I tried out a new 15 round 40 cal mag in my 40c today and the first time I shot it, the first four rounds failed to feed properly. Each time the rounds stopped half way into the chamber and with the base of the cartridge in the magazine and were cleared by rapping the back of the slide to force the slide closed or by tapping the magazine.

When I reloaded the mag and fired the first five rounds, it fired with no problem. I repeatedly topped off the mag after firing the top five rounds to try to replicate the malfunction and it worked fine.

I was thinking that maybe the spring wasn't seated right, as these were the first rounds loaded into it and was providing too much pressure.

Anyone else have FTF malfs with brand new mags?

I cant speak for .40's as all my mags are 9mm. The mags that came with a new gun at time of purchase and all add'l new mags picked up along the way; all have been just fine. I do clean them after awhile but never worried about it when just out of the box.
Same results for my Shield, compact, & full size mags.

I'd add that i only use new brand name ammo, no reloads or reman if that matters any.

When you put a full size magazine in a compact, without something to grip on, people tend to push the magazine back while gripping the gun. Try it with a grip adapter like X-Grip, or make sure you are not putting pressure on the magazine when you shoot it.

Great point. I should have added I used an xgrip when i ran full size mags in my compact as well. Never tried without to see what could have happened.

Check to make sure they didn't put the mag spring in backwards too just in case... I haven't had it happen to me but I've heard about it a couple times. It probably was just the spring needed to settle a bit. Whenever I get a new mag I run it through a program I made up to break it in. I take my uplula and after every round I load I "pump" the Lula up and down 2 or 3 times so it compresses the spring a few times at every interval of the compression cycle. It's a little quicker than loading and unloading 2-3 times and it seems to work pretty good for me when I get those kinda tight ones.

__________________
Equal opportunity shooter

Could have been a bit of junk inside the mag tube that prevented the follower from moving freely. I'd pull it a part and make sure it's clean and there's not some plastic flashing left on the follower or a nick inside the tube.

Ah-HA!

I took the offending mag apart and lo and behold, in the middle of the front face of the follower was a smashed, kind of bent over sprue (I believe that's the word for it) of plastic sticking out. There's also a corresponding mark in the paint or coating along the inside front of the magazine body.

I can only think that this sprue was dragging on or tilting the follower for the first few rounds and then it broke over or wore down enough to free up the follower to start working properly. I sanded it flat with some sandpaper and now moves smoothly.

Finding a possible cause is a relief as I was debating whether to carry a 10 rounder as my extra mag or the 15 rounder and I'd probably have had to buy another one if this one just mysteriously stopped working intermittently.

I'm going to take my other mags apart to check them.

It's always a good practice to inspect, clean and lube new mags. It's amazing what you find with some of them.

__________________
Centennial Every Day