@IIIWhiteWolfIIIThe SVK's standard and high power ammo deal 35 bodyshot damage and 56 headshot damage at range. I don't think you'll be using the HP ammo due to that measly mag size, and the standard issue is basically as fast as the DM7's HP.
The G428's standard ammo does 25/40, and high power 35/56 again, but has ~2x the reserve ammo count of the SVK. I can't really tell you much about its bullet velocity.
The DM7's high power ammo deals 35 bodyshot damage, but 77 headshot damage, it has a similar ammo count as the SVK and the same mag size as the G428's HP ammo - 10+1.
You WILL run out of ammo with the SVK/DM7 HP after shooting for a bit.
If you believe you can land headshots, go for the DM7, which is the strongest out of all the DMRs, however keep the ammo issue in mind. If not, G428.
For long range, I will always use the extended barrel and either the BCG light grip, or in rarer situations bipods. Bipods are ok, they will massively reduce recoil, they will not remove spread altogether like in BF4 but they seem to reduce it a bit. You can go prone only for a short while, shoot a bit, and then begin repositioning.