SIZE DOES MATTER: C-Products magazines in Rock River LAR-9
I purchased a new RRA LAR-9 9mm AR-15, (SBR).  It came with no magazines, so I used the two Colt magazines that came with my Colt AR6450 AR-15 9mm Carbine.  The Colt magazines ran flawlessly in the RRA.  I queried all my friends and associates regarding magazine choices for the new SBR.  They ALL ran C-Products magazines flawlessly in their AR’s.  They were highly recommended, from both a reliability and pricing standpoint.  So, I ordered 18 of them, in the 32rnd and 20rnd configurations.  
My new RRA LAR-9 9mm SBR became a ‘JAM_O_MATIC’.  (failure to eject issues).  I cycled through about half of the C-Products magazines, with 5 different brands of ammunition, in different configurations, both with and without the suppressor, still experiencing Failure After Failure ! ! !
I was beginning to believe the RRA LAR-9 had completely ‘crapped-out’.  Then, I noticed the huge wear marks, on the outside of the feed lips, of every C-Products magazine.  I compared them to the original Colt magazines.  There was zero wear on the Colt’s.  
I began to take some measurements.  The C-Products magazines were sitting higher in the receiver, causing drag on the bolt as it cycled.  Was this extra ‘drag’ enough to slow the bolt, and cause the consistent malfunctions ?!  
Solution:  I removed a few hundredths of material from the top of the magazine catch hole. This allowed these C-Products magazines to now sit at the same height as the original Colt mag’s.  Problem solved.  I ran every modified magazine through several brands of ammo, suppressed and non-suppressed.  300 rounds with ZERO FAILURES.
SIZE DOES MATTER . . .

Next time, get a sample and see how things go!  18 mags!!

I am very relieved for you that you got them running!

This was always a magazine issues, not an issue with the LAR-9.  RRA makes a quality products. I am a firm believer!
I meant no disrespect to RRA.

It has always been my belief that my dealer kept my two magazines.  

Steve,

Thank you for assisting with researching my "situation".

Best Regards,

Waldo-Jr

Here's the deal with C-Products magazines.  They are designed with a rather steep feed angle, and a LOT of people with dedicated lowers have problems with them because of that feed angle.  My RRA 9mm carbine is non-dedicated - it came with a magazine block that I can tweak a little for height.  That allowed me to use (old style, original company) C-Products magazines without a problem.  Dedicated lowers don't give you that ability, and C-Pro mags are going to be an issue because of it.

I'd like to add that my RRA carbine came with a magazine that looks like a modified Uzi magazine, and I bought a second to go with it.  Those magazines "just run."  No issues at all.  The pictures of their 9mm magazines on their web site look like their OEM might be Metalform...  That's a Very Good Thing, by the way.  Steve, can you comment on that?

Strange I have a GPI dedicated 9mm lower has a little bat or some shit on the side, takes colt mags, and have 10 ASC's with the orange followers and cant get this to jam if I try, and I always try by wiggling the mag as much as a can when I shoot it

I had the same problem with a new mag.  I pushed the follower down about ten times with a long screw driver and have never had another problem.