First thing if not mentioned is getting a reloading manual. I use the Lymans 49th. They have all case specs.
I ran into similar problems.
measure every aspect of the case and make sure it is within specs listed in the manual. Adjust your seating die by screwing in or out. You may have to get a couple of cases and play with it.
The dies instructions should give you a standard on how to set the die. most just lift the pillar up all the way then screw in the die til it touches plus 1/4 of a turn. From there size a case then measure ALL aspects. I mean all, and thats a lot of measuring. adjust die up or down and see how the measurements change. Though the only thing that will likely be off is going to be somewhere around the neck. That's where my problem was. So play with sizing die.
Trim AFTER you size
AND CRIMP. I hear people say they dont crimp for thier AR. If you have an M4 style ramp u HAVE TOO crimp. Those ramps are hard on ammo but do help keep dirt from getting into the chamber. Measure the nexk after crimping to make sure its not bulging past specs. I crimp heavily as a matter of fact. Keep in mind this could possible effect pressure a bit too, so watch for pressure signs if ur running hot loads. Thats a given anyways.
What is ur AR?
I ask because I spoke with a gentleman at the local gunshop that worked for Colt for a time. I happened to have bought a Colt AR from him so when I had the problem I went to talk to him. Hes says Colts have the slightest bit higher tolerances in all aspects of their weapons, including the chamber. If the case is even the slightest bit past specs and sometimes in a rare case just under them u might have problems with reloaded cases.Personally most people I know that load for ARs have had some difficulties getting started.
My experience. i had Lee die and for some reason it would not size to specs no matter how I played with it. Ended up getting a reading die and it works great! Could be your die just play with it measure, play with it measure, play with it measure. Good luck. Be safe and read a lot
O and the only real difference between 556 and 223 that I know of is chamber pressure. Its probably marked 223 on the lower but if your barrel is marked 556 or its a Colt or other milspec brand than its shouldnt matter... I think.
Wen u reload Mil brass like LC you have to keep in mind that the case walls are a tad thicker so the same pweder charges in those compared to lets say winchester cases are going to be higher, so BE CAREFUL. Whats safe on Win hornady and Rem cases may not be safe in mil brass