Sniping, Using the Mk12 SPR Options

[Noob-1]

Hi. Im new to the forums, I just got arma and I really love playing it. Lemme get straight to topic...the MK12 SPR... I tried using it, and I cant hit any long range targets, in fact its really hard for me (embarrassing as well) and Im doing this mission where you need to use it. Once I had to kill some friendly sniper so I could steal his m40 cos it's more accurate lol. Basically what Im asking for is there any guide for sniping for this game or if you have any tips for me on the Mk12...Any help would be appreciated..Thanks in advance. My problems are that Im a noob and I dont understand long range shooting...and how to use the scope for effectively in such situations.