AR LPK - 9mm vs .223

[gungate]

Is there any difference between a 9mm and .223 BCG when building an AR in 9mm? I have a milspec lower but see they sell BCGs as 9mm and .223. Can I use either?

Is there any difference between a 9mm and .223 LPK when building an AR in 9mm? I have a milspec lower but see they sell LPKs as 9mm and .223. Can I use either?

 

[E.Shell]

Yes, most/all 9mm are blowback and don't use a hollow gas key, rings, etc..

 

[mtlcafan79]

Within 9mm BCGs you also have ones that are Glock mag compatible, and some are not.

 

[Boats]

Is there any difference between a 9mm and .223 BCG when building an AR in 9mm? I have a milspec lower but see they sell BCGs as 9mm and .223. Can I use either?

Can you use either? No

The basic function of a 9mm AR is different than a .223 i.e. one is gas operated, one is blowback operated.

Gas operated on gas pressure. Blowback operates on mass and spring pressure.

Not to mention differences in bolt face diameter, cartridge size, ejector location, feed ramp location, weight, presence of a firing pin spring, etc etc etc.

 

[gungate]

Can you use either? No

The basic function of a 9mm AR is different than a .223 i.e. one is gas operated, one is blowback operated.

Gas operated on gas pressure. Blowback operates on mass and spring pressure.

Not to mention differences in bolt face diameter, cartridge size, ejector location, feed ramp location, weight, presence of a firing pin spring, etc etc etc.

Dammit! I screwed up my question. Yes, I know the BCG is different. What I actually meant to make this post about was the lower parts kit and trigger.

 

[outrider58]

Are we confusing upper parts with lower parts?

 

[gungate]

Are we confusing upper parts with lower parts?

I am confusing my acronyms...

 

[E.Shell]

The hammer is different, at least with my kit, as well as the buffer & spring. Been a long time, but that's all I can recall having to change out when I converted one to 9mm..

 

[GuitarmanNick]

I am using a standard, mil-spec LPK in my 10.5" blowback AR-9 pistol build. The BCG is a hybrid and works with Colt or Glock style mags. Using an 8.5 ounce extended length buffer, .308 spring, Stern Defense magwell adapter, and Glock mags in mine.

 

[gungate]

I am using a standard, mil-spec LPK in my 10.5" blowback AR-9 pistol build. The BCG is a hybrid and works with Colt or Glock style mags. Using an 8.5 ounce extended length buffer, .308 spring, Stern Defense magwell adapter, and Glock mags in mine.

Cool thanks! Sounds like I can use a basic 2-stage AR .223 trigger.

 

[Boats]

Cool thanks! Sounds like I can use a basic 2-stage AR .223 trigger.

Should be able to

 

[outrider58]

The hammer is different, at least with my kit, as well as the buffer & spring. Been a long time, but that's all I can recall having to change out when I converted one to 9mm..

AR9 hammers are beefed up because of the heavier shot impulse in them. If OP is going old school direct blowback, his buffer and springs will be heavier as well.

There are many solutions out there for taming the heavy impulse of direct blowback AR PCCs. The latest of which is the Maxim Defense roller delayed buffer system.

ROLLER DELAYED SYSTEMS – Maxim Defense

maximdefense.com

I would also recommend for anyone who is building their own AR9 go with an upper and lower receivers that are made for pistol calibers. More of a personal preference than anything born out of necessity.

 

[alucard0822]

Most AR9 lowers primarily use AR15 parts kits, but many have some proprietary parts, and there are considerations for buffer and triggers that are different from gas op builds.

Generally the takedown pins and detents are the same
Grip, selector, detent and spring are usually the same
The bolt catch/plunger/spring might or might not be the same, some include a LRBHO linkage that replaces it, some use a lighter spring or modified catch.
Mag catch/spring/button is usually proprietary, and usually included in GLOCK mag lowers
might have threaded pins for the bolt catch, and threaded holes to capture the rear detent.
The buffer retainer/spring is usually the same, but some buffer setups like the JP SCSS or Maxim RDB don't need it.
the buffer tube, end plate/castle nut are usually AR15 parts, but "enhanced" buffer tubes with an extended section to support the carrier better can avoid wear with some 9mm BCGs.
the pin/trigger guard are usually machined in, but if not AR15 parts usually fit.

The buffer/spring will be different. Being blowback, and lacking a gas system, 9mm ARs use mass or a delay mechanism to control blowback. Where 5.56 might have buffers ranging from 3-5.5oz, 9mms start around 8oz, and go up from there. They also have to be longer or use spacers as 9mm bolts have a shorter stroke than 5.56

The trigger group may or may not work with a standard AR15 FCG. 9mm being blowback has a quick recoil impulse, and it can cause issues with some bolts and FCGs. Ramped bolt designs do reset the trigger a little easier, and can reduce some of the issues, but they still can cause failures or wear out triggers faster than 5.56 builds. Purpose built 9mm FCGs can work well, lighter hammers, cushions, and a few other design changes can make them run better with AR9s, personally I like the Hiperfire PDI series.