I was looking at 9mm ammo and was amazed at the variety. I didn't see a thread discussing this and just wondered what the differences were. As labeled, there was Luger 9x19, Makarov 9x18, Largo 9x23 and no name 9x21. I assume these are measurements of length. Cabella's had only Makarov the other day but I didn't buy, not knowing if these rounds are interchangeable with 9mm Luger eating gun (XD). Looking for wisdom.