I bought the Langdon Tactical RDO slide for my M9A1 tonight and now I have to choose a red dot. I have a Holosun 503G on my Stribog and a 510C on my 7.62x39 AR that I love so I'm probably going to get a 507 but the problem is this; I don't know the difference between the V2, X2, 507C, 507K, etc. I'm heading to youtube now for some education but if anyone has any experience with those, let me know.