I have two two SBR'd lowers, one of which is housing a 7.5" Upper in .223/5.56.  I won a Larue Stealth Barrell, so that is going to replace the 7.5" and permanently claim the SBR'd lower.

I am still wanting to have a >11.5" PDW style weapon but am considering getting rid of the 7.5" .223 as it is miserable to shoot.

My question is, should I go 9mm?  With such a short barrell, is there any more lethality with .223/5.56 versus 9mm?  

The pro's I can think of for the 9mm would be lighter, quieter(maybe), cheaper ammo, traveling with a AR Pistol.  I do have have a 5.56 suppressor.

If you say yes to 9mm, would you go AR, MPX, or Zenith Clone?  Would definitely utilize a SB PDW Brace.

Also, is there any length savings with the SB PDW Brace versus a standard buttstock or the LWRC Compact stock?

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Do the 9mm

I have a 9mm, 7.5" Barrel and Shockwave Brace and Law Tactical Folder, All you would need is a Magwell Adapter (Hahn, KAK, Macon Armory) 9mm Pistol Buffer, and a suitable Upper. With the Law Folder you really reduce the overall length, since you have SBR'd lowers what ever stock you like, only draw back, you can't fire while folded.

Snellkid... when you installed the LAW spacer to connect to bolt to the buffer, did the 9mm bolt you were using just have a weight to remove and was then just a standard length AR carrier so you could just use the LAW part, or did you machine a special connector... thinking about the same set up with a 7" 9mm and PSA Glock lower with pistol tube and no blade

I used a CMMG Bolt and KAK 7.5oz Buffer w/ Carbine spring, runs great, PSA Colt lower, PSA upper

thank you... after I posted, I decided to just drill a hole to fit the connecting plug....  the PSA bolt has a solid extension to add mass..  the idea of an affordable MP5 size 9mm is kind of intriguing

I used a Blitzkrieg Components Hydraulic Buffer and an Armalite .308 spring

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What upper/lower ?

.223/5.56 pros and cons

Velocity wise, a 5.56mm M193 round will deliver around 2400 fps in a 7.5" barrel.  Commercial .223 in a 55 gr load will be closer to 2300 fps.  The 62 gr M855 will do about 2350-2400 in a 7.5" barrel.

You need around 2700 to get a .223 FMJ to tumble, and while you can achieve that velocity in a 10.5" barrel, any way you slice it the muzzle velocity will just not be sufficient to make that happen in a 7.5" barrel.  Consequently, you'd want avoid shooting FMJs in a 7.5" barrel PDW.  However, I've found that most .223 soft point bullets will still expand reliably at those velocities, so terminal performance is still very good - if you stay away from an FMJ.

On the other hand, in terms of noise and flash, as you've noted the muzzle blast and flash on an unsuppressed 7.5" .223 is obnoxious - and is definitely not something you want to fire in an enclosed space without hearing protection.


9mm Luger pros and cons

The 9mm Luger is available every where and if you build a quality 9mm AR pistol it will feed hollow points just fine.   You've also got lots of options from sub sonic 147 gr hollow points to 124 gr and 115 gr hollow points, all of which will expand, and all of which will be effective out to about 150 yards.

Accuracy with my 10.5" LAR-9 has bee superb - for a 9mm pistol or even a 9mm carbine, with 5 shot 50 and 100 yard accuracy between 1.5 and 2.0 MOA with jacketed hollow points and about 4 MOA with inexpensive heavy plated bullets.  

A 9mm AR pistol is also a lot less abusive on unprotected ears, in the event you ever have to fire it indoors without hearing protection.  Overall, I think a 9mm AR pistol is a lot more pleasant to shoot than a .223/5.56mm pistol, indoors or out.

9mm Luger is also less expensive and allows for more practice for a given dollar amount.    


Of course, if you decide on 9mm Luger, you then have to choose the magazine pattern.

Olympic arms used to make lowers for Sten pattern magazines back when they were common and very cheap.  Then they switched to a modified Sten mag that was not cheap, and eventually Oly switched to a polymer mag that had the same profile at the mag well as the .223 mag.  They obviously were not as easy to store or carry.  

Colt uses the Uzi pattern magazine with an insert in the magazine well.  Rock River Arms also uses the Uzi pattern magazine, but has a dedicated 9mm lower.  

The advantage with the Uzi magazine are that Uzi mags are still cheap (but supplies are drying up) and you can modify them by adding a cut for the AR mag catch on the side, and a clearance cut on the back for the ejector.   They are also very tough and very durable.  They are however primarily limited to 25 and 32 round variants, although there are some Uzi pattern 10 round magazines available.

Glock pattern  magazines are the third option, and 9mm AR-15 lowers that use Glock magazines are available from several companies - which might be nice if you also own a Glock pistol.  Some folks also prefer how pistol magazines load as opposed to SMG magazines.

Thank you for the great write up, it is greatly appreciated.  I do own Glocks, so a Glock magazine compatible lower would be ideal.  Are all 9mm ARs blowback operated, or are there DI on the market?

My understanding is that Colt messed around with DI in their development of a 9mm SMG, but went with blow back operation.  I've always asked myself why they did that and took it as a caution every time I ever thought about getting a 9mm DI AR-15.  My understanding has been that the major problem with an 9mm DI AR-15 has been factory ammunition gumming up the gas tube.  I've heard the guys that have them have better luck with hand loads.   I can see where Colt might not have wanted to go down the whole ammo/gas tube problem road again.   In any case, Colt started making their blow back operated 9mm SMG in 1982 and started selling the civilian blow back operated 9mm carbine in 1985.  


There are however some 9mm DI AR-15s options out there:

Stag Arms made a 9mm DI carbine a few years ago, but I think it's been discontinued. Still you might find one NOS, or in gently used condition.

Angstadt Arms, down the road in Charlotte NC, markets their UDP 9 which is their own take on a direct impingement 9mm AR-15.  It's been restyled a bit, but it operates on the same principle - and I believe it uses Glock magazines.

Ronald Williams, owner of RMW Extreme, is a member here and I believe he still sells a 9mm DI AR-15.  You could send him and IM and see what the wait time is for one.   He'd also be the guy to give you the skinny on how best to run one.

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Personally, I've been happy with blow back operation in mine.  When all is said and done, even after 500 so so rounds are put down range, the bolt, upper and lower are surprisingly clean - and that's using hand loads with Unique, which is a great powder, but one some folks refer to disparagingly as "flaming dirt".  I suspect it might be a bit dirtier with light loads, but it works great in my Uzi and LAR-9, where the loads are close to maximum.

Given that an AR pistol is likely to weigh upwards of 7 pounds, blow back operation doesn't pose any recoil issues like it would in a pistol. A DI pistol could be built a bit lighter as the blow back operated bolt and buffer assembly are heavier, but on the other hand, there's no gas block.