The DMRs in this game are ridiculously accurate. I love using an RDS with them. Heres a clip of me using the RDS with the SKS, the distance for kills I can get with such a simple scope is nuts:
If you're using straight pull bolt, you have to get good at pressing R3 to hold your breath before taking a shot. I almost never use straight pull bolt or the range finder. I use Bipod and 14x variable are on all my sniper rifles. If I'm using an 8x scope, I can switch to 14x for a more accurate headshot, or if I have a 40x scope I switch to the 14x to find my targets / spot and then zoom in for the kill. The bipod makes every shot pixel perfect. Go prone, bipod, line up a shot on a guy, take the shot, unzoom for the scope animation and when you zoom back in you'll be lined up exactly where you were just aiming.. then aim slightly up or down to take the next shot. If he moves, follow him and aim at him again when he stops moving. If you're in a good spot and don't see any enemies, hit and hold the options button, hit R1 to the large map and watch it for a while.. watch the edges of the maps for a random red enemy triangle to pop up, that usually means its a sniper. Aim towards that location, spot him and take him out. If the guy is too far off, aim for the flags or go aggressive with the sniper rifle.
A good way to get used to the bolt action is to get the timing down and dodge left or right during the animation. Don't move the R stick at all after firing.. Shoot, unzoom and immediately zoom back in.. this will make you zoom right back in after the animation.. while the animation is happening, strafe left or right, then immediately back to where you just where and when it zooms back in, you'll be aiming at what you were just shooting at for a follow up shot. When I do this, I usually dodge a shot and save my own ass in the process. This isn't the best example of my strafing but you can see how it still effective:
The range finder can be useful to figure out the distances on the maps until you get used to it or if you want to make sure that first shot is a kill. You can find the exact range of an enemy, if they are 170 away, get further back until you are exactly 200 away from the enemy, set your rifle to 200 and aim directly at their head for a perfect 1 hit kill.