Grab 5 of your LC brass and spit them in two (mouth to rim). You are trying to see the thickness of the case wall just above the web section.
The reason for this, if the ammo was first fired in a M-249 with a loose chamber, then the cases where way over fire form expanded, and when you re-sized the cases back down, the case wall just above the web section become very thin.
The second thing that you are checking when you slit the cases, just how brittle is the brass. So resellers of mil brass do an acid wash to clean the cases, instead of just tumbling, which make the case very brittle and more acceptable to splitting as well.
As for getting the split case out of the chamber if you don't have an slit case extractor, push a cleaning brush down the muzzle out the back of the receiver, install a 30 cal brass brush to the rod, pull the bush back until it in the case, then give the cleaning handle a good blow back down the barrel. The angled bristles of the brush will hold onto the case insides, as the brush and cleaning rod is driving back towards to the back of the receiver to remove the split case. As for getting the brush out of the case, hold the case and turn the rod/brush clockwise as you pull the case off the brush (angling the bristle to the sides, instead of straight forward).