I took my AR to the range today to zero my 552. I ended up popping off 280 rounds, 4 of which were FTF. I only used 1 of the 2 mags I brought. The mags are both NIW USGI 30 rounders that I used for the first time.

Anyway, when I had a FTF, the round in question would try to feed but wouldn't go all the way up. The bolt would then try to go forward but hit the back of the cartridge. The force of the bolt would slam the back of the cartridge causing the bullet itself to get pushed into the shell. Go figure. BTW, I was shooting Winchester 55gr FMJ whitebox.

I'm pretty sure the problem I'm having was related to the mag or my AR still needs to be broken in (I've only got @ 550 rnds through it so far) but I figured I'd check to see if I'm missing something.

TIA

Nope, it's a magazine problem.  The bullet nose is not being presented high enough to feed straight.

Thanks for the reply Ohio. As I said earlier, the mags were "NIW" (until I used them today). They had black followers but I replaced them with green ones. IIRC, the mags were made in '87'. I doubt the mag spring is the culprit so that would leave me to believe that the followers just need to be worked in a bit so they are "smooth". I've heard of some guys using a ruler (or something similar) to work the follower and break it in. Does that sound like a good course of action?

I had a similar problem.  Turned out the bullit was popping out of the mag too easily.  When the bolt returns the round was just laying on top of the mag with the nose too low to get fed properly.

In my case, I had to bend the lips of the magazine down to conform to the case of the top round.

I've used the same 20 round mag with minor adjustments now and then for about 4 years.  I estimate more than 6000 rounds have gone through the thing.  It's well worn but very reliable.

Billski told of us a magazine fountain test.  Fully load the mag and bang the bottom on something.  Not so hard the mag is damaged but hard enough to get the bullits to rebound into the mag a little.  In a bad mag, the bullits will come spewing out.

Thanks for the advice oldguy. I did fully load the mag once and, each time I load the mag, I do knock the back of the mag against something to firmly seat the rounds. I can try your suggested test at home. Since I've got a LULA, it should be pretty easy.