , i'd just go cmmg resolute ... basicallly a radial delay blowback 9mm upper that uses 9mm magazines that look like 556 mags and work in standard lowers.
[ame="https://www.youtube.com/watch?v=JG6L6QrnYZA&ab_channel=BlokeontheRange"]Rotary Delayed Blowback: CMMG 9mm AR15 upper on WWSD 2020 lower - YouTube[/ame]

[imyomama]

or you could go banshee if the ATF pulls their heads out of their ash. [ame="https://www.youtube.com/watch?v=9I2_CokEsAY&ab_channel=TacticalConsiderati ons"]CMMG Banshee 9mm Best AR9 Available - YouTube[/ame]

[stxhunter]

I have a 5” 9mm banshee on my SBR lower. It takes pmags with a conversion making them dedicated to 9mm use. I initially experienced some feeding issues, but it seems like the problem has resolved itself after a hundred rounds or so.

Fun gun to shoot and one ill be shooting a lot more going forward, wish I had purchased it sooner.

[RifleBowPistol]

I would do as much research as you have time to do. There are quite a few variations of 9mm ARs. As for the Colt, and Glock style mags, I have not found any Colt mags, Glock are easy to find. Glock style mags and lowers are easy to find, now factory Glock extended mags, are not always easy to find. I have aftermarket extended Glock style mags, and stock size factory mags. So far, I have not had any issues with either the factory or the aftermarket mags I bought. There is a brand, I read about that many people have had problems with, I stayed away from that brand. I don't remember what brand it is. If you do some research on the subject you should be able to figure it out.

As for the AR9 conversions, I would assume they will work like most other conversions. Probably some work, and some will give you problems, but I know nothing about the conversion kits, I did not find out about those, till after I bought both of my pistol caliber carbine lowers. I know I would have still bought specific pistol caliber carbine lowers. If you could only afford one guy and wanted to switch it back and forth, you would need to do some research on the various conversion kits. I have not experience with those, not much help there.

Most of the AR9 stuff is going to be blow back bolt design, there is also the delayed blow back design, then there are some gas operate bolts also, and I think there are also gas piston designs. I would almost like to do a delayed block back design or maybe gas operated gun, but not sure there is much advantage to either, there are some, but are the worth going that route. The bolts and the barrels are definitely harder to find. Then not having a gas block and gas tube would get nice, less pieces, simpler design.

My plan has been to get a JP Enterprises bolt, buffer kit, they make a short travel bolt. Since the pistol rounds are much shorter than rifle rounds, there is no reason for the bolt to travel so far back. But being based off of the AR15, the bolts of pistol caliber carbines still travel the same distance as a common AR15. That slows cycle rate down, which really is not important. I don't like the idea that the bolt travels a lot farther than it needs to.

Then when it comes up uppers and lowers, there are pistol caliber uppers that are designed for last round bolt hold open, and then lowers that are designed for last round bolt hold open. When either the upper or the lower says it is designed for last round bolt hold open, I have found it means that part will has a modification that will lift the bolt catch when the mag is empty. If you do like I did and get an upper and a lower that are both designed for last round both hold open, they both have components to lift the bolt catch to hold the bolt open. So they interfere with each other, so you have to remove one or the other. I found the upper piece of wire they use in the upper worked better than the design of last round bolt hold open, that was in the lower I bought. So I left out the piece that was in the lower for the last round bolt hold open. The gun works great. If you bought an upper and a lower from the same company, you should not have to deal with the redundant last round bolt hold open mechanism. I wanted an Aero upper and a Juggernaut lower. I am happy with the pieces I chose.

The more you search on the subject, the more options you are going to find. I found all types of crazy stuff in my searches. I have done one pistol caliber carbine, want to do at least one more, in 9mm. That would be a gun, I could burn through a lot of ammo with. I think an AR9 would be about the most fun version of an AR for plinking, and could be used for hunting at closer ranges, with the right ammo.