First, welcome to world of reloading! Second, the only stupid question is one not asked. You asked a very good question.
Someone said if you couldn't put a bullet into a fired case, it probably was too long. Not sure what he intended to say, but that statement isn't quite correct. If it won't drop in/through, the neck wall is too THICK. You'll need to turn the case necks then, not trim to length. Length and the neck thickness are set by your chamber (and throat and/or freebore). You need to check that neck diameter by dropping a bullet through a fired case. If it goes , good, if not , it needs turning. Unless your rifle is a custom build, the chances of that step being needed is slim. However, since it is a .243, you probably will run into a need for trimming later. The case has a lot of taper and the brass will flow at firing. Re-read some of these posts for good info. Then read the reloading manuals about reloading and they will give you some very good advise. Not just one book, but several to get other professional advise and opinions.
You have also discovered that brass quality varies from different mfg. Once you get more familiar with / into reloading , there's lots of things you can add to your basic steps to improve reliability and accuracy.
Don't stop asking questions!