AR-9 barrel length

[psalms144.1]

I've got my eyes on an AR-9. The options are 7" or 5" for barrels. I'd prefer the longer barrel, but it's only available with a side-charging system, and I'd prefer to stay with traditional AR controls for simplicity. How much "performance" would I be giving up going for the shorter barrel, or am I overthinking this?

Thanks!

[HeavyDuty]

Would you be happy with “normal” pistol ballistics? I personally am, and prefer my 9mm PCCs as short as practical.

[Jager]

There are a lot of factors involved, starting with what load and bullet weight you're using.

But assuming a generic 147gr projectile at 1066 fps from a 4" barrel - a typical barrel length used in spec'ing 9mm Luger ammo - you'd see something like 1122 fps in a 5" barrel and 1200 fps in a 7" barrel. Not a significant enough difference to not go with one's favorite weapon config, IMO.

[CleverNickname]

I've got my eyes on an AR-9. The options are 7" or 5" for barrels. I'd prefer the longer barrel, but it's only available with a side-charging system, and I'd prefer to stay with traditional AR controls for simplicity. How much "performance" would I be giving up going for the shorter barrel, or am I overthinking this?
Why would a 7" upper only be available with a side-charging system? It sounds like you're limiting yourself to a specific manufacturer for some reason. There's plenty of 7" 9mm AR uppers out there that use a normal CH.

Unless you specifically need a shorter gun for storage, I think a 7"-barreled upper is slightly better since it provides more room for your support hand, and more room on the upper to mount things. Also take a look at this: http://www.ballisticsbytheinch.com/9luger.html

[757_Magnum]

Unless you specifically need a shorter gun for storage, I think a 7"-barreled upper is slightly better since it provides more room for your support hand, and more room on the upper to mount things.

psalms144.1 I second this. My first AR-9 was a 5.5 inch build with a quad rail handguard. Adding sights and a light left little room for anything other than a magwell hold, and I'm a little guy. It might have been better with a VFG and today's slim M-LOK handguards and accessories.

What do you plan to do with it? Home defense, range toy, competition, trainer for a centerfire SBR? My current build is for the latter three, and I will be using an 8.5" CMMG Radial Delayed Blowback barrel and bolt on a standard SBRed lower using their 9mm adapters for PMags. It's a little pricier than a standard blowback build, but it's smoother shooting, has a bolt carrier that can be tuned, and I can train with full size AR mags. If I were to get a PCC for serious use in the smallest package available, I'd probably get an MP5K clone or B&T APC9K if a folding stock is important, or I'd consider the Glock mag version of the CMMG Banshee with the 5" barrel if you want the standard AR controls and don't care about a folding stock.

[Hot Cereal]

I'd go 8 inches. If I wanted a 30 round 9mm with a shorter barrel I'd just put a happy stick in a Glock.

I've stalked this territory for a while and I'm not sure I want to go with a blowback 9mm pistol or carbine. That type of action produces a good bit of recoil from the heavy bolt reciprocation, which I think defeats some of the reason I'd pick a 9mm PDW in the first place. If I make the decision to go this route I'm going to either choose delayed rollerlock and spend some cash, or really research what type of blowback operated design would suit me best.

[HeavyDuty]

For what it’s worth, I built a 5.5” AR-9 with a 4.25” HG a few years ago. I find the grip to be fine. It’s hard to take a pic to illustrate, but...

63332

[olstyn]

I've stalked this territory for a while and I'm not sure I want to go with a blowback 9mm pistol or carbine. That type of action produces a good bit of recoil from the heavy bolt reciprocation, which I think defeats some of the reason I'd pick a 9mm PDW in the first place. If I make the decision to go this route I'm going to either choose delayed rollerlock and spend some cash, or really research what type of blowback operated design would suit me best.

I keep leaning more and more toward the CMMG "radial delayed blowback" option, especially the one based around "conversion" PMAGs, as that would enable buying a 5.56 or .300 BLK upper for use with the same lower as well, and the whole setup would cost way less than HK money unless I'm grossly mistaken. I'm looking at 16" rifles rather than pistol/PDW configurations for my intended use, but RDB for less recoil/smoother operation seems like it would apply equally regardless of barrel length. Like any new/novel tech, though, there's always the question of reliability...

