After having quite a bit of bad luck with 20 and 32 round Metalform 9mm mags, I decided to give the current stainless 9mm magazines from ASC a try (formerly C Products).  The ASC mags have a reputation for being not as reliable as or functioning as well as Metalforms, however they're usually about $10 cheaper.  My initial impression was that the ASC mags were crap.  However, the problems I experienced were easily remedied and they're working fine for me now.  I do not recommend checking magazine function with live ammunition.  

Build details:    
Essential Arms lower
DPMS LPK w/ ALG ACT trigger
Hahn dedicated 9mm mag block
YHM 9mm marked upper
Colt Gas/Shell deflector and ejection port door
Wilson 16" 9mm stainless barrel
CMMG 9mm BCG
Spike's Tactical ST9X 9mm buffer
BCM receiver extension with DPMS carbine recoil spring

Here is what a 32 round looks like fresh from a retailer:


First, let's look at the feedlips.


If you notice, the feedlips are completey coated with whatever finish these stainless mags have.  At first glance, everything is fine.  I inserted the mag on a closed bolt, pulled the charging to the rear and the bolt did not fully cycle.  See the pic below:


If you look very closely to the right feedlip (closest to you on the screen), you'll see a tiny bit of the finish being scraped off.  It appears that the finish is thick enough to impede cycling of the bolt.  To fix this, I simply cycle the bolt several times.  Here's a shot after a few more cycles.  You can see the bolt has moved forward a bit more:


When it's done, one or both of the feedlips will be slightly shiny from the bare stainless steel:


Once the bolt is cycling fine, we can move on to the next problem.  The follower is not rising completely.  This causes feeding problems as the mag is down to the last couple of rounds, and also prevents last round bolt hold open.  Why isn't the follower rising to the top of the mag?  It's hitting the feed ramp on the mag block.


To fix this, the follower needs to be ground to size.  Using 120 grit sandpaper, simply grind off a few hundredths of plastic from the front of the follower.  I added a very slight bevel to the top/front of the follower as well.  Go slow, checking fit frequently.  


Once there is enough clearance, the follower will rise completely.


At this point, you've had the mag in and out of the gun several times and you may have noticed it can be very difficult to seat.  To make it a little easier to seat, use a small file to grind away some metal at the TOP of the mag catch hole in the magazine.  Be careful not to remove much or you'll have feeding problems.  The mag is upside down in this pic, so be careful of what you are filing.  


The ASC magazine operates the bolt hold open function using a step that's molded into the rear of the follower.  This design seems more reliable than the flimsy welded-on tab of the Metalform mags.


If your bolt stays open on an empty mag, that's it, you're done.  If it does not, chances are the follower needs to come up a bit higher to activate the bolt catch.  To accomplish that, you need to grind down the entire left edge of the follower since that is what limits upward travel.  Again, remove a little bit of material at a time, checking function frequently.  Once you achieve the desired result, stop grinding.  


At this point, I recommend cleaning your mag block.  The finish of these mags does not seem particularly durable and I noticed some build up in my mags that came off with a nylon toothbrush.  You can see the shiny spots where the finish has begun to rub off.  It's up to you if you want to use some fine steel wool to remove it now, or wait for it to come off on its own in those wear spots.  If you have a mag that seems a bit sluggish to drop free, the finish might be what's causing that.

Very nice. How many mags did you buy, and how many had to have all these modifications?

I bought five at the same time from Rainier Arms about two months ago.  I presume they are from the same batch and are "panic-era" mags.  All five had the same issues and were corrected in the same way.

It takes about 20 minutes to do all the mods, give or take.  It depends on how aggressive you are with the sandpaper.  I did not use any power tools.  Something like a benchtop belt sander could make it go very quickly, but you run the risk of taking off too much material.  

I think much of this can be attributed to the variety of 9mm parts on the market and tolerance stacking.

You might have some problems if you ever decide to get out of 9mm and sell the mags.  Disclosing that they were cut, sanded and filed on to fit your specific gun pretty much makes the resale value zero.

But, glad your setup is working.  I went through the same steps about 4 years ago with the original cproducts mags.