Normal Dies (FL included) do a poor job of squeezing down a belted mag case in the web area. If you're shooting load hot enough to expand the area above the belt you have a couple of options that I know of;
1) Reduce your load so they don't expand.
2) Buy one of Larry Willis' collet dies.
http://larrywillis.com/Watching this thread to see if the ingenious folk on the fire have different or better ways to manage the expansion on belted mags.
