I have a new M&P9 FS that I got two months ago. I had it out to the range last week and I shot 180 rounds of Winchester 115 grain target loads. There were no malfunctions of any kind, though quite a number of casings ejected straight back and hit me in the head. Does that indicate a problem?
Also, I had been only able to load 9 of 10 rounds into the magazine, even with a mag loader. Last night I was able to load 10 rounds into the mag, but it was really tight, and I couldn't get the mag to lock into place in the grip handle.
Is this common with new mags and how long does it take for the springs to loosen up?
I took the 10th round out and left only 9 in, just in case I have to use it in an emergency.
Thanks.

I'm assuming you live in a "restricted" state. The 10 rounds mags use the full 17 round springs so yes, they are tight.

As far as the ejection, try different ammo.

I have the exact same issue with my M&P9 FS when loading the mag. Even with my mag loader, I can only load 9 rounds and I've had it for 3 months now...shoot every weekend...and even left a few rounds in the mag so that the springs will loosen up. Same result. I can only fit 10 rounds after I load the 9 rds with the mag loader then I'd have to physically add the 10th round myself, and it's stiff as a board. I'll give it a few more months and let it sit over night (which I ended up stopping after a while because it remained stiff) before I try any modifications.

Yes the mag springs are very stiff, but they will soften-up as you continue to shoot! My FS9 has over 2,100 rds. through it and I no longer need to use the UpLula Mag loader. (17 rd. mags.) I can load them by hand!

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It's The Carbonaro Effect!

From my experience most magazines start out stiff and loosen up some. Leave them loaded with the full 10 when stored to help loosen them, storing with a few rounds probably won't make much of a difference. I had a Springfield with 16 round mags, and after awhile I was able to load 17 with the uplula...and never an issue.

The 10 round magazines are an abomination caused by govt. Consider that they have told the manufacturers that if they can somehow cram 11 rounds in a restricted magazine they are going to criminally prosecute everybody involved. So they are more difficult to load than the regular capacity magazines, and if you are clever enough to fix them and get caught.................

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Science plus Art

You may be on to something. I have zero problems loading 17 rounds into my mags. I never have to use a device to help load my mags. I only time I use a device is when I to the bottom of a 32 round Glock mag. I love living a free state.

Thanks for all your responses. I live in NJ, where the limit is 15 rounds, but S&W only sells the M&P's with 10 and 17 round mags. I believe I can order 15 round mags from them.
Last night I was able to get the mag with 10 rounds to seat into the grip handle, but it was real stiff. I'm only going to leave 9 rounds in the mags at home until I get to the range and see if I can chamber the 10th round safely with no problems.

This site says in stock.

Smith & Wesson M&P 9mm 15-Round Magazine