I just purchased a RRA 9mm Carbine with the flatop recvr (I ordered it 4 months ago). It came with a steel 25rd magazine but I thought I should have more than just one mag.  After shopping around, I was amazed to find some 32rd plastic Colt-compatible magazines from a major distributor in Texas.  The price was a very reasonable 19.99 per magazine.

When they arrived, I immediately tested them by inserting it into the RRA Carbine and pulled the charging handle to see if the bolt would lock to the rear.  The bolt locked to the rear, but the magazine fell out.  None of the mags would stay in the magwell and fell out if I moved or lightly shook the carbine.  The factory mag works perfectly but doesnt lock the bolt to the rear after the last round.

Has anyone else had this problem (non-functioning plastic mags) with their RRA 9mm Carbines? Is this one of those "proprietary manufacturing issues"?  Would the mags work in a Colt or other 9mm carbine? Or did I just get cheap mags and I got what I paid for?

I'm hazzarding a guess that the mags you purchased are the ProMag plastic/polymer ones from.  Depending on where you got them (CDNN (yes, I'll mention names)), there are two designs of these mags.  There were reported problems w/ the first generation (I do not recall what the issues were).  I'm sure someone will chime in.....

Dropping mags is an interesting issue.  I have some South African mags that drop out while others do not.  Even some modified Uzi's drop and some don't.  I've read that sometimes it's the mag, the adapter or the magazine catch (or a combination of the 3).

Been thinking of getting some, but I'm not sure if my mag dropping issue is mag related.  I switched my ASA upper to a different lower and certain mags would drop, but they worked fin in the other lower I was using.... go figure.

I too use an ASA upper and have found some interesting things about my mags!
First the upper does not have anything to do with the mags dropping or not, they don’t interact very much!  The only thing that touches the mag might be the carrier and differently the bolt to strip rounds off, but no other interaction is going on with the mags and upper.
I have been using a top loaded ASA mag block with my Bushy and now with a dedicated RRA lower.
Both had some issues with mags, first issue, always buy good mags!  All of my real Uzi mags work very well; some of the semi generic mags double feed and might fall out now and then.  But the no name shit cheap steel mags are the worst!  They don’t lock in, cant keep rounds in the pipe, and let more that two or three rounds feed correctly I should go and play the lotto that day!
Now one of the problems has been fixed by giving the Mag Catch one more turn to lock the mags in with, but the damn feeding problem is killing me.
I bought the gun for fun, I never envision myself defending my home with this, and it is for range and plinking duty only!  With that said I still want a reliable weapon to use and to bump fire at will!
I can’t tell you how much fun it is to bump fire a 9MM AR at the range!
I believe I will try a different mag adapter soon, I will keep you all posted!  

Here is a pic or two for refremce:

Well it seems like I will be forced to stay with the RRA 9mm 25rd magazines.  I took another look at the problem and the magazine-catch doesnt catch the notch in the plastic mags at all. It isnt a matter of alignment but the actual thing that is designed to keep the mags in place is very thin and doesnt extend out far enough to catch the additional 1 or 2mm needed to stay in the mag well.  Am I making sense or is the coffee taking its toll?

I guess I'll have to live with the fact the bolt will not lock to the rear after the last round.

Quoted: Well it seems like I will be forced to stay with the RRA 9mm 25rd magazines.  I took another look at the problem and the magazine-catch doesnt catch the notch in the plastic mags at all. It isnt a matter of alignment but the actual thing that is designed to keep the mags in place is very thin and doesnt extend out far enough to catch the additional 1 or 2mm needed to stay in the mag well.  Am I making sense or is the coffee taking its toll? I guess I'll have to live with the fact the bolt will not lock to the rear after the last round.

Yes, it makes sence.  Somewhere, before, someone posted that not all mag catches are "created equal".  This mean, the part that ACTUALLY engages the mag is "thinner".  There is not enough material there to extend into the mag well to catch the mag & the turning it in a few turns doesn't work either.  Someone said that DPMS mag catches worked for them since they extend the farthest into the mag well.... haven't realy looked....so....just relaying what I hear.

Quoted: Quoted: Well it seems like I will be forced to stay with the RRA 9mm 25rd magazines.  I took another look at the problem and the magazine-catch doesnt catch the notch in the plastic mags at all. It isnt a matter of alignment but the actual thing that is designed to keep the mags in place is very thin and doesnt extend out far enough to catch the additional 1 or 2mm needed to stay in the mag well.  Am I making sense or is the coffee taking its toll? I guess I'll have to live with the fact the bolt will not lock to the rear after the last round. Yes, it makes sence.  Somewhere, before, someone posted that not all mag catches are "created equal".  This mean, the part that ACTUALLY engages the mag is "thinner".  There is not enough material there to extend into the mag well to catch the mag & the turning it in a few turns doesn't work either.  Someone said that DPMS mag catches worked for them since they extend the farthest into the mag well.... haven't realy looked....so....just relaying what I hear.

Doing this helped keep some of my mags in the mag well!
I don’t know if it is sloppy dimensions or what, but a little more metal and tension on the mag won’t hurt it!

How do you turn in a mag catch?

Quoted: How do you turn in a mag catch?

Push the release button untill the catch clears the lower.  You then "screw it in" one or more turns (hopefully not too much that the threaded part sticks out of hte 'button'

I noticed that Bushmaster has started selling the plastic 9mm mags, so I e-mailed them to ask about compatibility.

Q:  Are the magazines for your 9mm carbine compatible with the Colt AR-15 9mm carbine,
    and do they have the bolt-stop feature?

A:  Yes, the polymer magazines are compatible with the Colt 9mm with a bolt hold open follower.
 
             Thank you,
             Jim Eden, Bushmaster Tech Support
             1-800-883-6229