Real newbie questions

[bonzaiii]

Could someone possibly explain the difference in the following three sights and help me with the advantages and disadvantages of each.
30 mm red dot sight
30 mm 2x red dot sight
red dot open 4 reticle

Thanks in advance

[HK Dave]

30mm = Size of tube, it is the dimension of the mounting device for the red dot tube.
2x = magnification.
2moa = size of dot, 1moa is 1" at 100 yards.
Red dot open 4 reticle = I have no clue where you saw this, but I will try to take a guess... red dot open reticle is probably something like a C-more sight or RMR type deal that doesn't have anything clunky on top blocking your line of sight... as for 4 reticle, it either means there are 4 reticles to choose from or it is a 4moa reticle.