There are two different style of 9mm ARs
First you have the Olympic pattern system that was used with standard AR lowers. These are harder to find and not quite as popular. They utilize a modified Sten / Uzi mag ( goofy block welded on ) or a aftermarket ( Hahn Precision mag block ). They also had complete carbines that utilize pistol mags
The most popular system is the Colt pattern. This is what the RRA and CMMG and Colt system uses
Although you can buy dedicated lowers ( no need for mag block ) I fail to see ( unless you have money to burn ) the purpose of that. You can get standard lowers with a mag block to work 100%
Mag Blocks
Here is where some 9mm Colt systems do not work 100% reliably if you don't set them up right. First off the problem is where the mag block is tightened. This is why on C-Products site ( one of the makers of the 9mm Colt style...and the people who make the Questar 10rd ) state plainly that you need to adjust your mag block .015 to .020" lower for RRA system
Secondly, there is a difference between mag blocks. The easiest available are the RRA / C-Products and can be identified by the squared off feed ramp
The harder to find ( Brownells ) is the Hahn blocks. As you can see from the pic below, the feed ramp is much different. The Hahn block is not finicky when it comes to the height your tighen it down at, even with C-Products mags
Having played with numerous 9mm setups, I can say that the Hahn block is much better than the other Colt style. They are also available in 3 styles. Top load, bottom load, dedicated
Bolt Hold Open
Not all Colt pattern systems will function the BHO with non Colt mags ( ie C-Products ) Too complicated to discuss here. For example, only 2 of the 3 Hahn variations has this function
UZI Mags
You can use UZI mags in Colt patter systems by milling / grinding new catch notch. You can also buy a V-M Hytec mag block that takes UNMODIFIED uzi mags.
Modification of 5.56 Lower FCG
RRA sells all the "parts" ( hammer, buffer, buffer spacer etc ) to allow you to convert your 5.56 lower to run the RRA / Colt style upper. You can use the 9mm hammer for 5.56 upper so this is not a bad way to go
Ramped Bolt ( not really a "carrier" )
You can have a RRA bolt ramped to allow the function of standard 5.56 hammers / match hammers etc.
The CMMG upper has a ramped bolt already. This alone would make me buy the CMMG over the RRA ( unless you have access to a milling machine )
In case you are wondering what happens with a non ramped bolt and a 5.56 hammer such as the RRA NM.....it locks the gun up the gun up as soon as you pull the bolt back so bad that you need to use both takedown pins to disassemble
9mm Buffer
Since these are blowback systems, you need a heavier buffer. A 5.56 upper may or may not run with this heavier buffer.
I would buy a RRA / CMMG system from Questars with a 5.56 lower. If you can get a dealer who imports from Brownells then order the Hahn bottom load or dedicated mag block. It pricey but worth the$$
I am interested in purchasing an AR in 9mm I have looked at the CMMG 9mm at Questar and they look good! Are there any other options in Canada.... (Can you use the 10 round LAR mags with these or are they still 5 round cap) and yes I would like to stay under the $1800 mark to start.