[Hot Cereal]

If I'm spending the coin CMMG wants for that RDB bolt technology I don't want to use converted pmags. I want a lower that takes MP5, Scorpion, or Colt/Uzi mags. Not sure I see the point in the money for the RDB when it uses a 5.56 footprint. If I was using it as a training analog I'd probably opt for the increased recoil/cost savings of the pure blowback operation.

[HeavyDuty]

To be honest, I’ve been tempted to divest my 9mm ARs. My Scorpion fills the need, and if I picked up a Ruger PC Charger I think I’d be covered. I built my first 9mm AR ca. 1998, and they run well, but...

[mrozowjj]

If I'm spending the coin CMMG wants for that RDB bolt technology I don't want to use converted pmags. I want a lower that takes MP5, Scorpion, or Colt/Uzi mags. Not sure I see the point in the money for the RDB when it uses a 5.56 footprint. If I was using it as a training analog I'd probably opt for the increased recoil/cost savings of the pure blowback operation.

100% agreed. I looked over the weekend because apparently New Frontier Arms is starting to make a lower that will take Scorpion mags but apparently making it work with the CMMG RDB system requires some major alterations on the bolt to be done which is off putting.


To be honest, I’ve been tempted to divest my 9mm ARs. My Scorpion fills the need, and if I picked up a Ruger PC Charger I think I’d be covered. I built my first 9mm AR ca. 1998, and they run well, but...

I have a Ruger PC Carbine and a Scorpion and every time I think about an AR9 I ask myself what it would do that those 2 guns don't do and other than be the same ergo and controls of the AR there really isn't anything.


psalms144.1 I second this. My first AR-9 was a 5.5 inch build with a quad rail handguard. Adding sights and a light left little room for anything other than a magwell hold, and I'm a little guy. It might have been better with a VFG and today's slim M-LOK handguards and accessories.

What do you plan to do with it? Home defense, range toy, competition, trainer for a centerfire SBR? My current build is for the latter three, and I will be using an 8.5" CMMG Radial Delayed Blowback barrel and bolt on a standard SBRed lower using their 9mm adapters for PMags. It's a little pricier than a standard blowback build, but it's smoother shooting, has a bolt carrier that can be tuned, and I can train with full size AR mags. If I were to get a PCC for serious use in the smallest package available, I'd probably get an MP5K clone or B&T APC9K if a folding stock is important, or I'd consider the Glock mag version of the CMMG Banshee with the 5" barrel if you want the standard AR controls and don't care about a folding stock.

How is the recoil on the CMMG RDB system? I keep thinking about messing around with it but I'm curious how much it would really reduce the recoil.

[757_Magnum]

If I'm spending the coin CMMG wants for that RDB bolt technology I don't want to use converted pmags.

If it helps, the CMMG Banshee originally came out using Glock mag lowers, polarizing as they can be. My first one was one of these, and it ran like a top. The relatively new Glock OEM 24 rd mags were the perfect length, as I found the 33rd ones a little bit too long.


How is the recoil on the CMMG RDB system? I keep thinking about messing around with it but I'm curious how much it would really reduce the recoil.

I found it to be significantly softer than the straight blowback build I mentioned earlier. When I threw a SiCO Octane 9HD on it, I bought the buffer tuning system and added the lowest weight. It ran flawlessly with my 147 gr reloads both suppressed and unsuppressed. I've never shot an MP5 or MPX, but the general interwebz consensus is that it's almost as soft as the former, and much more reliable than the latter.

[rdtompki]

I have two 8" CMMG RDB uppers on AR lowers (one for HD, one for competition). They run 100% using converted Pmags in the CMMG configuration. I like the idea of having an SBR lower than I can run multiple uppers on and the CMMG uppers use a standard carbine buffer; I may need to add some weight to the carrier when my suppressor comes in. Recoil seems pretty minimal; took me a bit to figure out that stouter loads actually reduce dot bounce, but this appears to be the case with straight blowback as well.

[EricM]

Can the upper from a factory complete CMMG Resolute RDB rifle that takes Glock mags be used on a standard AR lower with converted PMAGs? It wasn't clear to me whether anything about the upper (bolt, etc.) is specific to the mag type.

[CleverNickname]

Can the upper from a factory complete CMMG Resolute RDB rifle that takes Glock mags be used on a standard AR lower with converted PMAGs? It wasn't clear to me whether anything about the upper (bolt, etc.) is specific to the mag type.

An RDB upper is an RDB upper; there's only one type.