I received my 3 mpx mag with the pouch the other day and used in range today. But I found out that the three of them are not drop free. The only one drop free is the one came with the gun original. Also the two piece of my bolt carrier are kind loose and have play. I can actually move the piece which holds the spring rod a little bit. Does all these happened from you guys?   My MPX just got back from SIG where the original problem is loose hand guard. They fix it but now I found new problems.....

The part in red was my exact experience.  First mag fine, the next three would not drop free.  After a little use, they're better, but not perfect.  They drop free only sometimes.  I ordered 4 more magazines and they also won't drop free, and are about as tight as the first 3 were before use.  All 8 magazines will drop free from the lower only with no problem, but behave in the above-described ways when the upper is attached.  I've only fired the first 4 so far and they functioned fine.

I measured many dimensions of my "good" magazine and the others were identical.  It's been a puzzlement and I've been waiting for someone else to report this issue online.  You're the only other person I've found that reported this experience.  I'm hoping a few more do also, so we can put our heads together and figure it out, because I'm stumped.

I've just tested all 3 of my discounted mags and all are drop free.

Mine start to drop free sometimes after I used it for couple of days. Does your bolt carrier loose at all?  About to call sig tomorrow again.

 
Is this an issue? My MPX handguard has a bit of front to back play, but I figured that was inherent to the design of the handguard retainer.

Mine does it, NBD.

Does anyone here notice any difficulty when inserting a fully loaded mag and using the charging handle to chamber a round? When doing this on my mpx it seems like the bolt gets stuck when traveling over the cartridge. I also notice that it will scrape the casing. It seems to me to require twice the force to chamber a round this way compared to an AR15. Is this inherent with the mpx design?

What are you guy's using for a mag loader?

I called LulaLoader and they said one one going to be done.........but when? IDK

yeah, same feel here. Sent back to sig and they said its fine. Now I just have to pull harder...

All of mine (3) drop free every time

https://www.youtube.com/watch?v=y6qRbE5JJWU here is the video that I showed my mag is not drop free....

removed by user....

We need AXTS to make a charging handle for them. I'm sure they come make a good one.

Bumping this thread back up with an update.  My magazines drop free after being used a bit, glad to report.  However, I have noticed my carrier key is loosening up similar to what the OP reported.  Anyone else experiencing a loose carrier key?  Normal behavior, known issue, random fluke?