Funny you mention this, I just had this happen to me as well this morning. I have one Brownells magazine and it went into the well and I did not think I was going to get it out.
I have used other standard USGI spec aluminum magazines without problems. Just seems to be the Brownells magazines. You can also try the Magpul E-mags, they are the European cousin to the P-mags. The standard P-mags do not work that well in the 416.